Wetwood in Trees:A TimberResourceProblem

ReportPNW -112

August 1980

United StatesDepartment ofAgriculture

Forest Service

Pacific NorthwestForest and RangeExperiment Station

General Technical

J.C. Ward and W .Y. Pong

Abstract

Available information on wetwood is pre-sented. Wetwood is a type of heartwoodwhich has been internally infused withwater. Wetwood is responsible forsubstantial losses of wood, energy andproduction expenditures in the forest prod-ucts industry.

The need to evaluate these losses and tofind ways to eliminate or minimize them isemphasized.

Because of increased interest in wetwood,excellent opportunities exist in initiatinga comprehensive program of research. Bothshort and long-term studies are included inthe program with the short-term studiesanswering the immediate utilization prob-lems created by wetwood and the long-termstudies directed at examining the causes ofwetwood and its control and prevention.

Metric Equivalents1 inch = 2.54 centimeters1 foot = 0.304 8 meter1 cubic foot = 0.028 32 cubic meter1 pound = 0.453 6 kilogram1 pound/square inch = 0.070 38 kilogram/

square centimeter5/9(°F-32) = °C

Contents

Page

INTRODUCTION . . . . . . . . . . . . . . 1

CHARACTERISTICS OF WETWOOD. . . . . . . 1

CAUSES OF WET-WOOD FORMATION . . . . . . 2

WETWOOD IN TREES AND LOGS. . . . . . . 4Occurrence. . . . . . . . . . . . . . 4Types of Wetwood and PatternsWithin the Tree. . . . . . . . . . . 8

WETWOOD PROPERTIES--A MICROBIALIMPLICATION . . . . . . . . . . . . . . 10Odor . . . . . . . . . . . . . . . . . 10Hydrogen Ion Concentration or pH. .. 10Anaerobiosis. . . . . . . . . . . . . 12Strength Properties . . . . . . . . . 12Permeability. . . . . . . . . . . . . 14Chemical Brown Stain Precursors . . . 15

TIMBER UTILIZATION AND WOODPROCESSING PROBLEMS . . . . . . . . . . . 16

Shake and Frost Cracks. . . . . . . . 16Checking and Collapse . . . . . . . . 19Slow Drying Rates and UnevenMoisture Content . . . . . . . . . . 23Chemical Brown Stain. . . . . . . . . 28Plywood and Reconstituted Boards. .. 32Corrosion of Kilns. . . . . . . . . . 33

RESEARCH NEEDS. . . . . . . . . . . . . 34Short Term . . . . . . . . . . . . . 34Long Term. . . . . . . . . . . . . . 38

CONCLUSION. . . . . . . . . . . . . . . 41

LITERATURE CITED. . . . . . . . . . . . 41AUTHORS

J.C. Ward is research forest productstechnologist, U.S. Forest ProductsLaboratory, Madison, Wisconsin.

W.Y. Pong is research forest productstechnologist, Pacific Northwest Forestand Range Experiment Station, Portland,Oregon.

Introduction

This paper summarizes available informationon the timber quality characteristic des-cribed as wetwood. Included are some pos-sible causes of wetwood, where it occurs intrees, its properties, and the problemsassociated with it. The need for researchto solve utilization problems associatedwith wetwood and the processing of logs andproducts containing it is recognized.

With this report as a guide, a concertedresearch effort directed at the problem ofwetwood can be organized. Such researchwill, in the short run, target in on andprovide answers to wetwood-related proc-essing problems presently plaguing industryand, at the same time, build a base of sci-entific knowledge with which to solve theproblem of wetwood in timber stands.

Characteristics ofWetwood

Wetwood is a type of heartwood in standingtrees which has been internally infusedwith water (64, 79, 142). Wood infusedwith water from an external source (e.g.,logs exposed to spray, rain, or storageponds) does not fit this definition. Insome tree species, wetwood often has awater-soaked translucent appearance and isvariously designated as water core, sinkerheart, wet core, wet heart, and discoloredwood. When frozen, the water-soakedwetwood appears as a distinct, hard, glossysurface on the ends of winter-cut logs ofScots pine (Pinus sylvestris L.) (117) andbalsam fir (164).1/

In other species, the typical water-soakedappearance may be absent. In this case,wetwood has the appearance of either normalheartwood or it has an unusually darkcolor. This dark color leads to the term"false" or "pathological" heartwood. Redheart in white or paper birch is a type ofwetwood2/(31, 68). One type of wetwoodin white fir is called blackheart (146).

1/Scientific names of commercial NorthAmerican tree species mentioned in thisreport are listed in table 1 (conifers),page 5, and table 2 (hardwoods), page 6.

2/R. W. Davidson, Colorado State Univer-sity, personal communication to J. C. Ward.

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Causes of WetwoodFormation

In general, wetwood is higher in moisturecontent than the adjacent normal heart-wood. Reports of unusually high moisturecontent in heartwood may result from aninvestigator's inability to recognizewetwood. In comparison with sapwood,wetwood can be higher in moisture content(67, 79, 106, 117, 197) or lower or equalin moisture (20, 48, 121, 133, 203, 212,215). Regardless of external appearance,wetwood differs from normal wood inphysical and chemical properties andgenerally is more difficult to dry.

Wetwood in trees has been attributed to anumber of causes: microbial(bacterial), nonmicrobial (injury), andnormal age-growth formation. Associativerelationships have been the basis of manyevaluations of the causes of wetwood;actual tests to confirm cause-and-effectrelationships have had little success.

Much of the research on wetwood hasbeen directed toward investigating theassociation of bacterial infection and theoccurrence of wetwood in trees (20, 25, 46,49, 60, 78, 79, 93, 106, 158, 167, 168,190, 191, 202, 203, 206, 207, 214, 217,219). Not all of these studies have foundbacteria to be associated with wetwood.Bacteria have frequently been isolated fromhealthy sapwood and heartwood of livingtrees (12, 13, 40, 107, 144, 190, 214).

The concept that wetwood is the result of amixed population or invasion of more thanone species of bacteria could explain someof the differences in results. Thisconcept would be analogous to the suc-cessions of organisms in the development ofwood decay and associated discolorationsdescribed by Shigo (171) and Shigo andHillis (173). Strictly anaerobic bacteriaas well as facultative anaerobes have beenisolated from wetwood (158, 174, 199, 202,203, 205, 206, 222). Laboratory techniquesfor isolating andculturing strictlyanaerobic bacteria are different from thosefor aerobic and facultative anaerobes andfungi. This could explain why investiga-tive results do not always agree about therelative importance of bacteria to forma-tion of wetwood.

That wetwood formation or initiation may benonmicrobial in nature has been suggestedby some investigators (20, 45, 107, 144)and implied by others (12, 40 , 190). Theseinvestigations conclude that bacteria arepart of an indigenous microflora of normalwood, living in a viable but quiescentstate until some change occurs in the woodto provide a favorable substrate (wetwood)for the bacterial growth. Bacteria iso-

2

lated from wetwood of poplar utilizedcapillary liquid from sapwood and heartwoodfor growth (190).

Wetwood has been associated with physical,mechanical, and biological injuries3/(20,79, 186). Whether these injuries result inwetwood formation or initiate it has notbeen substantiated. Field observationshave shown that the normal cylindrical formof wetwood in white fir deviated radiallyand longitudinally to regions of naturaland silvicultural injuries (observations byW. Y. Pong). Similar deviations have beennoted in conjunction with injuriesapparently caused by insect attacks4/(78,79, 211) and stem cankers of dwarf mistle-

3/W. Y. Pong. 1967. Preliminary study ofgrade defects in true firs. Interim rep.USDA For. Serv. Pac. Southwest For. andRange Exp. Stn., 31 p. Berkeley, Calif.(Unpublished report on file at Pac.Northwest For. and Range Exp. Stn.,Portland, Oreg.)

4/ Personal communication to W. Y. Pongfrom K. R. Shea, Assistant Director,Science and Education Administration, U.S.Department of Agriculture, Washington D.C.,and from W. W. Wilcox, Forest ProductsLaboratory, University of California,Richmond.

toe in true firs5/(216). A fairly con-sistent association between wetwood anddecay has been noted in eastern Oregon truefirs/ and other tree species (79). Wehave observed that internal heart rot intree stems often has a peripheral ring ofsound wetwood, whereas stems with wetwooddo not necessarily have associated decay.There is some belief that wetwood mayinhibit decay (93).

The inability to consistently associatewetwood with micro-organisms has led to theconclusion that wetwood in conifers is onlya condition of excessive accumulation ofmoisture and not a symptom of disease(142, 165). There are opposing views on thesource of the excessive moisture inwetwood. Some suggest direct entry ofatmospheric moisture through stem openings,such as dead branch stubs (20); othersbelieve water in wetwood has an internalorigin from moisture sources in the rootand stem (45, 78). In Japan the occurrenceof wet heartwood is considered normal forsome species of hardwoods and abnormal forconifers (220). Whether formation ofwetwood is the result of a natural processof the tree or is bacterial in origin isdiscussed at length by Hartley, et al.(79). Until additional evidence isavailable, we will consider wetwood themanifestation of a syndrome of abnormalphysiological events in the living tree andnot a specific disease.

5/ J. R. Parmeter, Department of PlantPathology, University of California,Berkeley, personal communication to W. Y.Pong.

6/Field observations by P. E. Aho,Forestry Sciences Laboratory, Corvallis,Oreg.

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Wetwood in Treesand Logs

Occurrence

Wetwood occurs in both conifers andhardwoods, but its frequency can vary byspecies, age, and growing conditions oftrees. Occurrence of wetwood in the moreimportant commercial timber species ofNorth America is presented in table 1 forsoftwoods and table 2 for hardwoods. Eachspecies is listed by its apparent suscepti-bility to wetwood formation. Theseclassifications are derived from our obser-vations, personal communications fromcolleagues, office reports, and thepublished literature. Tree species inwhich wetwood either has never beenreported or is rarely observed are placedin class 1. Class 3 includes the mostsusceptible tree species or those that canbe expected to develop wetwood either withage or under less than optimum growingconditions. The intermediate class 2contains species in which wetwood is foundin one locality and rarely, if at all, inanother region.

For some species in both tables 1 and 2,the information is tentative; as additionalfield information becomes available aspecies may be moved from one class toanother. Sufficient information exists toindicate that wetwood will probably developin hemlocks, true firs, and cottonwoodsfrom most regions. For species such aswhite pine, red oak, and maple, wetwood isfound in many trees of one region andcompletely lacking in trees of anotherregion. Thus, if the rating of a speciesin tables 1 and 2 is based on reports froma limited area of the total range of thespecies, then its status could be reversedwhen additional information is available.

Not all the tree species in class 3 areaffected with wetwood to the same degree orfrequency. There are differences within agenus; for example, white fir and grand firare affected more than are noble fir andPacific silver fir. From his studies withwhite fir in northern California,Wilcox7/ found that all sample trees,including young ones, contained somewetwood. This appears to be the rule forwhite fir in northern California, but insome stands wetwood appears to be rare ornonexistent.8/ The presence of wetwoodwithin the hardwood genus Populus appearsto be the rule, but again there can beexceptions. Some investigators could notfind eastern cottonwood trees that werefree or wetwood9/ (191, 222). ForEuropean white poplar (Populus alba L.),the presence of wetwood is the rule inpistillate trees and the exception instaminate trees10/(71).

7/ W. W. Wilcox, Forest ProductsLaboratory, University of California,Richmond, personal communication to W. Y.Pong.

8/J. C. Ward, unpublished data on file atU.S. Forest Products Laboratory, Madison,Wis.

9/J. B. Baker , Southern Forest ExperimentStation, New Orleans, La., personalcommunication to J. C. Ward.10/J. C. Ward and J. G. Zeikus,unpublished data on file at U.S. ForestProducts Laboratory, Madison, Wis.

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Table l--Occurrence of wetwood in commercially important conifers of the United States1/

Frequency class2/

Species 1 2 3Occasional Scattered Generallyor none prevalence prevalent

California red fir (Abies magnifica A. Murr.)

Balsam fir (Abies balsamea (L.) Mill.)White fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.)Grand fir (Abies grandis (Dougl.) ex D. Don) Lindl.)Subalpine fir (Abies lasiocarpa (Hook.) Nutt.)Pacific silver fir (Abies amabilis (Dougl.) ex Forbes)Noble fir (Abies procera Rehd.)

