Effects of Acidic Depositionon Canada’s Forests

P. Hall, W. Bowers, and H. HirvonenNatural Resources Canada, Canadian Forest Service

Science BranchOttawa, Ontario

G. Hogan, N. Foster, and I. MorrisonNatural Resources Canada, Canadian Forest Service

Sault Ste. Marie, Ontario

K. Percy and R. CoxNatural Resources Canada, Canadian Forest Service

Fredericton, New Brunswick

P. ArpFaculty of Forestry and Environmental Management

University of New BrunswickFredericton, New Brunswick

Information Report ST-X-15

Published byScience Branch

Canadian Forest ServiceNatural Resources Canada

Ottawa, 1998

© Her Majesty the Queen in Right of Canada 1998Catalogue No. Fo29-33/15-1998ISBN 0-662-66384-9ISSN 1192-1064

Copies of this publication may be obtained free of charge from:Natural Resources CanadaCanadian Forest ServiceOttawa, Ontario K1A 0E4

Phone: (613) 947-7341

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Editing and Production: Paula IrvingLayout and Design: Danielle Monette

Canadian Cataloguing in Publication Data

Main entry under title:

Effects of acidic deposition on Canada’s forests

(Information report; ST-X-15)Text in English and French on inverted pages.Title on added t.p.: Effets des dépôts acides sur les forêts canadiennes.Includes bibliographical references.ISBN 0-662-66384-9Cat. no. Fo29-33/15-1998

1. Forest health—Canada.2. Forest conservation—Canada.3. Acid rain—Canada.4. Air—Pollution—Canada.5. Tree declines—Canada.I. Hall, J. Peter.II. Canadian Forest Service. Science Branch.III. Series: Information report (Canadian Forest Service. Science Branch); ST-X-15.

SD414.C3E43 1998 634.9’61’0971 C98-980044-XE

Cover: Photo courtesy of Canadian Forest Service,Great Lakes Forestry Centre, Sault Ste. Marie, Ontario.

Printed on alkalinepermanent paperPRINTED IN CANADA

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ContentsContents

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Acid Rain and Current Forest Decline . . . . . . . . . . . . . . . . . . . . . . . . 6

Effects of Acid Rain on Tree Physiology . . . . . . . . . . . . . . . . . . . . . . . 7

Effects of Acid Rain on Soil Chemistry . . . . . . . . . . . . . . . . . . . . . . . . 9

Results of Forest Health Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Critical Loads/Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

International Linkages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Research and Information Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Executive Summary

The maintenance of the health of Canada’s forest ecosys-tems is an important prerequisite to national and global forestsustainability. Remedial and preventative strategies formulatedto address forest health must consider as a key factor the poten-tial effects of atmospheric pollution, particularly acid rain, on for-ests. This assessment reviews findings, highlights current trends,provides direction to strengthen national programs aimed at address-ing acid rain issues, and focuses on the links between science andpolicy.

Evidence of Effects

The health of large portions of Canada’s forests is beingadversely affected by continuous exposure to a range of air pol-lutants. Air pollutants damage trees directly or influence eco-logical processes in a manner that impedes the development ofa healthy forest ecosystem. Both managed and unmanaged forestsare affected.

The range of effects may be minimal or severe dependingon the region of the country and on intensity and type of air pol-lutant. For example, in eastern Canada acid rain is extensive. Acidrain, used here as a generic term for all forms of precipitationincluding acid snow, acid fog, and acid vapor, damages the sur-faces of leaves and needles, reduces a tree’s ability to withstandcold, and inhibits germination of pollen. Consequently, tree vital-ity and regenerative capability are reduced. In western Canada,acid rain is less widespread and damaging pollutant levels arelocalized.

Forest Ecosystem Effects

Dry and wet acidic deposition alters the chemical andphysical characteristics of the leaf cuticle. The cuticle is a thin,waxy layer covering the leaf and needle surface of trees. It pro-tects the leaves and regulates many of the tree’s functions. Alter-ation of the cuticle structure may result in acceleration of the naturalageing process and reduced tree vigor. The ability of the tree tocope with other stressors such as drought, insects and diseases,and increased ultraviolet radiation is reduced.

