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The effect of tree dimension on the diversity ofbark microhabitat structures and bark use inDouglas-fir (Pseudotsuga menziesii var. menziesii)
Alexa K. Michel, Susanne Winter, and Andreas Linde
Abstract: The focus of this study was to investigate the role of tree dimension and associated bark structures for highstructural complexity and high natural biodiversity in forest ecosystems. Two-hundred and ninety-one Douglas-fir (Pseu-dotsuga menziesii var. menziesii (Mirbel) Franco) trees in two regions of the US Pacific Northwest were investigated forthe relationship between tree diameter and bark thickness (measured as bark fissure depth) and the relationships of both tobark microhabitats and signs of bark use. Our results emphasize the habitat function of tree bark of large-diameterDouglas-fir trees. Many bark microhabitat types and their total abundance significantly increased with increasing tree di-ameter and bark thickness. These were bark pockets with and without decaying substrate, bowls in the bark, and signs ofbark use, e.g., small holes from woodpecker drillings and large insects, large bark excavations from woodpeckers, spiderfunnel webs, natural cavities at the stem base without decay, and the occurrence of herb vegetation at the tree base. Inforest monitoring, tree diameter may be a good indicator of the number of bark microhabitats and of bark thicknessbecause it is strongly related to both of these variables. However, because of the high variability of bark thickness inlarge-diameter trees, we suggest monitoring bark fissure depth if an ecological evaluation of Douglas-fir forests is needed.
Resume : Cette etude porte principalement sur le role que jouent la taille des arbres et les structures de leur ecorce dans lagrande complexite structurale et la grande biodiversite naturelle des ecosystemes forestiers. La relation entre le diametre desarbres et l’epaisseur de l’ecorce (mesuree par la profondeur des fissures de l’ecorce), ainsi que les relations de l’un et l’autreavec les microhabitats de l’ecorce et les signes d’utilisation de l’ecorce, ont ete etudiees chez 291 tiges de douglas de Men-zies (Pseudotsuga menziesii var. menziesii (Mirbel) Franco) dans deux regions du Pacific Northwest, aux Etats-Unis. Nos re-sultats font ressortir la fonction d’habitat de l’ecorce des tiges de douglas de Menzies de fort diametre. Plusieurs types demicrohabitats de l’ecorce et leur abondance totale augmentent significativement avec le diametre des arbres et l’epaisseur del’ecorce. Il s’agissait d’ecorce incluse avec ou sans substrat en decomposition, de creux dans l’ecorce et de signes d’utili-sation de l’ecorce comme de petits trous d’alimentation creuses par les pics et de gros insectes, d’importantes cavites creu-sees par les pics, des toiles d’araignee en forme d’entonnoir, des cavites naturelles exemptes de carie a la base du tronc et lapresence de vegetation herbacee a la base du tronc. Pour la surveillance des forets, le diametre des arbres peut etre un bonindicateur du nombre de microhabitats de l’ecorce et de l’epaisseur de l’ecorce parce qu’il est etroitement relie a ces deuxvariables. Cependant, etant donne la grande variabilite de l’epaisseur de l’ecorce, nous recommandons d’examiner la profon-deur des fissures de l’ecorce si une evaluation ecologique des forets de douglas de Menzies est necessaire.
[Traduit par la Redaction]
Introduction
Tree bark and, more specifically, the dead outer tissuesthat accumulate during the secondary thickening of treesprovide protection of the inner living tissues against climaticeffects, air pollution, biotic agents, and mechanical damage.All tree species have a species-specific bark developmentand can be visually distinguished by their characteristicbark pattern, texture, and color (Harlow et al. 1979). For ex-
ample, Douglas-fir (Pseudotsuga menziesii var. menziesii(Mirbel) Franco) develops a thick layer of nondefoliatingand deeply furrowed bark (Fig. 1) that functions as an im-portant protection against damage from natural wildfires intheir natural range in the US Pacific Northwest. For this rea-son, many old Douglas-fir trees survive even large wildfiresand are retained as stand legacies at the beginning of a newforest succession cycle in these plant associations (Franklinet al. 2002).