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)Eastern hemlock (Tsuga canadensis (L.) Carr.)Western hemlock (Tsuga heterophylla (Raf.) Sarg.)Mountain hemlock (Tsuga mertensiana (Bong.) Carr.)True fir:

Shasta red fir (Abies magnifica var. shastensis Lemm.)Spruce :White spruce (Picea qlauca (Moench) Voss)Black spruce (Picea mariana (Mill.) B.S.P.)Red spruce (Picea rubens Sarg.)Engelmann spruce (Picea engelmannii Parry ex Engelm.)Sitka spruce (Picea sitchensis (Bong.) Carr.)

Soft pine:Eastern white pine (Pinus strobus L.)Western white pine (Pinus monticola Dougl. ex D. Don)Sugar pine (Pinus lambertiana Dougl.)

Hard pine:Jack pine (Pinus banksiana Lamb.)Lodgepole pine (Pinus contorta Dougl. ex Loud.)Red pine (Pinus resinosa Ait.)Ponderosa pine (Pinus ponderosa Dougl. ex Laws.)Shortleaf pine (Pinus echinata Mill.)Loblolly pine (Pinus taeda L.)Longleaf pine (Pinus palustris Mill.)Slash pine (Pinus elliottii Engelm. var. elliottii)Jeffrey pine (Pinus jeffreyi Grev. & Balf.

Western larch (Larix occidentalis Nutt.)Tamarack (Larix laricina (Du Roi) K. Koch)Redwood (Sequoia sempervirens (D. Don) Endl.)Giant sequoia (Sequoia dendron giganteum (Lindl.) Buchholz)Baldcypress (Taxodium distichum (L.) Rich.)Cedar:

Incense-cedar (Libocedrus decurrens Torr.)Western redcedar (Thuja plicata Donn ex D. Don)Northern white-cedar (Thuja occidentalis L.)Alaska-cedar (Chamaecyparis nootkatensis (D. Don)Port-Or ford-cedar ( Chamaecyparis lawsoniana (A. Murr.) Parl.)Atlantic white-cedar (Chamaecyparis thyoides (L.) B.S.P.)Eastern redcedar (Juniperus virginiana L.)

Juniper:

Spach)

Alligator juniper (Juniperus deppeana Steud.)Western juniper (Juniperus occidentalis Hook.)

1/From field observations by authors augmented with personal communications; also from literature,particularly Hartley et al. (79) and Lutz (126).2/Frequency class 1 includes tree species in which wetwood rarely, if ever, occurs and species inwhich wetwood has not been observed, reported, or recognized as wetwood. Class 2 includes species inwhich wetwood will develop on some sites and not on other sites, together with species that tend todevelop wetwood with increasing age on certain sites. Class 3 includes species very susceptible toformation of wetwood on most sites and in young- as well as old-growth timber.

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Table 2--Occurrence of wetwood in commercially important hardwoods of the United States1/

Frequency class2/

Species 1 2 3Occasional Scattered Generallyor none prevalence prevalent

Red oak group:Northern red oak (Quercus rubra L.)Black oak (Quercus velutina Lam.)Scarlet oak (Quercus coccinea Muenchh.)Pin oak (Quercus palustris Muenchh.)

Laurel oak (Quercus laurifolia Michx.)Shumard oak (Quercus shumardii Buckl. var. shumardii)California black oak (Quercus kelloggii Newb.)

Water oak (Quercus nigra L.)Nuttall oak (Quercus nuttalli Palmer)Southern red oak (Quercus falcata Michx. var. falcata)Cherrybark oak (Quercus falcata var. pagodaefolia Ell.)Willow oak (Quercus phellos L.)

Bur oak (Quercus macrocarpa Michx.)Overcup oak (Quercus lyrata Walt.)Swamp white oak (Quercus bicolor Willd.)

White oak group:White oak (Quercus alba L.)

Chestnut oak (Quercus prinus L.)Swamp chestnut oak (Quercus michauxii Nutt.)Chinkapin oak (Quercus muehlenbergii Engelm.)Post oak (Quercus stellata Wangenh. var. stellata)Oregon white oak (Quercus garryana Dougl. ex Hook)California white oak (Quercus lobata Nee)

Tanoak (Lithocarpus densiflorus (Hook. & Arn.) Rehd.)Hickory :

Bitternut hickory (Carya cordiformis (Wangenh.) K. Koch)Water hickory (Carya aquatica (Michx. f.) Nutt.)Pecan (Carya illinoensis (Wangenh.) K. Koch)Shagbark hickory (Carya ovata (Mill.) K. Koch)Shellbark hickory (Carya laciniosa (Michx. f.) Loud.)Mockernut hickory (Carya tomentosa (Poir.) Nutt.)Pignut hickory (Carya glabra (Mill.) Sweet)

Black walnut (Juglans nigra L.)Butternut (Juglans cinerea L.)Sweetgum (Liquidambar styraciflua L.)Maple:Sugar maple (Acer saccharum Marsh.)Silver maple (Acer saccharinum L.)Red maple (Acer rubrum L. )Black maple (Acer nigrum Michx. f.)Bigleaf maple (Acer macrophyllum Pursh)

Black tupelo (Nyssa sylvatica Marsh.)Water tupelo (Nyssa aquatica L.)Yellow poplar (Liriodendron tulipifera L.)Magnolias (Magnolia L. spp.)

Table 2--Continued

Frequency Class2/

Species 1Occasionialor none

2Scatteredprevalence

3Generallyprevalent

Ash:White ash (Fraxinus americana L.)Green ash (Fraxinus pennsylvanica Marsh.)Black ash (Fraxinus nigra Marsh.)Oregon ash (Fraxinus latifolia Benth.)

American beech (Fagus grandifolia Ehrh.)Poplar and aspen:

Eastern cottonwood (Populus deltoides Bartr. ex Marsh.)Plains cottonwood (Populus sargentii Dode)Narrowleaf cottonwood (Populus angustifolia James)Black cottonwood (Populus trichocarpa Torr. & Gray)Fremont cottonwood (Populus fremontii Wats.)Swamp cottonwood (Populus heterophylla L.)Balsam poplar (Populus balsamifera L.)Quaking aspen (Populus tremuloides Michx.)Largetooth aspen (Populus grandidentata Michx.)

Black willow (Salix nigra Marsh.)Red alder (Alnus rubra Bong.)Birch :

American elm (Ulmus americana L.)Slippery elm (Ulmus rubra Mühl.)Rock elm (Ulmus thomasii Sarg.)

Hackberry (Celtis occidentalis L.)Sugarberry (Celtis laevigata Willd.)Ohio buckeye (Aesculus glabra Willd.)Yellow buckeye (Aesculus octandra Marsh.)American basswood (Tilia americana L.)White basswood (Tilia heterophylla Vent.)Pacific madrone (Arbutus menziesii Pursh)Red mulberry (Morus rubra L.)Black locust (Robinia pseudoacacia L.)Honey locust (Gleditsia triacanthos L.)Water locust (Gleditsia aquatica Marsh.)Kentucky coffeetree (Gymnocladus dioicus (L.) K. Koch)

Paper birch (Betula papyrifera Marsh.)Gray birch (Betula populifolia Marsh.)Yellow birch (Betula alleghaniensis Britton)River birch (Betula nigra L.)

American sycamore (Platanus occidentalis L.)Black cherry (Prunus serotina Ehrh.)Northern catalpa (Catalpa speciosa Warder ex Engelm.)Elm:

1/From field observations by authors augmented with personal communications; also from literature,particularly Hartley et al. (79) and Lutz (126).2/Frequency class 1 includes tree species in which wetwood rarely, if ever, occurs and species inwhich wetwood has not been observed, reported, or recognized as wetwood. Class 2 includes species inwhich wetwood will develop on some sites and not on other sites, together with species that tend todevelop wetwood with increasing age on certain sites. Class 3 includes species very susceptible toformation of wetwood on most sites and in young- as well as old-growth timber.

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Even though considerable data may show thata species in class 1 is not susceptible towetwood formation, it may be important tocontinue looking for exceptions. Forexample, we have rarely found wetwood inDouglas-fir and then it was limited involume to small streaks or pockets.Nevertheless, one small stand ofDouglas-fir timber in the Cascade Range ofOregon contained extensive wetwood in thelogs, and the lumber was difficult to dryto a uniform moisture content.11/ InWashington, a Douglas-fir bridge timberwhich failed from ring shake was found tohave contained extensive bacterial wetwood(see footnote 8).

Age has some influence on formation ofwetwood. In general, there is less wetwoodin young trees: some foresters think thatthe problem will be solved when the lastold-growth timber is cut. In some species,such as cottonwood, aspen, elm, maple, andwhite fir, wetwood has been recorded invery young trees (see footnote 7) (79).

When wetwood is prevalent in young trees,site or cultural practices may be the majorcontributing factors. Field observationsby the senior author indicate that siteconditions may influence the incidence ofwetwood in young-growth western hemlock.Wallin (197) observed a relation betweenwetwood formation in balsam poplar and soiltype. Wetwood is most prevalent in thelower stems of western redcedar growing onwet, swampy sites (73, 89). These soil orsite factors may favor increases inpopulations of micro-organisms associatedwith wetwood. Similarly, cultural prac-tices could contribute to the establishmentand growth of these micro-organisms intrees. Most cottonwood trees grown inplantations on good soils in theMississippi delta develop wetwood within 2years. This is apparently related to the

11/C. J. Kozlik, Department of ForestProducts, Oregon State University,Corvallis, personal communication to J. C.Ward.

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planting of scions in moist soil (seefootnote 9). Second-growth aspen innorthern Minnesota contains more wetwoodthan aspen from the original forest.12/This may be due to the origin of thesecond-growth trees from sprouts and adifferent species composition of thesecond-growth stands favoring attacks byboth micro-organisms and stem-boringinsects. Eis (53) found that root graftsin natural stands of Douglas-fir, westernhemlock, and western redcedar can be amajor factor in the spread of decay organ-isms from stumps to neighboring trees.Root grafts may also be a factor in theinitiation and spread of wetwood. Cole andStreams (42) suggest that wetwood bacteriaare spread by slime-flux insects.

Types of Wetwood andPatterns Within the Tree

With few exceptions, formation of wetwoodhas been restricted to the heartwood andheartwood-sapwood transition zones ofliving trees. Essentially two types ofwetwood can occur in the living tree:wetwood formed from the aging of normalsapwood nearest the heartwood or injuredsapwood and wetwood developed in previouslyformed heartwood. The two types frequentlyoccur together and are difficult todistinguish.

12/E. M. Ballman, Chief Forester, DiamondInternational Corp., Cloquet, Minn.,personal communication to J. C. Ward.

Wetwood is generally not found in the outersapwood of living trees; reports to thecontrary indicate that such occurrence isassociated with insect attack, stemcankers, and wounds of various forms (79,91; also see footnotes 3, 5, and 7). Inthese instances, the wetwood zone of thecentral core deviated radially and longi-tudinally to the traumatized zone of thesapwood. Knutson (106), however, was notalways able to relate wetwood in thesapwood of aspen to visible wounds, but heconsidered wetwood to be generally a phe-nomenon of the sapwood. Takizawa et al.(186) found from the cytology of wetwood inthe Japanese fir Todomatsu (Abiessachalinensis Mast.) that wetwood was morecomparable to sapwood than heartwood but itexisted as streaks in the outer heartwood.

It is important that a distinction is madebetween the wetwood OK "sinker" stock(i.e., wood that will not float) formed inthe standing tree and the "sinker wetwood"resulting from storage of logs in water.In pond-stored logs, the affected wood ismainly the sapwood and the wood of thecentral core exposed on the log ends.Pond-stored logs cease to float because ofincreased permeability from bacterialdisintegration of pit membranes in thesapwood but not the heartwood (44, 50, 55,69, 72, 90, 95, 105, 119, 157, 180, 195,208). In comparison, wetwood in standingtrees is confined more to the heartwood andhas registered lower permeability than theadjacent normal wood, particularlysapwood. The permeability of wetwood fromtrees is discussed in the section onproperties of wetwood.

Although the spatial distribution ofwetwood in standing trees will vary bypoint of origin, duration of formation, andother such factors, there are essentiallythree general patterns. Most common is theconical pattern in the central core of thelower bole--the wetwood originates atinjuries in the roots OK at the root collarand tapers to an apex in the upper bole.This pattern has been found in true firs(45, 92, 147, 215), western hemlock (203),northern red oak (75, 202), easterncottonwood (see footnote 9), and westernlarch (see footnote 8).