Gaseous pollutants cause a decrease in net photosynthe-sis and nutrient uptake in mature red spruce (Picea rubensSarg.) trees; this effect increases with absorption of sulfate.Other studies with lodgepole pine (Pinus contorta Dougl. ex Loud.var. latifolia Engelm.) and trembling aspen (Populus tremuloidesMichx.) near point sources of pollution showed more recentlydead trees in areas of high pollution than in areas of less pollu-tion. Volume increments also decreased. These results may beprecursors of long-term trends in stand vigor attributable to con-tinued pollution at the regional scale.

Acid fog or mist, with a pH below 5.6, prevents germi-nation of pollen in white birch (Betula papyrifera Marsh.) andmountain paper birch (B. cordifolia Regel). Acid fogs are com-mon along the Bay of Fundy where these trees are most affected.Dieback within these species in this region is also extensive.

Acid mists also reduce a tree’s frost hardiness. Studiesassociated with acidic fogs that regularly blanket high eleva-tions of the Appalachian and Laurentian mountains show thata direct relationship exists between the amount of sulfate in theleaves and the ability of a tree to withstand cold temperatures.For example, in red spruce a 0.1% increase in sulfur in leavescauses a 2.7°C decline in frost hardiness. As pollution continues,trees can be expected to become more vulnerable to climaticperturbations.

Red spruce decline at high elevations has been stronglylinked to increases in aluminum/calcium ratio of woody tis-sues, and to increases in respiration. The mobilization of alumi-num through soil acidification impedes the uptake and transportof base cations by trees.

A continuing soil nutrient decline, due to acid rain, is occur-ring in certain forest ecosystems. In Ontario, ambient levels ofacid rain have accelerated the loss of base cations from soils thatsupport sugar maple-dominated (Acer saccharum L.) hardwoodforest. Studies in Quebec indicate that the nutrient status of sugarmaple seedlings declines as soil acidification increases and soilbase saturation decreases. These effects likely will be sustainedor increased at current deposition levels resulting in a long-termdecline in forest ecosystem productivity.

Identifying Areas at Risk

Eastern Canada has extensive areas where sulfate andnitrate deposition exceed the critical load. These areas reflectforest ecosystems sensitive to acid rain. Critical load values pro-vide useful information about sulfate and nitrate deposition thatmay lead to long-term damage of soils and forests. Preliminarymodeling, based on the Acid Rain National Early Warning Systemof plots (ARNEWS), suggests that exceedances of critical loadsof 1000 eq/ (ha•yr) would result in a 6–10% increase in mortal-ity over a period of 11 yr. Current exceedances are lower than thisamount, but reach well above 800 eq/(ha•yr) for portions of south-ern Ontario.

Areas with forest health problems coincide with areaswhere the annual rate of sulfate deposition exceeds the crit-ical load. Comparisons of maps outlining high exceedances ofsoil acidification with maps depicting symptoms of forest declineillustrate this relationship. For example, dieback of sugar maple ineastern Canada and the United States is higher in areas of exceed-ance than in areas of nonexceedance. In one study, through regres-sion analysis, the rate of forest decline increased about 40% foran exceedance of 1000 eq/(ha•yr).

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Initial signs of damage from acidic deposition areevident in forest ecosystems characteristic of a harsh climateand soil conditions, such as those ecosystems representativeof nutrient-poor soils or of the northern edge of the growing rangeof tree species. Given that the effects of continuous sulfate andnitrate deposition are cumulative, decline in forest health may bemore widespread in stressed ecosystems than currently documented.

Next Steps

Further analysis of sulfate and nitrate deposition isrequired including enhanced modeling of critical loads andrelated exceedance. If deposition continues to exceed criticalloads, it poses a serious threat to the sustainability of forests inOntario and large portions of the commercial forest in Quebec,Nova Scotia, and New Brunswick. In some areas, reductions ofeither sulfate or nitrate deposition will minimize the threat. In otherareas, both sulfate and nitrate deposition need to be reduced.

Develop critical levels for pollutants that trees could tol-erate without short-term and permanent damage. The UN-ECEhas established critical levels for several pollutants, developed fora few sensitive tree species. Some regional work is underwayin Canada. This effort needs to be expanded to include the majorCanadian species under threat from acid rain.