Received 15 January 2010. Accepted 19 October 2010. Published on the NRC Research Press Web site at cjfr.nrc.ca on 18 January 2011.
A.K. Michel1,2 and A. Linde. University of Applied Sciences Eberswalde, Faculty of Forest and Environment, A.-Moller-Str. 1, D-16225 Eberswalde, Germany.S. Winter.3 Technische Universitat Munchen, School of Forest Science and Resource Management, Department of Ecology andEcosystem Sciences, Hans-Carl-von-Carlowitz-Platz 2, D-85354 Freising, Germany.
1Corresponding author (e-mail: [email protected]).2Current affiliation: Botanic Garden and Botanical Museum Berlin–Dahlem, Freie Universitat Berlin, Konigin-Luise-Str. 6-8, D-14195Berlin, Germany.3Corresponding author (e-mail: [email protected]).
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Can. J. For. Res. 41: 300–308 (2011) doi:10.1139/X10-207 Published by NRC Research Press
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The thick layer of bark not only protects the tree itself butprovides many habitat functions and is used in several waysby different organism groups. Important factors are thechemistry and structure of the bark as they determine, forexample, the cover of bryophytes and lichens on several de-ciduous tree species (Kuusinen 1994; Frahm 2001). Barkstructures such as the large bark pockets of Douglas-fir areused by bats as roosting sites, e.g., by the silver-haired bat(Lasionycteris noctivagans) (Sondenaa 1991) and the Cali-fornia myotis (Myotis californicus) (Vonhof and Gwilliam2007), and by Brown Creepers (Certhia americana) as nest-ing sites and foraging substrate (Parks et al. 1997; Poulin etal. 2008). If they contain decayed substrate or litter, thesebark pockets with decay and bowls in the bark with freshresidue are also important for many bark-dwelling inverte-brate species (Winter and Moller 2008) on which woodpeck-ers forage (McClelland et al. 1979; Bull et al. 1997; Parks etal. 1997). Deep bark fissures have been found to be predic-tive for the presence of the endangered saproxylic longhornbeetle Cerambyx cerdo in oak (Buse et al. 2007) and otherbark-dwelling invertebrates, including arboreal spiders thatplay an important role in the regulation of insect pest popu-lations, e.g., the Douglas-fir tussock moth (Orgyia pseudot-sugata) (Jackson 1979; Fichter 1984; Mariani and Manuwal(1990) cited in Poulin et al. 2008). In a German study, thespider Cryphoeca silvicola even seemed to prefer the highlystructured bark of nonnative Douglas-fir compared withother investigated native tree species (Engel 2001). Heavyresinosis and partial bark loss are indicators of beginningtree death or internal decay. Depending on the decay type
(internal decay in the heartwood, external decay in the sap-wood, or root diseases), a tree will provide characteristictree and bark structures over time on which other speciesdepend (Parks and Shaw 1996; Parks et al. 1997). Douglas-fir bark can also be retained for a long time on snags andlogs, providing habitat even after tree death (Parks et al.1997). For a comprehensive literature review on the depend-ence of many more species on single-tree structures inDouglas-fir dominated forests and the importance of thesestructural attributes in forest conservation, please refer toMichel and Winter (2009).
Although studies have described forest structures associ-ated with later stand development stages, e.g., large livetrees, snags, stumps, logs, large dead branches, and decayedsapwood and heartwood, in mature or old-growth forests andtheir use as primary or secondary habitat by many vertebratespecies, including amphibians, reptiles, birds, mammals, andinvertebrates (Maser et al. 1979; Bruce et al. 1985a, 1985b;Harmon et al. 1986; Blaustein et al. 1995; Bull et al. 1997;Parks et al. 1997), we found no research that specifically ad-dresses the quantity of various bark structures on a single-tree basis and its habitat function in living Douglas-fir trees.Some publications, however, have already begun to discussthe importance of tree bark for forest biodiversity and themaintenance of large deciduous trees in forest conservation(Ranius and Jansson 2000; Gotmark 2009). Bark thicknessand tree diameter were highly correlated in European oak(Quercus robur and Quercus petraea; Buse et al. 2007), andwe had hypothesized the same relationship for Douglas-fir.Thus, the objective of our study was to investigate the effectof tree size and bark thickness on the diversity of bark mi-crohabitat structures and bark use in Douglas-fir to deter-mine the role of large-diameter trees on structuralcomplexity and biodiversity in forest ecosystems.