The second pattern consists of streaks OKcolumns of wetwood which can be traced toinjuries or to stubs of dead branches inthe upper stem. The basal portion of thesetrees may be free of wetwood. Upper stemwetwood has been noted in trembling aspen(106), cricket-bat willow (Salix alba var.calva G. F. W. Mey.) (49), elm (33),eastern white pine (123), silver fir (Abiesalba Mill) (20), California white fir,13/and eastern and western hemlock.14/(see footnote 11).

13/A. L. Shigo, Forestry SciencesLaboratory, Durham, N. H., personalcommunication to J. C. Ward.14/ Field observations by the authors.

9

The third pattern appears when a treecontains both basal and upper stem wetwoodformations that extend and coalesce into asingle massive formation. This pattern wasnoted in Scots pine and Norway spruce(Picea abies (L.) Karst) by Lagerberg (117)who also was the first to describe theother patterns. It is not uncommon foraspen in Minnesota and Wisconsin (seefootnote 8) and balsam fir (59) to have thecombined wetwood pattern.

Wetwood Properties--A Microbial Implication

Wetwood has distinctive physical and chemi-cal properties. These properties appearto result from microbial action on woodytissue in the living tree, but morecomprehensive tests are needed to validatethis relationship.

Odor

The odor of wetwood in the green conditiondiffers from that of normal wood and isreminiscent of fermentation processes.This indicates an influence by anaerobicbacteria. Odors in wetwood ranging fromrancid to a fetid rumenlike odor have beentraced to bacterial metabolism of woodycomponents, particularly extractives andhemicelluloses (1, 187, 222, 223). Usingpotato mash media in the laboratory, thesenior author (unpublished data) was ableto reproduce the rancid, fatty acid odorsof wetwood with pure cultures ofClostridium spp. isolated from wetwood ofcottonwood, red oak, and white fir. Bauchet al. (20) found acetic acid, propionicacid, and n-butyric acid present in thewetwood of silver fir, but not in thesapwood. They believe these fatty acidsindicate a high bacterial activity inwetwood.

Hydrogen Ion Concentration or pH

The prevailing belief is that wetwood hasa higher pH than normal wood. This canprobably be traced to the publication,"Wetwood, Bacteria, and Increased pH inTrees," by Hartley et al. (79), who citeresults indicating a higher pH in wetwoodof many hardwoods and conifers. Theycautioned, however, that not all wetwood isnotably high in pH and that it is unsafe toassume that all wood with high pH iswetwood or has been.

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A review of published data indicates that acomprehensive statement concerning the pHof wetwood cannot be made now. Reported pHvalues are generally on the alkaline sidefor wetwood in hardwoods and on the acidside for conifers, but there are exceptionsbetween tree species and even within singlespecies and trees. In comparison withnormal sapwood and heartwood, wetwood ofEuropean aspen (Populus tremula L.) wasmore acid (144), but wetwood in aspen fromMinnesota can be either more acid or morealkaline (41, 67, 106). All wetwood in elmtested by Carter (33) was more alkalinethan a slightly acidic normal wood.Seliskar (167) found that wetwood in elmcan be either acid or alkaline, whereas thewetwood of Lombardy poplar (Populus nigravar. italica Muenchh.) was consistently onthe alkaline side. Wetwood in balsam poplaris slightly basic, the sapwood slightlyacidic, and the heartwood essentiallyneutral (197). Red heart, a type ofwetwood in paper birch, is alkalineadjacent to the acidic sapwood and turnsacidic near the pith (31). Wetwood inconifers is more acid than the adjacentnormal wood for California white fir (212,215), silver fir (20), and western hemlock(111) . Another study of western hemlockreported that wetwood and normal heartwoodhad a similar range in pH values from 4.1to 5.5 (165). Any increases in the pH ofwetwood from conifers are small (79).

Until additional studies are made we canonly speculate on the causes fordifferences in pH between wetwood andnormal wood and between wetwood inindividual trees and species. Microbialpopulations in trees are possible causativeagents, but they must be considered inconjunction with site and chemistry of thetrees. Seliskar (167) noted that elms onpoor sites had alkaline wetwood, whereaswetwood from trees on good sites wasacidic. From laboratory test cultures,Carter (33) observed that the elm wetwoodbacterium, Erwinia nimipressuralis, willturn a nutrient broth containing a sugarstrongly acid, but growth on nutrient brothalone will result in an alkaline pH. Toole(191) reports that wetwood in easterncottonwood growing along the MississippiRiver between Vicksburg, Mississippi, andMemphis, Tennessee, is neutral to alkaline,and the sapwood is slightly acidic. Thesenior author found similar acidic pHvalues for sapwood of cottonwood growingalong the Mississippi River in Wisconsin,but the wetwood had either higher or lowerpH values than sapwood. The composition ofthe microbial population in wetwood varieswith either acid or alkaline pH conditionsfor eastern cottonwood (222) and Californiawhite fir (see footnote 10). Whetherdifferences in pH are the cause or theresult of differences in microbialpopulations has not been determined.

11

Anaerobiosis Strength Properties

Results from analysis of gas samples fromwetwood in hardwood trees range from nearanaerobic conditions in elm (33) and blackcottonwood (93) to strictly anaerobic withno oxygen present in eastern cottonwood(222). Carbon dioxide, nitrogen, andoxygen are the major gaseous components ofnormal wood, but oxygen was absent or invery reduced amounts in the wetwoodstudied, indicating that any bacteriapresent must be either facultative orobligate anaerobes. During the summergrowing season, high positive gas pressureshave been recorded in wetwood of standinghardwood trees, and these pressures areattributed to bacterial metabolism (33, 79,191, 221, 222). Trunk gases contributingto the positive pressures in wetwood arecarbon dioxide, hydrogen, methane, nitro-gen, and hydrogen sulfide. Zeikus and Ward(222) established that the methane gas isproduced by an autotropic anaerobe which isa secondary invader and not common to allwetwood populations even within the samehost species. This methanogen,subsequently characterized and namedMethanobacterium arbophilicum by Zeikus andHenning (221), is also found in soil andwater. It can only grow under strictlyanaerobic conditions and in the living treeonly in wetwood where it metabolizeshydrogen and carbon dioxide produced byClostridium and other anaerobic bacteria.These other bacteria must, in turn, derivenourishment from the woody tissue.

Van der Kamp et al. (93) consider wetwoodwith its near anaerobic conditions aperfectly natural phenomenon that impartsresistance to decay to the inner wood ofblack cottonwood trees.

There are conflicting reports in theliterature concerning the relative strengthproperties of wetwood compared with normalsapwood and heartwood. Some investigatorsfound wetwood weaker than normal wood;others report wetwood to be equal and evengreater in strength. We have found fromobservations made in conjunction withvarious tests on sawing, machining, drying,and mechanical strength properties thatwetwood is often weaker than normal wood inbonding strength of the compound middlelamella between wood cells. The secondarywall of wetwood cells does not appear to beweaker than that of normal wood withsimilar specific gravity. Conflictingreports on the comparative strengthproperties of wetwood can usually beresolved by normalizing test results forsample moisture content, sample size andmethods of preparation, and specificgravity of samples.

Results from mechanical tests showingwetwood of hardwoods to be weaker thannormal wood were all derived from thetesting of green wood (40, 41, 80, 106,133); furthermore, the wetwood was usuallylower in specific gravity than adjacentnormal wood. It is important that testspecimens be cut from dried wetwood ratherthan from green wetwood and then dried:during drying, deep surface checks, ringfailure, and internal honeycomb checks aremuch more likely to develop in wetwood thanin normal wood. Lagerberg (117) andThunell (189) found that Scots pine wetwoodwas weaker in bending, compression, andimpact strength properties than were normalsapwood and heartwood. They attributed the

12

lower strength of wetwood to weakness ofthe middle lamellae and seasoning checks inthe air-dried test specimens. Test speci-mens cut from green wetwood will not haveseasoning checks, but green wetwood is morelikely to have tissue with weaker bondsbetween cells than samples selectively cutfrom the defect-free portion of driedwetwood blanks or boards. Toughnessstrength of green wetwood samples fromwhite fir (212, 215) and from American andslippery elm (167) was not significantlylower than normal wood when differences inspecific gravity were accounted for.

If specimens used in mechanical testing canbe prepared from dry, defect-free wetwood,then strength differences between wetwoodand normal wood diminish. Stojanov andEnthev (181) found that air-dried specimensfrom wetwood of silver fir were equal instatic bending strength and stronger incompression strength than was normal wood.Table 3 shows that small test beams ofwetwood cut from kiln dried boards ofCalifornia white fir are as strong instatic bending properties as is normalwood. Differences between strengths ofwetwood and normal wood were related todifferences in specific gravity: however,during the preparation of the testspecimens, many wetwood samples developedshelling failures and could not be tested.Table 3 also shows that sugar pine wetwoodwas weaker than normal wood even thoughhigher in specific gravity.

Table 3--Comparison of specific gravity and mechanical strength properties of normal wood and wetwood1/(sinker heartwood) from California white fir and sugar pine

Specific gravity oven Static bending strength properties/dry weight, based on

Species and volume at test Stress atwood type Modulus of rupture Modulus of elasticity proportional limit

Mean Standard Standard Standard Standarddeviation Mean deviation Mean deviation Mean deviation

12-percent moisture Pounds per Thousand pounds per Pounds percontent square inch square inch square inch

White fir:Sapwood 0.353 0.012 9,601 787 1,313 90 5,968 611Heartwood .343 .018 9,054 870 1,355 97 5,578 628Wetwood .429 .072 11,329 2,347 1,649 338 5,997 1,253

Sugar pine:Sapwood .266 .016 5,643 587 708 106 3,588 430Heartwood .271 .024 6,049 702 686 87 3,883 491Wetwood .281 .020 5,529 606 674 68 3,041 369

l/Test samples from kiln-dried lumber. Unpublished data from J. C. Ward, U.S. Forest Products Laboratory,Madison, Wis.2/Strength values at 12-percent moisture content from test specimens measuring 1 by 1 inch in cross-section and 16 inches along the grain (bending test over a 14-inch span).

13

The intrinsic weakness of wetwood incell-to-cell bonding strength appears to berelated to the production of pectin-degrading enzymes by bacteria, especiallythe obligate anaerobe Clostridium. Theseenzymes degrade the pectic substances inthe compound middle lamella that holdstogether the wood cells. Wong and Preece(219) observed degradation of the middlelamellae of cricket-bat willow by thefacultative anaerobe Erwinia salicis whichalso produces pectolytic enzymes. Solublesugars could not be detected in the wetwoodof Abies alba, so Bauch et al. (20) assumedthat pectinaceous compounds and hemicellu-loses serve as substrates for wetwoodbacteria. Bacteria isolated from wetwoodhave not been found capable of degradinglignin and cellulose or the secondary cellwall under laboratory conditions (214,217). Furthermore, microscopic examinationof wetwood indicates that secondary wallsare not visibly degraded as are the wallsof wood decayed by fungi or attacked&wood-destroying bacteria (79, 106, 107,117, 122, 130, 158, 164, 213). Hillis etal. (83) consider wetwood bacteria respon-sible for shake in trees.

Sometimes a strong, pectin-degradingbacterium will be absent from the microbialpopulation in wetwood and shake, deepchecks, and honeycomb will not appear inthe wood. For example, when American elmwetwood was without Clostridium in itsmicrobial population, the wood did notdevelop deep surface checks, honeycomb, orring failure during kiln drying (24).American elm with wetwood infected byClostridium is highly susceptible to ringfailure and shelling when subjected todrying and machining stresses and isreferred to as "onion elm" in the lumbertrade.

Ordinarily, sapwood will not develop ringshake and honeycomb, but these defects canoccur if sapwood is submerged and infectedby bacteria that produce pectolytic enzymes(180). Salamon (160) found that honeycomband shake occurred in dried sapwood boardsfrom dead lodgepole pine and Engelmannspruce trees salvaged from flooded areas.He observed heavy concentrations ofbacteria in this sapwood under the electronmicroscope and considered bacteria respon-sible for the defects. Boards fromsprinkled or water-stored Scots pine andNorway spruce logs were also found byBoutelje and Ihlstedt (26) to be moreliable to checking than boards from newlyfelled trees. A wide variety of bacteria,including anaerobic pectin-degradingClostridium spp., were isolated fromsapwood of water-stored Scots pine byKarnop (95) but not from the heartwood.