Critical levels and areas of exceedance need to be mappedfor forest ecosystems that are at high risk from acid rain, forexample, in regions of frequent acid fog or clouds. Models thatrelate concentrations of pollutants in fog or clouds to emissionsneed to be developed.

Status and trend analysis of the health of Canada’s forestsrequire inclusion of a broad spectrum of ecological attributesof forest ecosystems. Much research to date has been on effectsof acid rain on forest trees and soils. Fauna and other flora, sur-face and subsurface waters among other attributes need to beincorporated.

A comprehensive approach to assessment of forest healththat links stressors and their cumulative effects is needed. Acidrain must be considered and assessed together with other stressorssuch as UV-B, toxic chemicals, and climate change. Such inte-grated assessment is key to determination of forest health.

Continued networks of baseline research and monitoringare essential to develop, enhance, and monitor key indicatorsof forest health and to facilitate extrapolation of research resultsto regional and national scales. Strong links need to be main-tained between ecological science, monitoring, and assessment.

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Introduction

Canada is custodian to 10% of the world’s forest resource. Thenation’s forests encompass 42% of our land area or 417.6 mil-lion ha. They are an integral component of Canada’s ecologicalmosaic. Forests continually respond to a combination of anthro-pogenic changes (that is, air pollution, climate change, enhancedUV-B radiation) and natural fluxes in the atmospheric environ-ment. Public interest in the health of Canadian forests is high andthere is a lingering concern about anthropogenic influences onthis health. The effects of air pollutants, particularly acid rain, areamong the major issues. The term “acid rain” includes acidic pre-cipitation in the form of rain, snow, mist, and fog. Research indi-cates that ambient levels of acid rain are affecting managed andunmanaged forests in parts of North America (Taylor et al. 1994.).The determination of effects of acid rain on forest ecosystems is akey component of a forest health assessment of Canadian forests.

This acid rain assessment reviews the state of the science asit has progressed since the last assessment carried out under theauspices of the Canadian Council of the Ministers of the Environ-ment (CCME) in 1990. Considerable progress has been made inaddressing issues identified in the 1990 assessment and on issuesthat have emerged since. Some of the gaps identified in 1990 areaddressed and the direction of future research is outlined. Wherepossible, results are based on peer-reviewed literature. The assess-ment also highlights key policy issues and the uncertainties asso-ciated with addressing them.

Specifically, the following questions are addressed:

a) What are the current effects of acid rain on forest trees, soils,and ecosystems?

b) What are the long-term effects of acid rain on forests?

c) What policy actions will emerge from current science?

Acid Rain and Current Forest Decline

Forest decline denotes a continued and sustained deterio-ration of condition ending in the death of the tree (Manion andLachance 1992). Decline may result from climatic events, anthro-pogenic influences such as air pollution, or insect and diseaseinfestation indigenous to forests. These stressors may act singlyor together to effect decline. In fact, declines are frequently ini-tiated by one or more factors and these causal factors are oftendifficult to determine. In some instances, forest decline is part ofthe natural regenerative process for healthy forests. In other situa-tions, decline is triggered by anthropogenic stressors. Determina-tion of deterioration of forest health due to anthropogenic stressorsis essential to forest health assessment. The role of air pollutionincluding acid rain is a key element in this assessment. Currentforest decline due to anthropogenic influences is evident in sev-

eral forest ecosystems of eastern Canada and involves severaltree species.

Coastal Birch Forests

Historically, declines of birch have been associated with resid-ual white birch stands subsequent to harvesting of other species.White birch (Betula papyrifera Marsh.) has a shallow root systemthat is very sensitive to site disturbance. Studies of a protracteddecline of yellow birch (B. alleghaniensis Britt.) that began in east-ern Canada in the late 1920s and continued intermittently untilthe early 1950s have revealed no definitive causes for the decline.It was triggered supposedly by winter climatic conditions (Braathe1995; Cox and Malcolm 1997) which continued to adversely affectthe species at irregular intervals until a sustained recovery beganin the early 1950s (Braathe 1995).