DataAll research stands were Douglas-fir dominated and lo-
cated in the H.J. Andrews Experimental Forest (HJA)(44815’N, 122810’W) in the Southern Cascade Range and inthe McDonald-Dunn Forest (McD) (44837’N, 123819’W)near Corvallis in the Willamette Valley in Oregon, USA. AtHJA, mean annual temperature is 8.5 8C and mean annualprecipitation is 2300 mm. At McD, mean annual tempera-ture is 11 8C and mean annual precipitation is 1100 mm. Inboth regions, precipitation occurs primarily (75%–85%) asrain between October and March (Franklin and Dyrness1973). One-hundred and sixty-one trees in 10 unmanagednatural stands at HJA and 130 trees in seven managed standsat McD were investigated. All investigated trees were liveDouglas-firs and selected from a database for tree data fromlong-term Douglas-fir research and monitoring stands (Mi-chel and Winter 2009). Trees from this database were div-ided into diameter classes of 20 cm width, and in eachclass, 20 trees were randomly selected by using a randomnumber generator in SPSS 15.0 (SPSS Inc. 2004). If thenumber of trees per class was less than 20, all trees in thatclass were selected. This resulted in 20–21 trees per class atHJA and 14–19 trees per class at McD. Diameter at breastheight (dbh, i.e., 1.37 m) of the investigated trees rangedfrom 3 to 212 cm at HJA and from 1 to 275 cm at McD.
Fig. 1. Douglas-fir (Pseudotsuga menziesii var. menziesii) withlarge bark excavations.
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Two kinds of tree circumference were recorded: (i) simpletree circumference and (ii) bark contour circumference. Sim-ple tree circumference was measured with a circumferencetape at 1.37 m height fixed at the upper slope side of thetree. Bark contour circumference was measured with a tapearound the tree including the circumference increase causedby bark fissures, bark pockets, and bark excavations up to adepth of 1.2 cm fissure width (one finger thickness). Barkfissure depth was measured with a ruler, and the deepest fis-sure at 1.37 m was used for analyzing the relationship be-tween dbh and bark fissure depth and width. Other barkmicrohabitat variables and signs of bark use were recordedas counts and given a level of intensity (Table 1) used forthe c2 test of independence and homogeneity of differentmicrohabitats and signs of bark use and diameter classes.Counts were taken after surrounding the tree and visuallyexamining the stem base for microhabitats and signs of barkuse, with the exception of woodpecker holes. These weresearched for all along the tree stem using binoculars.
MethodsTree dbh was calculated from simple tree circumference
according to the formula dbh = circumference/p. Diameterclasses were defined as, e.g., class ‘‘20’’ equals trees with adbh between 20.1 and 40.0 cm. Because of the limitedamount of trees in the highest diameter classes, all treeswith a diameter >140 cm were combined in the diameterclass ‘‘140’’. Bark fissure depth classes were defined simi-larly. The class ‘‘2’’ represents trees on which the deepestbark fissure was between ‡2 and <4 cm deep. All trees witha bark fissure depth of ‡14 cm were combined in the highestbark fissure class ‘‘14’’.
If not stated otherwise, the significance level in all analy-ses was set at a = 0.05. All correlation analyses, regressionanalyses, and the Mann–Whitney U test were performed inSPSS 15.0 (SPSS Inc. 2004). Chi-square tests for independ-ence and homogeneity (Ramsey and Schafer 1996) were cal-culated in MS Excel.