P e r m e a b i l i t y

Wetwood has long been considered to havelow permeability because it is extremelyslow to dry and wet pockets are oftenpresent in dry lumber (79). Severalstudies have correlated slow drying oflumber with restricted flow of liquids orgases through small cores of wetwood fromaspen (98, 106), western hemlock (111, 120,121), and western redcedar (188). On theother hand, the absorptive capacity ofwetwood can be greater than that of normalwood. In the green condition, white firwetwood is slow drying, but it will absorbmore oil than normal wood does after drying(9, 215, 217). Air-dried wetwood fromScots pine has greater water absorptioncapacity than normal wood and the capillaryrise of water is much more rapid, yet thelumber will contain wet pockets withmoisture content as high as 100 percent(117). The moisture-holding capacity ofelm wetwood does not exceed that of normalwood (167), but elm lumber with wetwooddoes not dry abnormally slowly either (24).

14

For a better understanding of the totalconcept of permeability for wetwood,studies should be designed to evaluate therelative contributions of wetwood extrac-tive, pit aspiration, tyloses, and bac-terial metabolism. Wetwood can haveconcentrations of extractives that not onlymay block cell lumens and pits, but canalso have greater osmotic pressures thannormal wood (6, 20, 45, 83, 106, 112, 117,136, 139, 165, 169). Reduced permeabilityof wetwood has been associated with aspira-tion of bordered pits in softwoods (111,121, 203) and tyloses in hardwoods (98,106, 205). Bacteria have produced slime orextracellular polysaccharides in thewetwood of both conifers and hardwoods(158, 203, 205). Bacterial slime cancontribute to blockage of cells andincreased osmotic pressure.

In contrast to wetwood, the permeability ofwater-stored logs is increased, rather thandecreased, by bacteria. Porosity of water-stored wood is increased by bacterial degra-dation and destruction of pit membranes andthin-walled parenchyma cells, especially inthe rays (21, 26, 50, 55, 63, 72, 90, 94,95, 105, 119, 157, 195, 208). To a limiteddegree, cell walls in water-stored sapwoodmay be pitted and corroded (44, 69, 70, 72,76, 119), suggesting that bacteria may beattacking the wood under aerobic ratherthan anaerobic conditions. Karnop (94)found that the anaerobic Clostridiumomelianski can attack unlignified cellulosein the sapwood of water-stored pine, butthis activity is reduced with low watertemperature and acidic conditions.

Chemical Brown Stain Precursors

Dark discolorations or chemical brownstains frequently develop on the surface ofwetwood which, after being removed from theanaerobic atmosphere of the tree, isexposed to aerobic or oxidative condi-tions. The intensity of these chemicals oroxidative brown stains varies with wooddrying conditions, tree species, andindividual trees. Wetwood has beenassociated with dark discolorations in bothhardwoods and softwoods (79), particu-larly in these species: aspen (79, 84,106, 133),pine (35),

white popular (71), eastern whitewestern redcedar (136), redwood

(5), western hemlock (18, 165), "Sugi"(Cryptomeria japonica (L.F.) D. Don) (66),paper birch and sugar maple (79), andcricket-bat willow (47, 209, 219).

Bacterial degradation of the normal woodsugars and polyphenols may cause wetwood todevelop dark oxidation stains when dried.Chemical brown stain has been attributed tothe bacterial attack on extractives insugar pine (182), western hemlock (61), andPacific silver fir (19). The ForestProducts Laboratory's (FPL) investigations(see footnote 8) found that the formationof chemical brown stain in freshly sawedgreen lumber from California white fir,sugar pine, eastern white pine, westernhemlock, aspen, and cottonwood is asso-ciated with and confined to the wetwoodzones of these trees. This stain, however,did not develop uniformly in all wetwood ofthese species; variations in composition ofbacterial populations appear to beimportant. FPL data suggest that thepresence of phenyloxidizing bacteriasimilar to those described by Greaves (70)is required before very dark chemical brownstain develops in wetwood.

15

Timber Utilization andWood Processing Problems

Chemical brown stain in wetwood isidentical to the so-called sour log brownstain that develops on the surface ofsoftwood lumber sawn from logs stored underwater or in moist, shaded log decks.Millett (141) observed that brown stainprecursors can be present in sugar pinetrees, or the precursors can form byenzymatic or hydrolytic degradation instored logs. Both types of brown stain mayhave similar biochemical origins. Bacteriaare considered factors in the formation ofsour log brown stain on sapwood of westernpines (55, 182, 183). Knuth (104) dis-covered that chemical brown stain, but notdecay, will develop on the surface of woodsamples submerged in liquid cultures ofbacteria. Formation of stain was enhancedby autoclaving the wood before inoculation,and aerobic conditions were necessary forfinal development of stain. Evans andHalvorson (61) studied the chemistry ofbrown stain in water-stored sapwood andwetwood of western hemlock. From theirresearch, they were able to postulate thatbacterial enzymes (probably polyphenoloxidases) condense monomeric leucoantho-cyanins to water soluble polymers. Thesepolymers migrate to the wood surface duringdrying and on exposure to the atmosphereundergo oxidative condensation to darkbrown polymers.

The presence of wetwood can cause sub-stantial economic losses when the affectedtimber is converted into logs and end-useproducts. Since wetwood is usually limitedto inner sapwood and heartwood, it seem-ingly is not detrimental to growth oftrees. In trees where wetwood radiatesinto the outer sapwood from the centralcore because of insect attack and otherstem injuries, the function of sapwood maybe impaired. To what extent this mayaffect the growth of the tree has not beeninvestigated.

Shake and Frost Cracks

The weaker bonding between the cells ofwetwood can result in radial and tangentialgrowth ring separations when the affectedtrees are subjected to stresses from wind,growth, and freezing. These separations,known as shake (radial and ring) andsometimes spangles, are internal defects ofthe trunk that often go undetected when thetree appears vigorous and healthy. Shakecan make boards with otherwise sound andclear wood, worthless for structural andfactory grade lumber. The constructionindustry is the largest wood-usingindustry, and the losses in availableconstruction grade lumber resulting fromshake in both trees and boards areconsiderable.

16

Many logs, both hardwood and conifer, areexcluded from the most valuable log gradesfor lumber and veneer only because theycontain shake (81, 117, 124, 125, 126, 127,128). Shake in western hemlock logs fromAlaska has caused staggering losses in thevolume of wood intended for export toJapan.15/ An association between shakeand wetwood has been proposed by manyinvestigators (31, 33, 37, 79, 83, 96, 110,111, 117, 202, 203, 204). Even thoughwetwood is present in conjunction with muchof the shake observed in trees and logs,the actual formation of shake is a complexprocess involving other factors, such astree growth stresses, stem injuries, andanomalous wood tissue (38, 83, 91, 113,114, 115, 124, 129, 130, 140, 172, 173).

15/R. O. Woodfin, Pacific Northwest Forestand Range Experiment Station, Portland,Oreg., personal communication to J. C. Ward.

Frost cracks on the outer stem surfaceappear to be an extension of shake fromwithin the bole; they result not only inloss of usable wood volume but also in areduced yield of high quality lumber orveneer from the logs. Wood from trees withfrost cracks is likely to contain wetwood(see footnote 3) (46, 79, 88, 117). InEuropean poplar plantations, frost crack isa common and serious defect consistentlyassociated with bacterial wetwood inaffected trees (57, 62, 82). Surveys ofdefect in western conifers do not indicatea close relationship between frost cracksand fungal decay (27, 65), yet frost crackscertainly offer a favorable infection courtfor decay fungi. Aho (2) found that lessthan 6 percent of the frost cracks in grandfir from Oregon and Washington wereinfected by fungi, and decay losses weresmall. He did find that all grand firtrees with frost cracks invariably hadwetwood, and 46.7 percent of all trees 11.0inches or more in diameter contained frostcracks.16/ From an extensive review ofthe literature, Schirp (163) found that theassociation of frost cracks with wetwood intree stems dated back to 1765.

16/P. E. Aho, Forestry Sciences Labor-atory, Corvallis, Oreg., unpublished data.

17

Rapidly freezing air temperatures are alsonecessary for formation of frost cracks.In Macedonia, frost cracks developed in thestems of poplars (Populus x euramericana(Dode) Guinier) growing in an area with acontinental climate (warm summer tempera-tures and cold oscillating winter temp-eratures), but not a single stem crack wasobserved on trees growing in an area with aMediterranean climate (mild winters and hotsummers) (74). Robert Hartig (77), the"father" of forest pathology, observedfrost cracks to be most abundant on thenortheast side of trees where sudden andlarge drops in temperature often occurredin winter. He also noted radial andperipheral cracks in the interior of oldoaks; these cracks did not extend outsidethe stem, and he was uncertain whether theywere due to frost. A survey of damage fromfrost cracks on walnut (Juglans regia L.)growing in the Ukraine showed that thenumber of stems affected ranged from 1.1to 83.4 percent in stands on southslopes with fairly moist soils (185).White fir stands of the Sierra Nevada inCalifornia have two to three times morefrost cracks in trees on the east slopewith greater drops in temperature than onthe milder west slope (196). Freezing ofredwood boards results in breakage ofheartwood with wetwood but not with normalheartwood below 150-percent moisturecontent (58).

Losses in product volume and value asso-ciated with shake and frost cracks areespecially important because these defectsare most prevalent in the lower two logs oftrees and, in many instances, are spiraled(see footnote 3) (163, 196). In the truefirs, for example, these logs have thepotential for producing not only thehighest quality boards but also nearly halfthe total lumber volume of the tree (215).It is in these logs that reducing wetwood-related losses has its greatest potential,both in volume and value (147). Theoccurrence of combinations of shake, frostcrack, and spiral grain in white sprucetimber growing in northern Alberta seri-ously affected the yield of 1- and 2-inchdimension lumber (131). Shake caused 7 to12 percent of the lumber output per tree tobe graded utility or lower. Degrade washigher, 15 to 22 percent, when frost crackswere present; and trees with both frostcracks and spiral grain suffered thehighest grade losses--23 to 26 percent.

18

Checking and Collapse

The unexpected occurrence of checking andcollapse in lumber and veneer during dryingcan be attributed to the same weakening ofthe compound middle lamella that predis-poses the wood to shake and formation offrost cracks in trees. In wetwood, dryingchecks can develop in both a radial and atangential direction. Radial checks may bedeep surface checks, internal rupturescalled honeycomb, or bottleneck checks--deep surface checks that develop into ahoneycomb. Tangential checks known as ringfailure appear to be an incipient form ofring shake that starts in the tree: thesechecks do not actually rupture untilsubjected to shrinkage stresses (202). Adescription by Kutsche and Ethington (116)of various shelling failures during themachining of wood suggests that somefailures may be related to wetwood and maypossibly be an incipient form of ringshake. Collapse is essentially a col-lective internal failure of cell wallsresulting in depressions on the surface ofthe dried board or veneer which cannotalways be removed with surfacing. Figures1 and 2 show examples of honeycomb, ringfailure, and collapse that developed inwetwood during drying. Deep surface checksare present in some pieces but are notvisible because they close up toward theend of drying when the surface is below25-percent moisture content (MC) while thecore is still above 30-percent MC.

The dry kiln operation is often blamed forshake in dried lumber. Results from inves-tigations with softwood dimension lumberreveal, however, that much of the observedshake was initiated in the tree and onlybecame apparent after drying (37, 87, 140,203). Shake in western hemlock lumber hasbeen associated more with wetwood than withdrying conditions (111, 203).

Green lumber with wetwood is more likely todevelop collapse during the early stages ofdrying than lumber with normal sapwood andheartwood. Collapse can be expected inwetwood when the bonding strength betweencells has been weakened and the rate ofinternal moisture loss through cellcavities is restricted. A generally heldtheory is that collapse can develop only incell cavities that are completely saturatedwith water (142), but Kemp (98) was able toinduce collapse in wetwood cells of aspenthat were not completely saturated. He wasalso able to relate collapse to portions ofthe board with the lowest rates of moistureloss. Honeycomb and ring failure are oftenassociated with collapse in wetwood.Reconditioning of collapsed lumber bysteaming is possible when internal rupturesare not present (108).