Currently, white birch and mountain paper birch (B. cordifoliaRegel) decline is occurring in the Bay of Fundy area on Canada’seast coast. This decline is widespread in relatively undisturbedforest stands that experience frequent fog. Observations since 1989reveal a prevalent browning of leaves followed by premature leaffall. Twig and branch death follows the browning often culminat-ing in the death of the trees. Devastation by insects or diseases hasbeen ruled out. Significantly, tree mortality in areas affected byacid fog is several times that of death rates in areas not character-ized by acid fog. The degree of damage and rate of recovery overthe years coincides with the frequency of these acid fogs. Thesefacts clearly point to acid fog, a common occurrence in this area,as the causal factor of the decline (Cox et al. 1997). Of the twospecies affected, the foliage of mountain paper birch is more sen-sitive to acid fog, the tree crown condition is slower to recover,and as a result this species is more prone to mortality than whitebirch.

Wet deposition via fog in causing direct foliar damage differsfrom that of acid rain. With fog, acidic pollutants accumulate onleaf surfaces in higher concentrations because of almost instan-taneous evaporation and may remain longer before being washedoff (Jacobson et al. 1990). Wetting and drying cycles exacerbatedamage from acid fog as has been shown for red spruce (Picearubens Sarg.) under laboratory conditions simulating acid rain(Percy et al. 1990, 1992).

The birches of the Bay of Fundy area, as well as other treespecies, are also subjected to elevated levels of tropospheric ozonefrom long range transport of gaseous pollutants (Cox et al. 1989;T.P.W. Williams 1994, unpublished data, Use of passive monitorsto assess bioindicators for ground-level ozone, M.Sc. Thesis, Univ.New Brunswick, 130 p.). Ozone causes carbon accumulationsin the foliage at the expense of the roots (Temple and Riechers1995). In addition, it damages cell membranes increasing perme-ability and nutrient leaching from foliage (Swanson and Thomson1973). Ozone also increases the acidity of marine fogs as well as

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the water film on the leaf surfaces by the oxidation of sulfite tosulfate (Eatough et al. 1984) and plays a role in the region-widealteration of needle surfaces (Percy et al. 1993; Cape and Percy1996; Turunen et al. 1997; Cape and Percy 1997).

Sugar Maple Forests

Extensive crown dieback and mortality of sugar maple (Acersaccharum L.) occurred throughout Quebec during the early 1980s.Decline symptoms were evident across almost 2000 km2 of sugarmaple forest by 1982 (Roy et al. 1985). A sustained recovery ofthese maples has been taking place since this time and mortalityand dieback have reverted to historical levels.

Analysis of climatic data around the time of the decline sug-gested a strong link with winter weather conditions and the die-back. An unusual warming occurred in the affected area duringFebruary 1981 as regional temperatures hovered around 20°Cresulting in a heavy snow melt leaving much of the ground snowfree. Subsequently, temperatures dropped to levels normal forFebruary and the ground froze to a depth of 1.0–1.5 m. Snowfallagain covered the ground. At the onset of the growing season,the trees became physiologically active. However, the roots wereunable to tap water from the soil as it was still frozen. The lack ofwater and damage to the conducting vessels is assumed to haveresulted in crown dieback and occasionally tree death.

To test the validity of these conclusions, similar frozen soilconditions were duplicated experimentally. Forest soils associatedwith sugar maple were allowed to freeze to comparable depths bypreventing normal snow accumulation on the ground (Bertrandet al. 1994). Maple decline symptoms resembled closely thoseof the sugar maple associated with the climatic anomaly of 1981.The experimental freezing resulted in a 27% increase in dieback3 yr after treatment and a 32% increase in canopy transparency1 yr following treatment. Sap flow was also impaired in the stressedtrees; trees with greater than 50% dieback had a mean sap flowrate 12% lower than healthy trees associated with unfrozen springsoils (Robitaille et al. 1995). The experiment also revealed thedevelopment of smaller leaves on the stressed maples (Robitailleet al. 1996) These experimental results reinforce the conclusionthat sugar maple decline of the early 1980s throughout southernQuebec was caused by abnormal climate conditions. The mapleswere also under additional stress from acid rain which, togetherwith climate stress, undoubtedly resulted in more widespread dam-age than that attributable to weather alone.

Ryan et al. (1994) compared the specific volume incrementsof sugar maples over their range in Ontario. After adjusting fortree age, rainfall, and temperature, the results indicate decline ingrowth beginning around 1960 in the areas of acid sensitive soils.Growth in the southern part of Ontario, where soils adequatelybuffer incoming acidic precipitation, has been relatively unchangedsince the early 1900s. The authors propose that the decreasing

growth on acid-sensitive soils is related to air pollution and possi-bly to reduced buffering capacity of the soils.