The relationship between simple tree circumference andbark contour circumference, i.e., the curvilinear increase intree circumference caused by bark structure, was estimatedwith a quadratic polynomial regression after checking theassumptions of independence, normality, and equal variance.The x variable was centered around the mean to reduce mul-ticollinearity. No significant difference was found betweensites (p value > 0.05, from a quadratic polynomial regres-sion), and the site variable was removed in the final regres-sion analysis. Both variables in the regression equation (i.e.,simple tree circumference and bark contour circumference)were not controlled but random; therefore, a model II re-gression was applied instead of a model I regression. In ourcase, a model I regression would have underestimated theslope of the linear relationship between the two variables asthey both may contain error. ‘‘Techniques for handlingmodel II cases have not been developed’’ for multiple re-gression problems (Sokal and Rohlf 2000, p. 610); therefore,caution in interpreting the results is advised, and the coeffi-cients of this part of the analysis were only used for predic-tion.
The relationships between tree dbh and bark fissure depth
and width, respectively, were back-transformed and pre-dicted as the median on the original scale after first log-transforming (ln) both the explanatory and response varia-bles (Neter et al. 1990, p. 146). The association of the depthand width of the deepest bark fissure recorded was describedwith the Spearman rank correlation coefficient r after viola-tions of the assumptions of homoscedasticity had been de-tected. Relationships of bark microhabitat structures andsigns of bark use of a tree with dbh class were tested usingc2 tests for independence and homogeneity (Ramsey andSchafer 1996).
ResultsAbundance of microhabitats and signs of bark use steadily
increase with bark thickness, measured as bark fissure depth,and with tree dbh (Fig. 2). Beginning at a bark fissure depthof around 6 cm, the average abundance stays at around 6–8microhabitats or signs of bark use per tree at both sites. Be-ginning at a dbh of 120 cm, the abundance of different mi-crohabitats and signs of bark use is, on average, around sixper tree. The abundance of microhabitats for the highestmeasured bark fissure depth class and the highest diameterclass level off at different abundances, which means thatnot all of the trees with a large dbh also developed verydeep bark fissures.
Single microhabitats such as the level of abundance ofspider funnel webs, bark pockets, pockets with decay, barkbowls, signs of bark use, natural cavities at the base of atree (only at McD), resinosis (only at HJA), natural regener-ation (only at HJA), and herbal vegetation also increase sig-nificantly with dbh, i.e., their occurrence is not independentand not homogenous across dbh classes (p < 0.001, from ac2 test for independence and homogeneity) (Ramsey andSchafer 1996) (Fig. 3; Table 2). Signs of wood use wereonly found on two trees, and natural cavities with decaywere only found on three trees and do not depend on treedbh class in this study.
Both tree dbh and bark thickness are significantly relatedin that we found a strong curvilinear relationship betweentree dbh and bark fissure depth and width (Fig. 4). The ef-fect of dbh on bark fissure depth and bark fissure width wassignificantly different at the two investigated sites (p <0.001, from a Mann–Whitney U test). A doubling of thedbh is associated with an approximate 2.5-fold (21.32-fold)increase in median fissure depth for HJA and an approxi-mate 2.8-fold (21.49-fold) increase for McD. Regarding barkfissure width, a doubling of the dbh is associated with an ap-proximate 2.3-fold (21.18-fold) increase in median fissurewidth for HJA and an approximate 2.6-fold (21.35-fold) in-crease for McD. Both bark fissure depth and width, how-ever, become increasingly variable in large-diameter trees.For example, bark fissure depth of trees around 180 cm dbhranged from 13 to 21 cm, and a bark fissure depth of 7 cmwas found on trees with a diameter between 78 and 172 cm.