19

Figure 1 .--Cross-cut sections from softwood boards with wetwood that developed

collapse (C), honeycomb (HC), and ring failure (RF) in drying: A, old-growtheastern hemlock; B, young-growth eastern hemlock; C, old-growth western hemlock;D, young-growth western hemlock; E, California white fir.

20

Figure 2. --Cross-cut sections from hardwood boards with wetwood that developed

collapse (C), honeycomb (HC), and ring failure (RF) in drying: A and B, northernred oak; C, eastern cottonwood; D, red maple; E, American sycamore; F, largetooth

aspen.

21

Two major sources of stress consideredresponsible for collapse during drying areliquid water tension produced by capillaryforces in the cell which can be very highwith rapid drying and compression stressperpendicular to the grain caused by asharp moisture content gradient across theboard (108, 142, 151). Little informationis available on the comparativesusceptibility of wetwood and normal woodto collapse. Kemp (98) found that aspenheartwood, artificially saturated withwater, will collapse but only at a highertemperature than that required to firstinitiate collapse in wetwood. High dryingtemperatures can plasticize wood and makeit less able to withstand the stressesassociated with collapse (108). Thin-walled cells in the newly formed earlywoodof outer sapwood have collapsed when theliving tree is subjected to extrememoisture stress (198). Results from dryingtests at the Forest Products Laboratory(see footnote 8) indicate that wetwood issusceptible to collapse undertemperatures less than 212°F,wood is not.

Collapse, honeycomb, and ring

kiln-dryingbut normal

failure arecostly defects associated with the dryingof wetwood from white fir17/(145),western hemlock (100), redwood (138),western redcedar (51, 73, 188), aspen (41,98, 133, 134) ,and west coast hardwoods(176)--particularly Pacific madrone (30),tanoak (154), and California black oak(205). The plugging of cell cavities withtyloses increases the tendency of wetwoodto collapse in such hardwoods as aspen(98), overcup oak (126), California blackoak (205), and red gum (151).

17/B. G. Anderson. Study of change ingrade and footage loss in kiln dried whitefir lumber from the green chain to thecar. Report presented at the meeting ofEastern Oregon-Southern Idaho Dry KilnClub, La Grande, Qreg., Nov. 19, 1954. 4 p.Oregon Forest Products Laboratory,Corvallis, Greg.

The severity of honeycomb and collapse inwetwood usually increases with an increasein temperatures during the early and middlestages of drying. Temperatures at whichthese defects begin to form in wetwoodapparently vary with tree species, siteconditions, and types of wetwood. Whengreen lumber is kiln dried, honeycomb inthe wetwood of California black oak andnorthern red oak can be minimized by lowerinitial dry bulb temperatures (202, 205).Wetwood in southern bottom-land oaks,however, honeycomb and collapse under themildest kiln schedules; this lumber mustinitially be dried in the open air ordried in kilns at low temperatures to atleast 25 percent MC before it can be kilndried.l8/ Collapse in aspen wetwood canbe reduced by using mild kiln conditionsin the early stages of drying (98).Western redcedar lumber containing aparticularly dark heavy type of wetwoodwill sometimes collapse after only 1 day ofair drying.19/ With high temperaturekiln schedules (temperature of 212°F orhigher) most wetwood will honeycomb orcollapse, but usually adjacent normal woodwill not20/(36, 132, 134, 159).

18/Don Cuppett, Northeastern ForestExperiment Station, Princeton, w. Va.,personal communication to J. C. Ward.

19/Jack McChesney, Louisiana PacificCorp., Dillard, Oreg., personalcommunication to J. C. Ward.

20/Unpublished data from drying studies:on file at U.S. Forest Products Laboratory,Madison, Wis.

22

Honeycomb and ring failure are majordefects associated with kiln drying of oak,our most important hardwood lumberspecies. Annual wood losses from dryingdefects associated with wetwood in oakfactory grade lumber produced in theEastern United States are estimated to beat least 3 percent. It is not unusual,however, for 10 to 25 percent or even halfthe lumber charges from individual kilnsto be lost because of honeycomb and ringfailure.

FPL studies on kiln drying northern red oaklumber, green from the saw, showed 8- to25-percent losses in volume from honeycomband ring failure in wetwood (201). Mone-tary losses ranged from $34.28 to $139.42per thousand board feet of rough drylumber. Three studies of kiln dryingCalifornia black oak (205) at a commercialmill showed losses from defects in wetwoodto be 7 to 48 percent of the total volumeof Number 1 Common and Better lumber.Total monetary loss in rough, dry lumberfor the three studies was over $9,000.This can explain why one large Los Angeleslumber company fills a standing monthlyorder for one-half-million board feet of4/4-inch kiln-dried oak lumber with oakimported from the Eastern United States.

Smith (180) estimates that, for BritishColumbia, the development of honeycomb andring shake in softwood studs with bacteri-ally infected wood will result in a reduc-tion in grade of $7 per thousand boardfeet. We can calculate from the averagegrade prices in "Random Lengths" (150) thatdrying defects associated with wetwood cancause grade losses of $38 to $40 per thou-sand board feet for hem-fir dimensionlumber.

Slow Drying Rates andUneven Moisture Content

The wetwood of many species has lowpermeability and requires much longerdrying times than normal wood to reach adesired moisture content. Even whenwetwood boards reach the desired average MC,there is an uneven distribution of moisturewhere the shell is very dry but the corecontains wet pockets or streaks that arestill above the fiber saturation point.Sometimes after supposedly dry wetwoodlumber is surfaced, the internal wetpockets dry and collapse, resulting indegrade of the lumber (134). Wet pocketscan be a problem when dried wetwood fromaspen and hemlock are used for core stockin the manufacture of doors and panels.Although these wet pockets may be pencilthin, they will build up enough steampressure during electronic gluing opera-tions to explode and shatter the surface ofthe pieces.21/ Internal wet pockets inkiln-dried western hemlock causes erroneousdeterminations of MC when electronic metersare used (109).

21/ Elmer Cermak, Algoma Hardwoods, Inc.,Algoma, Wis., personal communication to J.C. Ward.

23

A serious economic problem can result fromthe presence of impermeable wetwood insoftwood dimension lumber for constructionpurposes. The same problem applies toaspen and poplar studs and light framinglumber graded under softwood rules. Tomeet moisture specifications for kiln-driedlumber softwood dimension lumber must bedried to at least 19-percent MC; the desir-able range is 12 to 16 percent. Boardswith wetwood may require 50 percent or moretime in the dry kiln to reach 19-percent MCas do boards with sapwood and heartwood.When the volume of boards exceeding the19-percent MC specification is greater than5 percent, a kiln charge or shipment ofkiln-dried lumber is considered to be innoncompliance with specifications formoisture22/ (28, 178, 210, 218).

Slow drying of construction lumber con-taining wetwood can be expected for thesesoftwood species: redwood (103, 138, 151),true firs (101, 103, 145, 151; also seefootnote 17);western hemlock (29, 103,110, 151),159), and white pines (151).

western redcedar (29, 103, 151,Hardwood

species with impermeable wetwood and slowdrying rates are: aspen (36, 86, 133, 200),red gum (151), and water and swamp tupelo(137). Redwood with heavy sinker heart isespecially difficult to dry (138), andl-inch-thick boards require 146 days of airdrying to reach 20 percent MC (52). Incontrast, 2-inch-thick redwood with lightand medium heartwood can be air dried to19-percent MC in 74 days (8). Arganbrightand Dost (7) found that development ofchemical brown stain decreases the dryingrate of sugar pine and, at the end ofdrying, retards moisture movement back intothe board surface during conditioningtreatment.

Table 4 shows that the slow drying rates ofwetwood vary with tree species. Wetwoodfrom aspen and the conifers is much lesspermeable than normal wood, whereas wetwoodfrom cottonwood, elm, and oak takes onlyslightly longer to dry than normal wood.The effect of wetwood on reducing dryingrates tends to decrease as thickness ofboards decreases. For some species, thetime required to dry veneer containingwetwood and veneer containing sapwooddiffers little for a given MC: but forspecies such as red gum, true firs,hemlock, larch, and redwood, there aredecided differences (126). A comparison ofdrying times for green veneer from balsamfir and white fir (table 5) indicates thatwetwood has an adverse effect on dryingrates for even thin material.

Where wetwood boards occur in kiln chargesof softwood and aspen dimension lumber, theprocessor is confronted with five possiblealternatives; each may result in losses ofwood and energy and in higher manufacturingcosts.

1. The kiln residence time can be extendeduntil the wet test boards reach therequired 19-percent MC. This can causeoverdrying of nonwetwood stock (i.e., below11-percent MC), which is then subject toplaner splits and costly degrade in subse-quent machining operations (100, 101, 103,110, 145, 218; also see footnotes 17 and22).

22/G. Reinking. 1971. Kiln drying lumberto the moisture provisions of the newlumber standards. Forest Products ResearchSociety News Digest File J-1.1, 2 p.Madison, Wis.

24

Table 4--Drying times for green wetwood boards compared with drying times for boards containing normal WOOd1/

Initial kilnBoard specifications temperatures 2/ Wetwood Sapwood Heartwood

Species Source of data Dried Green Green Greenmoisture moisture Drying moisture Drying moisture Drying

Thickness content D8 WB content time content time content time

Softwoods:Western hemlockWestern hemlockEastern hemlockWhite firWhite firWhite firWhite firSugar pine

Inches Percent - - °F - - Percent Hours Percent Hours Percent Hours

Ward and Kozlik (203) 1-34 15 180 176 150 139 156 82 54 59Kozlik et al. (111) 3/4 10 100 69 153 170 -- -- 66.1 90J. C. Ward3/ l-3/4 15 180 170 148 185 137 95 75 96Smith and Dittman (177) l-7/8 20 160 145 193 158 145 84 57 42Smith and Dittman (177) 1-7/8 4/15 160 145 193 145 4/97 57Pong and Wilcox (147)

4/195160 -- 174 -- 106 -- 60

J. C. Ward3/1- 1-3/4 15-16

1-3/4 15 180140170 155 150 170 105

4/63

66 73J. C. Ward3/ l-1/2 15 120 5/75 194 226 204 148 51 72

Hardwoods: 6/Aspen Cech (36), Huffman (86) l-3/4 15 140 133 170 4/210+ 140 175 -- --

Quaking aspen J. C. Ward (200) l-3/4

Eastern cottonwood J. C. Ward3/ l-1/8157

180180

170 115-128144

17975

76-122131

9085

81-106 115170 --

American elm J. C. Ward3/ l-l/8 7 120 113 106 134 90 121 69 113--

California black oak Ward and Shedd (205) l-1/8 7 120 115 95 387 -- __ 85 379Northern red oak J. C. Ward3/ l-l/8 7 125 121 97 324 -- -- 86 312

1/Comparisons are within kiln runs and not among kiln runs or between species.2/DB = dry bulb temperature; WB = wet bulb temperature.3/ J. C. Ward, unpublished data on file at U.S. Forest Products Laooratory, Madison, Wis.4/Extrapolated from published data.5/Kiln vents open with steam sprays off.6/All wetwood developed honeycomb, collapse, or ring failure or a combination of these.

Table 5--Drying times for green veneer with wetwood compared with veneer containing normal sapwood and heartwood

Species Source of dataVeneer specifications Wetwood Sapwood Heartwood

Dried Dryer Green Drying Green Drying Green DryingThickness moisture temperatures moisture time moisture time moisture time

content content1/ content1/ content1/

Inch Percent ° F Percent Minutes Percent Minutes Percent Minutes

Balsam fir Dokken and Lefebvre (48) l/10 4 300 154 18-1/2 201 15-1/4 76 11Dokken and Lefebvre (48) 4 450 154 7-3/4 201 7-l/4 76 5-l/2Dokken and Lefebvre (48) l/6 4 300 154 35 201 31 76 25-l/2Dokken and Lefebvre (48) 450 154 15 201 14-l/4 76 9-l/2

White fir Vern Parker2/ 1/8 5 3/ 150 10 150 8 4/

1/ Average value.2/ Superintendent, Plywood Mill, Bendix Corp., Martell, Calif., personal communication to W. Y. Pong.

3/ 4-stage gas jet-dried: 450°, 450°, 400°, and 350°F.4/ Heartwood dried according to sapwood schedule of 8 minutes will be overdried with a moisture content of 1 percent.