High Elevation Forests

Polluted cloud and rain water is a threat to forest health at midto high altitudes where forests are already under severe naturalstress. Red spruce decline at high elevations in the AppalachianMountains has been strongly linked to increases in the aluminum/calcium (Al/Ca) ratio of woody tissues, and to increases in respi-ration (Mclaughlin et al. 1991). High Al/Ca ratios were also foundfor red spruce root tips confirming that increases in aluminumavailability are associated with the decline (Smith et al. 1995).

The Chemistry of High Elevation Fog (CHEF) project beganin late 1985 to study the chemical deposition by clouds to highelevation forests in southern Quebec. Initial sites for the CHEFproject were located at Montmorency, Mont-Tremblant, and MontsSutton. At present, only the latter two sites are still in operation.Researchers have examined trends (1985–91) in concentrationsof sulfate, nitrate, ammonia, and hydrogen ions. Consistent increasesin several ions were found during winter while trends in the otherseasons were variable. Significant nutrient leaching occurred frombalsam fir (Abies balsamea [L.] Mill.) foliage at one site duringthe winter when deposition was highest. Lin et al. (1995) haveshown that trace metal deposition on balsam fir foliage is signif-icantly increased by cloud and fog immersion at high altitudes.The chemistry of the acid rain and associated forest decline atMont-Tremblant and Monts Sutton is similar to the high elevationred spruce decline in the Northeastern United States (Eagar andAdams 1992).

Effects of Acid Rain on Tree Physiology

Acid rain affects trees directly by altering the physical andchemical characteristics of leaf cuticles, inhibiting reproductiveprocesses, hampering the ability of trees to respond to low tem-peratures, and by influencing tree physiology. Tree response to acidrain varies depending on dose–response relationships, and the pol-lutants and species involved.

Foliar Surfaces

The leaf surface is covered by a thin, waxy layer called thecuticle which has many essential protective and regulatory func-tions. Its integrity is vital to the maintenance of a healthy tree. Thiscuticle is exposed to the atmosphere and, as such, is the initial con-tact point between air pollutants and the tree. Its waxy layer haschemical (Percy et al. 1994) and physical (Huttunen 1994) char-acteristics that can be deleteriously altered by dry and wet acidicdeposition.

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Under laboratory conditions simulating acid rain, the structureof the needle wax of red spruce and Sitka spruce (Picea sitchensis[Bong.] Carrière) degrades from a crystalline to an amorphousform at pH levels approximating ambient forest conditions (Percyand Baker 1990; Percy et al. 1990). This change is usually accom-panied by alterations to wax chemical composition as air pollu-tants interact with chemicals that are naturally produced by the tree(Percy et al. 1992; Krywult et al. 1996; Bytnerowicz et al. 1997).

Once wax chemistry is altered, essential leaf surface proper-ties are altered. Increased water loss results from increased cutic-ular transpiration mainly at night. Decreased vigor in xerophyticspecies like spruce results. Also, an increase in leaf wettabilitytakes place. Consequently, water droplets spread more easily andcover a larger surface area of the leaf, putting more leaf epider-mis in contact with acid in the precipitation than for an unalteredcuticle. An increased retention of solutes following droplet dry-down occurs. This increased concentration of ions causes toxicityover localized areas of the leaf surface resulting in increased uptakeof damaging ions (Percy and Baker 1991; Percy et al. 1992; Capeand Percy 1993, 1996). Increased water retention also leads toenvironments favorable for germination of fungal spores and forinfection from foliar diseases.

The rate at which acid rain causes change in foliar chemistryis often rapid, less than 6 weeks in some instances, during theearly part of the growing season (Huttunen 1994). Such effects areimportant measures of disruption. Consequently, cuticle changemay help determine critical levels of acid rain for sensitive treespecies (Cape 1994).