On trees that have very pronounced bark, bark fissuressignificantly increase the circumference of a tree. In ourstudy, this was represented by the significant relationshipbetween bark contour circumference (tree circumference in-cluding the contour gain by bark fissures and bark excava-tions) and simple tree circumference. To quantify the extent
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to which the depth and width of bark fissures lead to a morecomplex bark structure, predictions of the bark contour cir-cumference from simple tree circumference can be madefor the trees in our study. The average bark contour circum-ference of trees with a dbh of 20 cm, for example, was 11%larger (62.8 cm compared with 69.8 cm) than their simplecontour circumference (Fig. 5). Of trees with a dbh of80 cm, the average bark contour circumference was 20%larger (251.3 cm compared with 302.8 cm), and of treeswith a dbh of 200 cm, it was 39.3% larger (628.3 cm com-pared with 875.4 cm) than the recorded simple tree circum-ference. This relationship was curvilinear and did not differat the two study sites (HJA and McD) (p > 0.05, from aquadratic polynomial regression analysis).
The highest recorded depth of a bark fissure in this studywas 28 cm, and the highest width was 58 cm. Bark fissuredepth and width were strongly correlated on the investigatedtrees on both study sites (Spearman’s r = 0.88 at HJA andr = 0.94 at McD, both p < 0.001, significant site difference
from Mann–Whitney U test (p < 0.001)) (Fig. 6). However,the spread in bark fissure width increases substantially withincreasing bark fissure depth so that, for example, the widthof an 11 cm deep bark fissure ranged from 7 to 32 cm.
Discussion
Abundance and species diversity often increase with theavailability of forest structures and, more specifically, sin-gle-tree microhabitat structures, which are needed by numer-ous organisms for breeding, feeding, and resting (shelter)(Bruce et al. 1985a, 1985b) The monitoring of these struc-tures can be regarded as a meaningful alternative to thecostly determination of single species from various organismgroups because the occurrence of microhabitat structures ishighly correlated with the abundance of many forest speciesand ecosystem functions (Thomas et al. 1979; Harmon et al.1986; Bull et al. 1997; Remm et al. 2006; Parsons et al.2003). Especially in highly structured and complex forests
Table 1. Investigated bark microhabitats and signs of bark use.
Bark microhabitat or sign of bark use Details
Number of funnel webs from arboreal spiders No. of spider funnel webs on lower 2 m of the stem0: none1: 1 to 52: 6 to 103: more than 10
Bark use Presence or absence of, e.g., insects borings, small holes from woodpeckerexcavations in bark on lower 2 m of the stem
0: none1: £2.0 cm in diameter2: 2.1–4.0 cm in diameter3: 4.1–10.0 cm in diameter4: >10.0 cm in diameter
Wood use Presence or absence of, e.g., woodpecker excavations in wood on lower 2 m ofthe stem
0: None1: £2.0 cm in diameter2: 2.1–4.0 cm in diameter3: 4.1–10.0 cm in diameter4: >10.0 cm in diameter
Natural cavity at stem base without decay Natural cavity at the base of the tree stem without decay0: None1: 10.0–20.0 cm diameter of cavity entrance2: >20.0 cm diameter of cavity entrance
Natural cavity at stem base with decay Definition of natural cavity see above, additionally filled with decayingsubstrate
0: None1: 10–20 cm diameter of cavity entrance2: >20 cm diameter of cavity entrance
Resinosis 0: None1: Heavy flow of resin of at least 30 cm length or >5 flows of resin of smaller
size on tree stemBark pockets Presence or absence of space between loose bark at least 5 cm wide and 2 cm
deep on lower 2 m of the stemBark pockets with decay Same as above but with decaying substrate on lower 2 m of the stemBark bowls Presence or absence of bowl structure in bark at least 5 cm wide collecting litter
on lower 2 m of the stemNatural regeneration Presence or absence of tree regeneration on bark on lower 2 m of the stemHerbal vegetation Presence or absence of herbs on bark on lower 2 m of the stem
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Fig. 2. Average number (±SD) of microhabitats and signs of bark use of Douglas-fir (Pseudotsuga menziesii var. menziesii) trees of differ-ent bark fissure depth classes (left) and diameter (dbh) classes (right). HJA, H.J. Andrews Experimental Forest; McD, McDonald-DunnForest.
Fig. 3. Percentage of all investigated Douglas-fir (Pseudotsuga menziesii var. menziesii) trees from different tree diameter (dbh) classes withselected microhabitats or signs of bark use at H.J. Andrews Experimental Forest (HJA) and McDonald-Dunn Forest (McD).