25

2. The wet stock can be sorted and redriedafter a normal kiln run. This approachwill increase drying and handling costs forthe wetwood lumber. For white firdimension lumber containing wetwood,redrying can result in a 40-percentincrease in the kiln-drying costs.23/

Table 6--Freight costs for shipping white fir lumber1/

3. Boards greater than 19-percent MC can bemarketed as surfaced green lumber and lessreturn will be received for the product.For hem-fir dimension lumber, this canamount to approximately $26 per thousandboard feet in selling price (149), not tomention drying and extra handling losses.Shipping weights and the resulting shippingcosts of green lumber are important consid-erations in this third alternative. Intable 6 are calculated weights for 1,000board feet of white fir at differentmoisture contents. Also included are costsfor shipping this lumber to Los Angeles andChicago from Portland, Oregon. The pricedifferential of $26 between green and drylumber barely offsets the shipping costdifferential of dry lumber (15 percent) andgreen lumber (150 percent) to Los Angelesbut not to Chicago.

23/Donald O. Prielipp, general manager,Roseburg Lumber Co., Anderson Division,Anderson, Calif., personal communication toJ. C. Ward.

Cost of shipping 1,Ow board feet

TO LOS Angeles To Chicago(base rate, (base rate,$1.27/100 PO"rldS~ $3.07/100 pounds)

- - - - - - - - - Dollars - - - - - - - -

Moisture content Calculatedweight per 1,000board feet

Percent Pounds

0 1,271 16.14 39.0212 1,423 18.07 43.6915 1,463 18.58 44.9119 1,513 19.22 46.4525 1,590 20.19 48.8175 2,226 28.27 68.34

100 2,543 32.30 78.07115 2,734 34.72 83.93150 3,178 40.36 97.56

1/F.o.b. Portland; base rates (Dec. 15, 1978) are for railroad shipments of 85,000pounds or less (western Wood Products Association, Portland, Oreg.).

4. The lumber can be dried by a normalschedule and not sorted for wet stock. Allboards, both above and below 19-percent MC,would be surfaced and sold as kiln dried.This option could be exceedingly costly ifthe volume of wet stock exceeded the5-percent limitation (210) and was detectedby the buyer and reinspection by thegrading association requested. Not onlywould the surfaced lumber have to beresorted, regraded, and metered formoisture, but the boards that exceeded themaximum allowable moisture would beconsidered substandard surfaced green andregraded accordingly. Losses in excess of$50 per thousand board feet are possiblewith reinspection.

26

5. The green lumber could be segregatedinto uniform drying sorts (sapwood,heartwood, and wetwood or sinker) and eachthen dried under different kiln schedulesor treatments. This presorting wouldminimize the extremes in final moisturecontent within a given kiln charge and haslong been advocated for western softwoods(103, 178), particularly white fir24/(101,177, 179; also see footnote 17), westernhemlock (110), redwood (138), andincense-cedar (152). Presorting wouldminimize energy costs. With the energycrisis, it becomes imperative that costs ofenergy used in drying and redrying wetwoodbe given careful consideration, especiallysince 60 to 70 percent of the total energyused in manufacturing most wood products isconsumed in drying (43).

Commercial presorting of green lumber fordrying by visual detection and hand methodshas been used with some success for westernhemlock (110), incense-cedar (152), andCalifornia white fir (145, 178, 215). Forpresorting to be effective, however, eachboard must be examined on all sides duringthe segregation operation. Under highproduction mill conditions, manualpresorting is generally not possible.Examination of individual boards will alsobe limited where tray and drop sorters areused. Mechanical, electrical, and opticalinstruments for automatic presorting ofwetwood lumber on a commercial scale havenot been developed (207).

Another obstacle to obtaining maximumeffectiveness in presorting is the mixtureof wetwood and normal wood that occurs inmany boards on the commercial green chain(110, 147). It is possible to developoptimum schedules for drying boards havingmixtures of normal sapwood and heartwoodwith wide differences in green moisturecontent (161), but not for mixtures ofsapwood and wetwood even though there maybe little or no difference in greenmoisture content (162). Studies of whitefir indicate that boards containingmixtures of wetwood, sapwood,and corky heartwood will have more dryingand surfacing problems than boards that drymore uniformly (147, 215).

Steaming of green lumber before dryingincreased the drying rate of wetwood fromwhite fir (175) and redwood (52), but wasnot effective with western hemlock (110) oreastern hemlock (see footnote 8). Thechemical nature of extractives in thewetwood seems to be an important factorcontrolling the effectiveness ofpresteaming.

24/J. Steel. 1953. Kiln drying whitefir. Wood drying committee news digest.(Aug.) 3 p. Forest Products ResearchSociety, Madison, Wis.

27

Chemical Brown Stain

Chemical brown stain in wetwood is anespecially serious defect in lumber,veneer, and wood fiber products graded onappearance. Brown stain also affects themarketability of softwood dimension lumberbecause consumers think the wood looksdecayed. Wood decay fungi are notconsidered causal agents for chemical brownstain (51, 56, 85, 103, 148, 151). Brownstain in wetwood usually develops in thezone between sapwood and heartwood orwithin bacterially infected heartwood.Sapwood of many species develops brownstain if taken from logs stored for anextended time under humid conditions.Brown stain developed in eastern white pinelumber from logs stored in the woods for 42and 93 days, but not in lumber sawn andseasoned within 24 hours of felling (14).Depth of stain in boards increased as-storage time increased. Extended storageof ponderosa pine and sugar pine logs underwater sprays will initiate development ofbrown stain in sapwood boards and furtherintensify darkening in wetwood zones.25/

Timber species noted for developingchemical brown stain during dryingusually tend to have wetwood. Processingproblems associated with chemical brownstain in lumber are considered importantcommercially for the following species:sugar pine, eastern and western whitepines, ponderosa pine, Pacific silver fir,noble fir, western hemlock, redwood,western redcedar, and Sitka spruce (5, 7,18, 19, 23, 29, 35, 56, 61, 89, 99, 103,141, 151, 165).

Brown stain can develop in both air-driedand kiln-dried wood, but it is usually morepronounced under warm, humid kilnschedules. Considerable brown stain canoccur in boards air dried at temperaturesas low as 80°F (141). Chemical brownstain may also develop just below, but noton the surface of, air-dried lumber andwill not be noticed until after the boardsare surfaced (155). This concealed brownstain, referred to as yard brown stain, hasbeen observed in ponderosa pine, sugarpine, and the white pines (85). Wetwoodprone to developing brown stain should besegregated from other green lumber. Thewetwood stock would be air dried; thenormal stock, kiln dried. For minimumdevelopment of brown stain during airdrying, the lumber should be from freshlyfelled trees and drying should be fast, withrelative humidities below 65 percent (14,61, 66, 141, 155).

If faster drying of wetwood is desired orpresorting is not feasible, reduction orprevention of chemical brown stain duringkiln drying may be accomplished by eithermanipulating kiln schedules or dipping thegreen boards in enzyme-inhibiting orantioxidant solutions. There are, however,drawbacks to these preventive methods thatare important.

25/Del Shedd, Quality Control Supervisor,Kimberly Clark Corp., Anderson, Calif.,personal communication to J. C. Ward.

28

Low initial temperatures in kilns and lowhumidities with good air circulation arenecessary to successfully reduce brownstain or prevent it (23, 29, 99, 102, 105,141). Rosen (156) found that hightemperature jet drying eliminated brownstain in cottonwood, but the wetwoodcollapsed and checked. With lowtemperatures, the kiln usually has to bevented to attain low relative humidities(29, 102, 103). Venting dry kilns resultsin a great waste of energy. Sometimes thekiln doors must be opened during drying tolower the humidity, and the duration of theventing can be from 3 days to 1 week.Manipulating kiln schedules has not beensuccessful in controlling dark stains inredwood unless the lumber was first steamed(5, 52, 54). Solvent drying of redwoodwith acetone has been proposed forelimination of stains (6).

Kiln schedules to prevent stain are notalways successful if the drying operationis regulated either on a time basis or onkiln samples containing only normal wood.Figure 3 shows chemical brown stain thatdeveloped only in sugar pine boardscontaining wetwood but not in sapwood ornormal heartwood. These boards were driedunder an antistain schedule based on theaverage drying rate of boards with normalwood. The operation could have been basedon the drying rate of the wetwood boards,but the normal boards would have been over-dried and the subsequent surfacing of theseboards would result in planer splitting.The solution is to segregate the normalboards from the wetwood boards and then dryeach board sort under different schedules.

Dipping green lumber in enzyme-inhibitingor antioxidant solutions usually preventsbrown stain and permits higher initial kilntemperatures and humidities (34, 35, 61,102, 103, 170, 182). Such reagents willnot reduce brown stain if they are not ableto sufficiently penetrate the wood surfaceduring the dip treatment (141). Proper useof antistain dips results in substantialreductions in drying and energy costs, butmost chemicals used are highly toxic tohumans. Because of Occupational Safety andHealth Administration inspections, thedipping of boards in brown stain retardantsolutions has been discontinued by manymills.

Stain-producing extractives from wetwoodcan cause problems with finishes. Figure 4shows the undesirable darkening of alacquer finish from an underlying oaklaminate containing wetwood. Water-solubleextractives in wetwood of redwood andwestern redcedar cause serious problemswhen stains penetrate finishes on exteriorsiding. Connors found that extraction ofsequirins from the redwood can effectivelyprevent the problem.26/ On a commercialscale this treatment could be cumbersomewith lumber, and a disposal problem wouldbe created. Connors also found thatdipping the boards in lead acetate beforeapplying finishes is effective butexpensive, and lead is now prohibited forsuch use.

The chemical precursors causing brown stainproblems in drying wetwood in westernhemlock lumber are also responsible forproblems when this species is used forground woodpulp (15, 17). The cost per tonof raising the brightness of the groundhemlock woodpulp can be increased by 15 to20 times when brown stain occurs. Aspenwetwood is objectionable for pulp becauseof added bleaching costs (79).

26/G. L. Connors. 1968. Considerationsfor reducing extractives staining inredwood. Office report, 11 p. U.S. ForestProducts Laboratory, Madison, Wis.

29

Figure 3. --Kiln-dried sugar pine shows chemical brown stain in center board that

contained wetwood but not in boards with sapwood (left) or normal heartwood (right).End sections (top) were cross cut from rough, dry boards, and one-sixteenth inch was

planed from board surfaces (bottom).

30

Figure 4.--Darkening of a white lacquer finish overlying a kiln-dried red oak laminatecontaining bacterial wetwood. Ring failure (RF) within the laminate extended to thesurface during air drying of the lacquer finish.

31

Plywood and ReconstitutedBoards

Little is known about the effect of wetwoodon the quality of plywood and reconstitutedwood products. We believe that investiga-tions of the use of wetwood in theseproducts may well provide insight onunsuspected causes of some processingproblems. Also, these products areincreasingly promoted for use as componentsin light frame construction (184), and theeffect of wetwood on strength propertiesshould be investigated.

There are several ways that wetwood causesproblems with the processing and quality ofplywood. Shake and frost cracks cause"spinout" of bolts on the lathe, resultingin splits and splintering of veneer (71,79, 125, 126). Commercial experienceindicates that wetwood in "sinker" logs ofspecies like redwood is undesirable forveneer because of cutting and dryingproblems (125). The assembly-timetolerance ofconventional plywood adhesionrestricts the moisture content of veneer todefined upper and lower limits. Control ofmoisture content in veneer from wetwood isa major technical problem and can lead to"undercured," "washed out,” or "starved"glue lines and to “blows” and "blister" inthe pressing operation (32). Nearly allthe "blows" during production of white firplywood are related to wetwood in theveneer. 27/

The effect of wetwood on the strength ofthe glue bond is usually associated withwet pockets, and there is a dearth ofinformation on the influence of extractivesand pH. Poor adhesion because of unevendistribution of moisture in dried veneerscontaining wetwood has been reported foreastern cottonwood (191), western hemlock(79), balsam fir (32), and white fir(126). Oxidation stains are objectionablecharacteristics for face veneers, and thesestains may possibly interfere with properadhesion of the glue to the wood. Somewetwood species most susceptible tooxidation stains during production ofplywood are: sugar pine, western whitepine, ponderosa pine, Jeffrey pine, redmaple, tanoak, bottom-land oaks, tupelo,and willow (126).