Similar cuticular changes have occurred on mature trees inthe field. Epicuticular wax chemistry and needle/water dropletcontact angles were altered to varying degrees related to deposi-tion levels along gradients stretching more than 300 km (Percyet al. 1993; Turunen and Huttunen 1990; Turunen et al. 1997; Capeand Percy 1997). The degree of change was more severe on asymp-tomatic needles sampled from trees exhibiting decline than onneedles collected from “healthy” trees. In general, needle surfacesin the areas of higher acid rain “aged” at a more rapid rate than nor-mal, often mimicking changes induced by natural factors (Hadleyand Smith 1994; Hoad et al. 1994). Scientists are building on thisresearch to develop the needle/droplet contact angle and epicu-ticular wax chemistry as early-warning indicators of tree declinedue to acid rain (Meyer et al. 1996; Cape and Percy 1996, 1997).

The role of altered epicuticular waxes due to acid rain in cutic-ular water loss is unclear (Rinallo et al. 1986; Barnes and Davison1988). Internal tree water movement and transpiration in needle-leaved trees is unaffected by acid rain under conditions of normalwater supply (Eamus et al. 1990). Cuticular water loss is morerelated to changes in the cuticular surface than the stomata. Assuch, acid rain at ambient levels is unlikely to induce water lossof trees unless they are already water-stressed. Changes in needlesurface characteristics due to natural environmental causes do

correlate with changes in rates of needle water loss (Cape andPercy 1996).

Studies looking at relationships between acid rain and thefoliar surfaces of deciduous species are limited. An increase indrought stress has been observed in white oak (Quercus alba L.)when simulated acid rain was applied. However, this stress wasattributed to adverse effects on root function rather than at the foliarsurface (Walker and McLaughlin 1991).

Foliar Leaching, Nutrient Concentrations,and Buffering Capacity

Acid rain increases foliar leaching which results in reductionsof foliar nutrient concentration and growth unless compensatedthrough increased uptake or internal redistribution (Morrison 1984;Hogan 1992). Increases in foliar nitrogen, and particularly sulfur,occur in response to uptake of sulfur from highly acid rains (Hogan1997). Increases in foliar aluminum and, in some cases, calcium,can occur in response to nitrate and sulfate pollution, or throughcation displacement brought about by soil acidification.

Base cations in foliage vary in their response to acid rainlargely based on their physiological properties or location withinthe leaf. Unlike potassium and magnesium, calcium is readilyleached from maple foliage over the range pH 5.0 to 3.0 (Woodand Bormann 1975; Hogan and Foster 1989; Hogan 1997). Potas-sium and magnesium are more often associated with intercellularstructures of the leaf than is calcium. Estimates of leaching fromthe leaves of red maple exposed to pH 4.6 indicate that 1–22% offoliar base cations could be lost from this species during a growingseason (Potter 1991).

Studies reveal that simulated insect damage to foliage increasesthe loss of potassium and magnesium, a finding that is noteworthygiven that trees affected by pollutants are predisposed to second-ary attack by insects and diseases (Cobb and Stark 1970; Smith1981). As an example, Maynard et al. (1994) noted that tremblingaspen (Populus tremuloides Michx.), adversely affected by sulfurdeposition, had increased incidences of armillaria root rot andhypoxylon canker.

External neutralization capacity (ENC) is a measure of theability of the leaf to buffer acid rain while the buffering capacityindex (BCI) is a measure of the ability of the leaf to resist inter-nal acidification. ENC is related to foliar nutrient levels in a gen-eral way and BCI is correlated with magnesium, nitrogen, andcalcium (Liu and Côté 1993). Although preliminary, research withsome native species suggests species-specific differences in thesecharacteristics, an observation that may be used to assess speciesat risk. Generally, late successional species, such as sugar maple,have a low ENC and BCI which make them susceptible to acidrain. Early successional species like birch and largetooth aspen(P. grandidentata Michx.) with higher ENC and BCI are lesssensitive to acid rain when compared with these latter species.

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These traits establish a clear physiological basis for developmentof a range of measures of tree sensitivity to acid rain.

Photosynthesis and Chlorophyll Concentration

Studies with sugar maple and hybrid poplar reveal that chloro-phyll a and b concentrations are unaffected by increases in acid-ity of rain from pH 5.6 to 2.0 (Reich et al. 1986; Hogan and Taylor1995; Neufeld et al. 1985). Certain findings indicate a naturalselection for tolerance to pollutants. Adverse effects on photosyn-thesis are greater on pine clones originating from nonpolluted areasthan on clones growing in polluted areas (Charland et al. 1996).Similarly, eastern white pine (Pinus strobus L.) from Sudbury,Ontario, is more tolerant to both sulfur dioxide and ozone thaneastern white pine from Acadia Forest Experimental Station, NewBrunswick, where ambient pollution is less.