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such as in the Pacific Northwest, biodiversity is expressedthrough organisms that greatly depend on small-scale foreststructures and, moreover, generally are not easily observed,e.g., fungi (Smith et al. 2002), lichens (McCune et al.2000), bats (Thomas 1988), and invertebrates (Schowalter1995; Addison et al. 2003).
In this study, all variables that describe Douglas-fir barkstructure (bark contour circumference, bark fissure depthand width) significantly increase with dbh up to the highest
measured diameter of 275 cm. Total abundance and the sin-gle occurrence of most microhabitats and signs of bark usealso increase with dbh and bark thickness, but both leveloff at about six to eight microhabitats. Therefore, more po-tential habitat for, e.g., bark-dwelling insects and foragingbirds is provided with increasing tree size up to a certainlevel. As a consequence, microhabitats and bark use and itsassociated function as an ecological niche can be inferredfrom tree dbh recordings. However, although dbh is a more
Table 2. Chi-square test of independence and homogeneity of different intensities of microhabitats andsigns of bark use and of tree diameter classes.
HJA McD
Bark microhabitat or sign ofbark use c2 df
One-sidedp value c2 df
One-sidedp value
Bark use 173.84 28 <0.001 199.71 28 <0.001Wood use 6.13 28 1 0 28 1Natural cavitiy at stem base
without decay13.34 14 0.500 40.80 14 <0.001
Natural cavitiy at stem basewith decay
6.13 14 0.963 5.89 14 0.969
Resinosis 15.93 7 0.026 6.05 7 0.534Bark pockets 105.43 7 <0.001 81.06 7 <0.001Bark pockets with decay 99.92 7 <0.001 81.05 7 <0.001Bark bowls 96.19 7 <0.001 95.02 7 <0.001Natural regeneration on bark at
tree base51.51 7 <0.001 8.35 7 0.303
Herb occurrence 34.53 7 <0.001 33.48 7 <0.001Number of spider funnel webs 143.73 21 <0.001 177.36 21 <0.001
Note: Moss and lichen occurred on all investigated trees and were excluded from the analysis. HJA, H.J. AndrewsExperimental Forest; McD, McDonald-Dunn Forest.
Fig. 4. (A) Association between the deepest measured bark fissure and tree diameter at breast height (dbh) at H.J. Andrews ExperimentalForest (HJA) and McDonald-Dunn Forest (McD). (B) Log–log scatterplot of depth of deepest bark fissure of a tree versus tree dbh, alongwith the estimated line for the regression of log fissure depth on log dbh. HJA: m{ln(bark fissure depth)j ln(dbh)} = –4.238 + 1.318 ln(dbh),R2 = 0.92. McD: m{ln(bark fissure depth)j ln(dbh)} = –5.044 + 1.486 ln(dbh), R2 = 0.92. (C) Association between bark fissure width of thedeepest measured bark fissure and tree dbh at HJA and McD. (D) Log–log scatterplot of width of deepest measured bark fissure versus treedbh, along with the estimated lines for the regression of log fissure width on log dbh. HJA: m{ln(bark fissure width)j ln(dbh)} = –3.6868 +1.1762 ln(dbh), R2 = 0.78. McD: m{ln(bark fissure width)j ln(dbh)} = –4.2168 + –4.2168 ln(dbh), R2 = 0.85.
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common measurement and a relatively reliable indicator forthe abundance of microhabitats, large-diameter trees show ahigh variability of bark thickness. As bark thickness alsostrongly corresponds to the abundance of microhabitats andsigns of bark use, we suggest recording bark fissure depthinstead of tree dbh in studies that specifically aim at describ-ing the ecological role of Douglas-fir forests.
Abundance of many of the investigated bark microhabi-tats, i.e., bark pockets with and without decay, bark bowls,and bark use by insects and birds, increased significantlywith increasing tree size and age at both investigated sites.If a tree is beginning to decay, it will be used heavily bywoodpeckers for nesting, foraging in the bark, and foragingin the interior for carpenter ants, which often colonize de-caying wood at the base of a living tree (McClelland et al.