A small amount of research data exists tosuggest that wetwood may lower the strengthproperties of reconstituted boards thatinclude particle board and hardboards.Table 7 shows that wetwood from aspen andwestern larch will not make as strong aflakeboard as normal sapwood andheartwood. Linear stability was notadversely affected by wetwood, butthickness swelling was greater for aspenboards containing wetwood and least forlarch. Wet process hardboards from aspenwetwood have a lower modulus of elasticity,modulus of rupture, and internal bondstrength than boards from sapwood (67).Dimensional stability of the hardboardsfrom wetwood was greater than boards fromsapwood, however.

28/J. C. Ward and J. Chern. 1978.Comparative properties of flakeboard madefrom normal wood and from bacterial wetwoodof trembling aspen and western larch. Apreliminary report. 6 p. U.S. ForestProducts Laboratory, Madison, Wis.

27/Vern Parker, Bendix Corp., Martell,Calif., personal communication to W. Y.Pong.

32

Table 7--Physical and mechanical properties of 1/2-inch-thick flakeboards1/ from normal wood and wetwood of

trembling aspen and western larch?/

Specific gravity?’ Mechanical properties Swelling in 30- to 90-percent

Species and relative humidity

type of wood

Solid wood Flakeboard Moisture Modulus of Modulus of Internal

content rupture e l a s t i c i t y b o n d Linear T h i c k n e s s

Thousand

Pounds per pounds per Pounds per

P e r c e n t square inch square inch square inch Percent Percent- -

Aspen (Wisconsin):

Sapwood 0 . 4 7 4

W e t w o o d4/ .504

0 . 6 5 7 7 . 2 6 9 2 1 2 8 0 . 0 7 6 . 8 85,541

.652 7 . 4 4 , 4 9 9 6 1 8 9 5 .08 9 . 5 8

Aspen (Colorado):

H e a r t w o o d .35 .715 6 . 2 6 , 3 9 0 9 9 4 7 6 .14 9 . 5 6

W e t w o o d5/ .34 .634 6 . 8 4,335 6 7 7 4 8 .14 12.76

Larch:S a p w o o d

Heartwood

W e t w o o d 6/

.480 .652 9 . 3 4 , 9 9 2 6 5 5 8 9 .09 1 1 . 4 5

.454 .648 8 . 9 6 3 5 1 1 2 .10 9 . 8 74,520

.701 .628 9 . 2 3,542 5 4 9 7 4 .08 7 . 8 0

1/Boards made from 0.02- by 2.0-inch flakes with 4-percent phenolic resin.2/Data from J. C. Ward and J. Chern. 1978. Comparative properties of flakeboard made from normal wood and from

bacterial wetwood of trembling aspen and western larch. A preliminary report. 6 p. U.S. Forest Products

Laboratory, Madison, Wis.3/Specific gravity of wood based on ovendry weight and ovendry volume. Specific gravity of board based on ovendry

weight and volume of board at mechanical test moisture content.4/ Bacterial wetwood formed where sapwood changes to heartwood.5/ Bacterial wetwood formed in previously developed, normal heartwood.6/Formed from microbially infected inner sapwood--heavily saturated with water-soluble extractives that included

arabinogalactans.

Corrosion of Kilns

Reports of accelerated corrosion of drykilns have increased in frequency in recentyears. Most cases of corrosion can betraced to highly acid atmospheres withinthe kiln during drying. Not only are metalwalls, pipes, fasteners, etc., attacked bythis corrosive mixture of air, butunprotected concrete (16) and wood stickersare also subject to accelerateddeterioration. Several explanations can begiven. Drying charges of green lumberrather than partially air-dried lumber

considerably increases the liability ofwood to evolve corrosive acid vapors (10,11, 39). Use of high temperature schedulesin drying green wood increases corrosion.Contact between dissimilar metals in thekiln, such as aluminum and iron in anacidic atmosphere, will produce a "batteryeffect" and corrosion of metal will begreatly accelerated (16). The drying ofwood treated with aqueous solutions ofpreservatives will cause corrosion problems(39).

33

Research Needs

Although investigations are needed, webelieve that the presence of wetwood ingreen lumber charges can be a definitefactor in many reported cases of increasedkiln corrosion. Oak, hemlock, and the truefirs are species likely to have wetwoodwith a pH as low as 3.5. Metal corrosionis greatly accelerated by a pH of 4.0 orless (10, 11), and acidic vapors of pH 3.5will attack concrete (16). Barton (16)related the presence of normally occurringchelating compounds in the extractives ofwestern redcedar with the corrosion of ironbut not of aluminum in dry kilns. Not allcharges of western redcedar causedchelation corrosion.

Wetwood in western hemlock lumber may be animportant factor in kiln corrosion. In1951, MacLean and Gardner (135) reportedthat an increase in the deterioration ofwooden dry kilns was due to increased usefor drying western hemlock lumber. An acidcondensate of aqueous vapors from thehemlock resulted in deterioration of thecellulose in kiln boards and corrosion ofmetal fittings. Although Douglas-firheartwood is acid, the vapor condensateswere not as acid as those from hemlock.Also, Douglas-fir heartwood has a highercontent of resin, and the vapor condensatesform a protective layer. The acid vaporsfrom drying one or two charges of hemlockwill remove this protective layer ofDouglas-fir condensates. Most lumber isnow dried in metal kilns, and Barton (16)estimates that at least 90 percent of thecorrosion in these kilns is caused by vaporcondensates.

Wetwood is responsible for substantiallosses of wood, energy, and productionexpenditures in timber-using industries.There is a clear need to more thoroughlyassess these losses and to find ways ofeliminating or minimizing them.

An effective research program on wetwoodshould include planning short- andlong-term solutions to the overall problemand providing for them. Solid woodproducts, particularly lumber and veneer,are most adversely affected by processingdefects related to wetwood. Lumber andveneer are expected to be prime economicproducts from future timber harvests (97).In the long term, the best solution to thewetwood problem may well result from timbermanagement research to prevent futureoccurrences of wetwood.

Short-Term

The initial research effort must beprimarily concerned with attacking existingutilization and processing problems, but itshould also provide a scientific basis forlong-term research planning. A short-termresearch program should concentrate on: (1)assessing the occurrence of wetwood instanding timber and its effect on productyields and losses, (2) defining thechemical and physical properties of wetwoodas a basis for detecting and segregatingaffected wood products, and (3) developingoptimum utilization and processing methodsfor timber and wood products containingwetwood. Item 1 would be the pivotal areaof endeavor for the short-term researchprogram and also the area most likely toprovide continuity with long-term research.

34

These studies should be carried outcooperatively by timber managers, timberprocessors, and forestry researchers. Themanagers would provide the timber and themill operators the processing facilities.Researchers would record information ontimber quality for selected sample trees.Bucked logs from the sample trees would befollowed individually through the millingprocess and information, including thegrade and amount of product cut from eachlog, tallied. The yield data would becompiled and analyzed in conjunction withtimber quality data to provide the basisfor predicting the product yield fromsimilar timber. Measurements of wetwood inthe timber and the resultant defects inboth green and dried wood products would beincorporated into the beginning and finalphases of a product yield study.

Studies on timber products and yields areimportant to the initiation of research onwetwood. Now we can only guess, but withsome confidence, that the utilization andprocessing problems associated with wetwoodare costing the wood-using industries manymillions of dollars annually. Nostatistical or economic surveys havebeen initiated either for the purpose ofdetermining the amount of wetwood in timberstands or for estimating the losses thatunquestionably result from the processing ofwetwood timber. Government agencies havenot initiated and carried out the necessarysurveys on wetwood because they are notaware that a problem exists: this stemslargely from industry's reluctance to makea concerted effort to publicly recognizethe problem.

The major reason the wetwood problem is notpublicized is that most foresters and manywood processors have not related defects--such as shake and frost cracks in trees andcollapse and honeycomb in lumber--to wetwood.This is understandable because, until justrecently, wood scientists have related theunexpected occurrence of honeycomb and ringfailure when drying wetwood lumber to aninherent and perfectly normal variability inwood properties (75). Thus, lumber gradingassociations have been reluctant torecognize wetwood as a precursor toprocessing defects when even scientificexperts are often unable to discern it ingreen lumber. In fact, there appears to bemore recognition of wetwood and its problemsby sawmill and dry kiln operators andfurniture manufacturers than there is in thescientific community.

Logical species for the initial short-termstudies are western hemlock, western truefirs, eastern oaks, and the white pines,including sugar pine. These species arevery susceptible to formation of wetwoodand constitute a sizable volume of timberon commercial forest lands in the UnitedStates (192, 193). Comparisons of thesespecies with other commercial species areshown in table 8. In spite of theirsusceptibility to wetwood, western hemlock,western true firs, eastern oaks, and whitepines are tree species of high value andcapable of producing valuable lumber items(table 9).

35

Table 8--Comparison of softwood and hardwood timber volumes from USDA Forest Service (192), andsusceptibility of species to formation of wetwood

Sawtimber volume Growing stock volume

Species Billion Billion Wetwood Studyboard feet Percent cubic feet Percent class1/ priority2/

Eastern softwoods:Shortleaf and loblolly pineLongleaf and slash pineWhite and red pineSpruce and balsam firCypressEastern hemlockOther eastern species

TotalTotal softwoods

Eastern hardwoods:Red oakWhite oakHickorySoft mapleHard mapleSweet gumTupelo and black gumYellow poplarCottonwood and aspenAshBeechBlack cherryBasswoodYellow birchOther eastern species

Total

Western hardwoods:Red alderCottonwood and aspenOakOther western species

TotalTotal hardwoods

65.27323.52020.87240.89731.25623.627

3.41.21.12.21.61.2

25.5304.3443.9938.1066.7534.428

5.91.0.9

1.91.61.0

Western softwoods:Douglas-fir 520.640 27.3 96.861 22.4Western hemlock 251.012 13.2 47.540 11.0True fir 218.772 11.5 45.326 10.5Ponderosa and Jeffrey pine 189.897 10.0 38.292 8.9Spruce (Sitka, Engelmann, etc.) 132.225 6.9 26.296 6.1Lodgepole pineSugar pineWestern white pineWestern redcedarWestern larchRedwoodOther western species

Total31.362 1.7 6.886 1.6

1,549.353 81.3 314.355 72.8

196.50244.24826.87423.48519.11216.17829.534

355.933

10.3 53.571 12.42.3 13.855 3.21.4 8.349 1.91.2 17.322 4.01.0 5.033 1.2.9 5.781 1.3

1.6 13.611 3.218.7 117.522 27.2

1,905.286 100 431.877 100

106.21778.68930.91423.87125.75726.31825.50625.09316.77115.95715.6496.9048.5027.324

46.313459.785

20.6 39.309 18.115.3 32.099 14.86.0 12.583 5.84.6 15.070 6.95.0 11.731 5.45.1 10.528 4.85.0 9.817 4.54.9 8.570 4.03.3 12.096 5.63.1 7.736 3.63.0 5.794 2.71.3 3.488 1.61.6 3.434 1.61.4 3.249 1.59.0 22.178 10.2

89.2 197.682 91.1

15.713 3.1 5.043 2.355.696 10.8 19.330 8.9

515.481 100 217.012 100

24.842 4.8 7.638 3.512.077 2.3 5.043 2.33.064 .6 1.606 .8

13 1

2-3 21-2

112 42 42221

11

1-21-3

131

2 31-2 31-221222

2-322112

1-2

1-22-3

21-2

1/Class 1 species are least affected with wetwood; class 3 are most susceptible.2/1 = highest; blanks indicate species with a priority lower than 4.