Several scientists have reported stimulation of net photosyn-thesis or no effect from acid rain with a pH 3.0 to 3.5 for severaldeciduous and conifer species (Taylor et al. 1986; Reich et al. 1987;Sasek et al. 1991; Seiler and Paganelli 1987; Barnes et al. 1990;Hogan and Taylor 1995). Studies with red spruce have revealedincreases of net photosynthesis at rain acidities of pH 4.1 and noapparent effect at a mist of pH 3.6 (Taylor et al. 1986).

Exceptions to this body of research exist. A reduction in netphotosynthesis after treatment with simulated acid rain at pH 3.0occurred for yellow-poplar (Liriodendron tulipifera L.) (Roberts1990). Also, Meng et al. (1994) reported a decrease in net photo-synthesis and stomatal conductance for mature red spruce withincreasing absorption of sulfate. Mclaughlin et al. (1992) reportedthat net photosynthesis for the same species decreased by 19%and dark respiration increased by 51% following 16 wk of treat-ment by acid mist of pH 3.0. Increasing dark respiration is a com-mon response of plants exposed to air pollutants and is considereda reliable indicator of stress.

Forest Plant Reproduction

Reproductive systems of wind-pollinated plants are vulner-able to atmospheric pollution. Changes in pollen structure mayoccur that affect the genetic composition of the progeny (Tanksleyet al. 1981; Sarcy and Mulcahy 1985; Cox 1989). Natural selectionfor pollution-tolerant individuals may have occurred in easternwhite pine and red pine (Pinus resinosa Ait.) (Cox 1992). Whitebirch and mountain paper birch near the Bay of Fundy frequentlyintercept acidic (pH <3.5) marine advection fogs. In both species,pollen germination and basic physiological responses are inhibitedbelow pH 5.6 (Hughes and Cox 1994).

Frost Hardiness

Several researchers have studied relationships between acidrain and changes in the inherent ability of trees to cope with nat-

ural stresses. In one study, red spruce was exposed to mist of vary-ing acidity. A correlation was found between leaf sulfate contentand frost hardiness. A 0.1% increase in foliar sulfur content causeda 2.7°C decline in frost hardiness (Sheppard 1994).

A toxic effect of sulfate on tree vigor occurs when other nutri-ents such as nitrogen are insufficient to direct the extra sulfateinto protein synthesis for tree growth. Photosynthate productionis reduced, and sugar available for synthesis of anti-freezing com-pounds decreases, resulting in reduced frost hardiness. This pro-cess is likely responsible for large-scale decline of red spruce inthe Appalachians (Eagar and Adams 1992). Sugar maple declinein Quebec and links to acid rain and frost hardiness are discussedin the section “Results of Forest Health Monitoring.”

Tree Growth and Physiology

Studies carried out with red oak (Quercus rubra L.), sugarmaple, hybrid poplar, and jack pine (Pinus banksiana Lamb.) indi-cate that many growth parameters are not affected by acid rain atpH above 3.0 (Reich et al. 1986; Hogan and Taylor 1995). Amongthe growth parameters unaffected, except within extreme pHranges, are height growth, number of leaves, stem diameter, andleaf area.

In contrast, reductions in the growth rate of root biomass havebeen documented for white ash (Fraxinus americana L.) and jackpine following acid rain treatment at pH 3.0 (MacDonald et al.1986; Chappelka and Chevone 1986; Amthor 1986 ). Anotherstudy revealed shifts in carbon allocation from coarse roots to anutrient absorption function in fine roots on red spruce with appli-cation of simulated acid rain (Deans et al. 1990). These resultsneed to be replicated under natural conditions.