1979; Bull et al. 1997; Parks et al. 1997). Resinosis was sig-nificantly more abundant in larger trees only at HJA, possi-bly because of the absence of tree removal of trees withstem decay or wounds in the natural stands at HJA com-pared with the managed stands at McD. In managed stands,in general, the removal of old decadent trees has been a tra-ditional forest practice. Excavations in the sapwood or heart-wood were very uncommon in the investigated live trees inour study, which may support the finding that these micro-habitats primarily occur on Douglas-fir snags and logs (Bullet al. 1997). An additional study, therefore, may be neededto study the effect of dbh on these rare structures in livetrees. Hollow chambers in the butt of a tree that can beused for cover by small animals, for roosting by bats, andas den sites by black bears (Ursus americanus) (Noble et al.1990; Parks et al. 1997) and stem cavities in a living treewith decay substrate (mould) that are important for inverte-brates and secondary cavity nesters (Martin et al. 2004;Remm et al. 2006; Winter and Moller 2008) were also rarein the live trees in this study. Large-diameter trees facili-tated the establishment of herbal vegetation and natural treeregeneration on their bark. Bark structure at the base of atree, therefore, may provide an important ecological nichein Douglas-fir stands with competing understory vegetation,as has been found for Western hemlock (Tsuga hetero-phylla) regeneration on logs (Harmon and Franklin 1989).However, this relationship was found only at one of thesites. Lichen and bryophytes were found on all investigatedtrees, presumably because of the high level of precipitationin this region.
Although bark thickness has been described to reach asmuch as 60 cm on old trunks of Douglas-fir (Harlow et al.1979), bark fissures on trees in our study were only up to28 cm deep. The recorded variable bark fissure depth isonly an indication for bark thickness as only the depth offissures but not the total depth of the bark up to thecambium layer is measured. However, although tree bark inour study may not have been as developed as in other inves-tigations, a steady increase in the abundance of microhabi-tats and signs of bark use was already found in trees up tothe highest diameter class of this study.
In general, structural attributes at the tree level, especiallyof the stem and crown, such as woodpecker excavations,broken tops, etc., increase with tree size and age (Spies etal. 2002; Winter and Moller 2008), and this applies to barkthickness and the development of bark microhabitats inDouglas-fir as well. The results of this study emphasize thehabitat function of large-diameter Douglas-fir trees and theirimportance for a high structural complexity and natural bio-diversity in Douglas-fir forests in the Pacific Northwest. Asa consequence, we suggest the preservation of suitable forestlands for forest conservation and the adaptation of forestpractices on managed lands that enhance the developmentof large-diameter trees, i.e., increasing the rotation length,tree retention, and thinning to increase growth of the re-maining trees (DeBell et al. 1997), to maintain a high natu-ral biodiversity in these forests.
AcknowledgementsThe authors are grateful to the following colleagues: M.
Flade, D.L. Johnson, K. Keable, C. Mollnau, J. Moreau, K.
Fig. 5. Relation between simple tree circumference and bark con-tour circumference of the investigated Douglas-fir (Pseudotsugamenziesii var. menziesii) trees: y = –6.604 + 0.0239x + 0.0005x2 +3; R2 = 0.99. HJA, H.J. Andrews Experimental Forest; McD,McDonald-Dunn Forest; Sig., significance; Dep., dependent; dbh,diameter at breast height.
Fig. 6. Association between bark fissure depth and width at HJA(Spearman’s r = 0.88) and McD (r = 0.94). HJA, H.J. AndrewsExperimental Forest; McD, McDonald-Dunn Forest.
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O’Connell, R.J. Pabst, D. Pflugmacher, T. Sengewald, andT.A. Spies. The authors also thank two anonymous re-viewers for their helpful comments on the manuscript. Fund-ing was provided by the University of Applied Sciences inEberswalde and the Dr.-Ing. Leonhard-Lorenz-Stiftung atTechnische Universitat Munchen, Germany.
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