36

Table 8--Comparison of softwood and hardwood timber volumes from USDA Forest Service (1921, andsusceptibility of species to formation of wetwood

-

Sawtimber volume Growing stock volume

Species Billion Billion Wetwood Studyboard feet Percent cubic feet Percent class& priority21

Western softwoods:Douglas-fir 520.640 27.3 96.861 22.4Western hemlock 251.012 13.2 47.540 11.0True fir 218.772 11.5 45.326 10.5Ponderosa and Jeffrey pine 189.897 10.0 38.292 8.9Spruce (Sitka, Engelmann, etc.) 132.225 6.9 26.296 6.1Lodgepole pineSugar pineWestern white pineWestern redcedarWestern larchRedwoodOther western species

Total

65.27323.52020.87240.89731.25623.627

3.41.21.12.21.61.2

25.5304.3443.9938.1066.7534.428

5.9i.0

.91.91.61.0

Eastern softwoods:Shortleaf and loblolly pineLongleaf and slash pineWhite and red pineSpruce and balsam firCypressEastern hemlockOther eastern species

TotalTotal softwoods

Eastern hardwoods:Red oakWhite oakHickorySoft mapleHard mapleSweet gumTupelo and black gumYellow poplarCottonwood and aspenAshBeechBlack cherryBasswoodYellow birchOther eastern species

Total

Western hardwoods:Red alderCottonwood and aspenOakOther western species

TotalTotal hardwoods

31.362 1.7 6.886 1.61,549.353 81.3 314.355 72.8

196.50244.24826.87423.48519.11216.17829.534

355.9331,905.286

10.32.31.41.21.0

.91.6

18.7100

53.57113.8558.349

17.3225.0335.781

13.611117.522431.877

12.43.21.94.01.21.33.2

27.2100

106.21778.68930.91423.87125.75726.31825.50625.09316.77115.95715.6496.9048.5027.324

46.313459.785

20.615.36.04.65.05.15.04.93.33.13.01.31.61.49.0

89.2

39.30932.09912.58315.07011.73110.5289.8178.570

12.0967.7365.7943.4883.4343.249

22.178197.682

18.114.85.86.95.44.84.54.05.63.62.71.61.61.5

10.291.1

24.842 4.8 7.638 3.512.077 2.3 5.043 2.33.064 .6 1.606 .8

15.71355.696

515.481

3.110.8

100

5.04319.330

217.012

2.38.9

100

13

2-31-2

112222

T

11

l-2l-3

131

2

is,'

:222

2-322112

l-2

l-22-3

2l-2

1/Glass 1 species are least affected with wetwood; class 3 are most susceptiole.1/l = highest; blanks indicate species with a priority lower than 4.

36

Table 9--Values of sawtimber stumpage and typical kiln-dried lumber prices for tree species with high priority for incorporationin a research program on wetwood

SpeciesStudypriority 1/

Value ofsawtimberstumpage2/

Dollars

Western softwoods:Western hemlock 1Mountain hemlock 1

TotalWhite, grand, andmiscellaneous firs 2

Noble fir 2Shasta red fir 2

46,245,409344,395

46,589,804

24,234,6475,046,8262,097,222

Subalpine fir 2Total

Sugar and western white pine 4

601,37831,980,07340,004,600

Eastern hardwood:Oak 3 1,328,917

Kiln-dried lumber

Grade Thickness

Inches

1978 price3/

Dollars perthousand

board feet

Finish FG, C and Better selects 4/4 569No. 2 and Better, Hem-fir Dimension 8/4 244

Moulding and Better, rough dry 5/4, 6/4 526-562No. 2 and Better Dimension 8/4 236

C and Better Selects, S2S 4/4, 5/4, 6/4, 8/4 944-1,010Moulding, S2S 4/4, 5/4, 6/4, 8/4 463-761

No. 1 Common and Better 4/4, 5/4 800-1, 100

1/1 = highest.2/Value of sawtimber stumpage sold from National Forests in 1976 (143).3/Averaged prices for softwood (Western Wood Products Association, Portland, Oreg.) and range of prices for oak (HardwoodMarket Report, Volume 56, Memphis, Tenn.).

Material for the corollary studies onwetwood properties and processing should beselected from sample trees, logs, and woodproducts of the product and yield studies.Studies of wetwood properties should bedesigned to aid development of practical andaccurate methods for detecting wetwood intimber and solid wood products in the greencondition. Studies on processing methodsshould determine, both qualitatively andquantitatively, the wood losses, gradechanges, and energy requirements forproducing end products containing wetwood sothat comparisons could be made with productscontaining normal wood. Results andfeedback from the corollary studies wouldincrease the efficiency and accuracy oftimber quality evaluation and prediction ofproduct potential for the commercial woodprocessor.

High priority should be given to three itemsof concern in the area of research todevelop improved processing methods fortimber and wood products containing wetwood:

1. Develop an accurate and fast method foridentifying wetwood in green lumber andveneer to segregate the pieces into dryingsorts.

2. Determine optimum methods for processingsorts of lumber and veneer containing pureand mixed amounts of wetwood and normal wood.

3. Compare mechanical strength properties oflumber containing wetwood with similarlumber of normal wood content for lightframe construction.

There are good possibilities to designstudies which could examine all three items.

37

Investigations to determine optimum methodsof drying boards and veneers containingwetwood should probably be conducted intandem with investigations to characterizeand sort these products before they aredried. It would be useful to design theseprocessing studies so that any developmentof drying defects could be related back tomicro-organisms in the tree. The spread ofbacteria from wetwood zones to sapwoodduring log storage needs to be investigatedwith respect to formation of chemical brownstain and changes in permeability of thesapwood. Definitive biochemical studies ofcontributing micro-organisms can also beconsidered a long-term research need.

A particular short-term research need mustbe the processing of boards and veneercontaining mixtures of wetwood and normalwood. As was pointed out for white fir(147) and western hemlock (110), boardscontaining wood of a mixed drying sortpresent additional problems for drying andplaning which have been ignored. Greenboards and veneer containing mostly wetwood,if properly presorted, can be dried underspecial schedules. No data are presentlyavailable, however, for delineatingcharacteristics of mixed wood sorts thatwould properly identify and permitpresorting these products for drying.Techniques for drying mixed wood boards andveneer for optimum yields and energy con-sumption must be considered a short-termneed of high priority.

The influence of wetwood on strengthproperties of lumber for light frameconstruction is not known. We can safelyassume, though, that wetwood probably has adetrimental effect because of the frequentoccurrence of deep surface checks,honeycomb, and shake in the dried lumberthat reduces the resistance to shear.Shake is closely limited in stress-gradedlumber for construction, particularly inmembers subject to bending (194). The datain table 7 suggest that wetwood may causeparticleboard intended for structuralpurposes to have lower strength values thanboards made entirely from normal wood. Theimportance of investigating the influenceof wetwood in structural lumber and otherbuilding components cannot beoveremphasized. According to Suddarth(184), the light-frame building is thepredominant form of constructionworldwide. Consequently, technologicalgains in light-frame construction canvitally affect large segments of theworld's population.

Long-Term

The objectives of a long-term researchprogram should be mainly concerned withattacking the causes of wetwood. Additionalbasic research aimed at reducing energy andwood losses during the processing of wetwoodmay also be needed. Specific plans forlong-term research will depend somewhat onthe initial information derived fromshort-term research, together withrecommendations from workers in the fieldsof forest biology and timber management.

38

Knowledge of specific patterns and locationof wetwood in standing trees gained largelyfrom short-term research will provide thebasis for planning long-term investigationsof the causes of wetwood. The role ofmicro-organisms, especially anaerobicbacteria, need to be investigated. To bemeaningful, though, research on microbiologymust be coordinated with investigations ofother possible causes. There is a need toinvestigate possible contributions by thefollowing agents: insects, fungi, mistletoe,oxygen and water stress, fire, andmechanical wounds associated with timbermanagement and recreational uses of theforest.

Identification and biochemical character-ization of the bacterial populationsassociated with wetwood can help to solvecurrent wood processing problems and providea basis for future control of wetwoodformation in trees. The biochemicalcontribution of pectolytic andphenyloxidizing bacteria to shake and frostcracks in trees and to surface checking,honeycomb, collapse, ring failure, andchemical brown stain in wood products needsto be more thoroughly defined and related tothe initial formation of wetwood.

It is important to explore the possibilitythat bacteria in wetwood may enhance itssubsequent decay by fungi if anaerobicconditions are lost, such as by frostcracks. Hartley et al. (79) noted thatfungal wood decay can be accelerated by thepresence of bacteria but did not considerthe possibility that wetwood bacteria mayprovide a source of nitrogen and vitaminsfor fungal growth. Nitrogen-fixing bacteriahave been considered in more recentpapers29/ (3, 4, 22). Nitrogen fixationby bacteria has even been associated withsporophores of decay fungi on westernhemlock trees (118). In addition, Bourchier(25) found thatbacteria from wetwood canproduce vitamins which enhance the growth ofwood-decaying fungi.

Studies of bacterial populations in wetwoodmust also be concerned with distinguishinginocuous plant and soil bacteria frompathogenic and fecal coliforms by serology,cell wall analysis, and deoxyribonucleicacid homology studies.30/ The presence ofcoliform bacteria in living wood tissues(13) can be a problem in some uses of woodproducts, such as redwood for water storagetanks (166).

29/S. D. Spano, M. F. Jurgensen, M. J.Larsen, and A. E. Harvey. 1978. Nitrogenfixation in decaying Douglas fir residue.Paper presented at 70th Annual Meeting ofthe American Society of Agronomy, 11 p.Madison, Wis.30/J. G. Zeikus, University of Wisconsin,Department of Bacteriology, Madison,personal communication to J. C. Ward.

39

Once the causes of wetwood are understood,there are two methods for control: directand indirect. Long-term research shouldevaluate the relative merits of eachmethod. In the practice of forestry, directcontrol of tree diseases and insect injuriesis generally not as practical or economicalas indirect measures of control. Directmethods, such as chemical spraying, are morepractical with agricultural crops than withlong-term timber crops which have a lowervalue per unit of ground. Still, directmeasures of control have been successfullyused in forest management and must beconsidered for preventing wetwood. Treespecies susceptible to wetwood during earlystages of growth may require aerial sprayingand soil fumigation. Special thinningmethods and fertilization may also be neededin these young-growth stands, but wetwood issometimes reported to be more prevalent indominant, fast-growing trees than insuppressed or slow-growing trees (45, 59,181).

The rotation age for sawtimber could bereduced for species that develop wetwoodwith advancing age. When guidelines aredeveloped for this type of control, it isalso necessary to determine the influenceof site factors and genetic makeup of thetree species on its resistance orsusceptibility to wetwood with aging. Ifwetwood develops in young trees beforesawtimber size is reached, direct controlmeasures could be aided by research on theutilization and processing of the timber.Pole-size or presawtimber stands might beconverted into products of higher unitvalue than the present end uses for pulpand paper. The development of moreefficient drying and machining methodscould even allow young trees with wetwoodto profitably grow to sawtimber size.

With current economic conditions, the goalof timber managers must be to growsawtimber that will not develop wetwoodbefore financial maturity. Indirect orpreventive control measures necessary toachieve this goal can be obtained fromforest biology and tree genetics research.This research must correlate the mechanismsof genetic resistance to wetwood formationwith the influences of site and culturalpractices. Benefits from a long-termresearch program could include:

1. Determination or development of treesthat are genetically resistant to formationof wetwood.

2. Determination of growing conditionsthat will prevent or minimize formation ofwetwood in susceptible tree species beforethe trees reach financial maturity forsawtimber.

3. Developmentpractices thatfor wetwood toregeneration.

of cultural and harvestingwill minimize the tendencyform in residual trees or in

40

Conclusion Literature Cited

From this overview, it is apparent thatwetwood is responsible for substantiallosses of wood, energy, and productionexpenditures in the forest productsindustry. These losses result from a rawmaterial anomaly that in the past haseither not been recognized or has beenignored. Though there is a definite needto assess more thoroughly these losses andto find ways of eliminating or minimizingthem, there is also a need to examine thecause; i.e., wetwood. The fact that moreand more problems in utilization andprocessing of timber are recognized asrelated directly or indirectly to wetwoodclearly demonstrates the importance of thisphenomenon.

An effective program of research is neededto determine the significance of wetwood inthese problems. This program should planand provide for obtaining both short- andlong-term solutions to the overallproblem. Because of increasing interest inwetwood, excellent opportunities now existto initiate such a comprehensive researchprogram.

There can be little doubt that the effectsof wetwood on the forest products industryare real, and the impact is far reaching.With dwindling timber supplies, increasingcosts of available energy, and a scarcityof low cost capital, the timber industrycan no longer afford to ignore the problemsassociated with wetwood, for wetwood has animpact on all these issues.

It is time we examine, in depth, thedetrimental effects of wetwood during allphases of timber production--from the stumpto the finished product. Losses related towetwood, which in the past were accepted aspart of the package of doing business orthought to be the result of inadequateprocessing, can no longer be viewed in thatcontext. A fuller understanding of thecharacteristics of wetwood, whether in thetree, log, or product, will provide timbermanagers and processors with the tools tohandle the wetwood problem.

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