Carbon Exchange and Storage

Carbon storage and storage products are reduced in sev-eral species due to acid rain. Jensen and Patton (1990) reporteddecreased starch in roots of yellow-poplar treated with rains ofpH 3.5 to 4.5. In another study, levels of ethanol-soluble carbohy-drates in current and 2-yr-old conifer needles were reduced by acidmists (Barnes et al. 1990). Increased dark respiration rates havebeen measured within high elevation forests affected by high levelsof acid rain resulting in carbon balances not conducive to sustainedgrowth (Mclaughlin et al. 1993).

Effects of Acid Rain on Soil Chemistry

Strong acid anions, including sulfate and nitrate, depositedby acid rain interact with forest ecosystems in both a beneficialand harmful manner. Both sulphur and nitrogen are essential toplants. Excess amounts, however, hinder growth and vigor of soil

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biota, impede organic decomposition and nitrogen and sulfurcycling, and increase base leaching and mobility of trace metals.Excess amounts of acid anions also weaken host tree species andcause deterioration of forest ecosystems leading to infestation byinsects and diseases.

Soil Biota, Decomposition, and Nitrogen Turnover

Increased acidity directly influences the soil nutrient cycle byreducing microbial diversity, nitrification, ammonia volatilization,and microbial respiration (Mahendrappa 1982, 1989, 1991). Micro-bial respiration and biomass carbon:organic carbon (Cmic/ Corg)ratios are lowered in the litter and humus surface layers of the soilwith application of sulfate singly or in combination with nitrate(C.Thirukkumaran and I.K. Morrison 1997, unpublished data,Impact of simulated acid rain on microbial respiration, biomassand metabolic quotient in a mature sugar maple, Acer saccharumMarsh., forest floor). Continued reductions in these processes even-tually reduce forest productivity. Acid rain at ambient rates onnorthern soils is probably not harmful to microbial processes inthe short term. However, in areas of higher deposition, or long-term exposure, the possibility of ecosystem deterioration increases.

Soil Acidification and Base Leaching

In northern Ontario, on nutrient-poor soils, most nitrogen fromacid rain is absorbed by foliage or retained by the soil (Foster etal. 1995). Retention of atmospheric nitrogen has a positive effecton tree growth, because much of the vegetative growth of theboreal forest is limited by lack of nitrogen. Vegetative uptake ofnitrate helps to buffer soils against incoming acidity. Broadleaftrees neutralize up to 80% of rain acidity, compared with lessthan 50% for needleleaf trees (Mahendrappa 1983). However, thecapacity for these sites to retain additional nitrogen is limited andcontinued deposition will lead to soil acidification and the even-tual leaching of nutrients from the soil.

Any change in sulfate amounts in soil solution with nutrient-poor forest soils depends on the ability of the soil to retain it, whichis, in turn, related to the amorphous inorganic aluminum content ofthe soil (Bhatti et al. 1997). At many nutrient-poor sites, the outputof sulfate from the soil rooting zone is as great or greater than inputsfrom the atmosphere (Foster and Hazlett 1991; Mitchell et al. 1992).

Studies in Ontario indicate that ambient levels of acid rainhave accelerated the loss of base cations from soils that supporta sugar maple forest (Foster et al. 1992). Sugar maple is sensi-tive to levels of soil nitrate (Yin et al. 1993, 1994). An increasein nitrogen deposition may create nitrogen/base cation imbalancesin this species. For example, on some ARNEWS plots in areas ofhigh sulfate and nitrate deposition, depletion of base cations fromforest soils is occurring. This leaching will probably increase ascurrent deposition levels continue (Morrison et al. 1995).

Acid rain reduces concentrations of soil calcium, magnesium,and potassium. Effects are greatest where deposition is the high-est (Morrison et al. 1995). The year-to-year supply of base cationsto forest vegetation may be interrupted on sites where replenish-ment from the weathering of primary minerals is slow, or affectedby natural disturbances such as drought, flooding, or severe frosts.Simulated long-term biomass projections, based on critical loadsresearch, suggest that when cumulative losses of base cations occur,forest productivity declines with continued sulfate and nitrate depo-sition (Arp et al.1996).

Based on analysis of nutrient fluxes, substantial losses of basecations linked to acid rain have occurred and are projected to con-tinue at the Hubbard Brook Experimental Forest located in thenortheastern United States (Likens et al. 1996). Continuous deple-tion of base cations from the forest floor layers has been docu-mented at Hubbard Brook over a 30-yr period. Depletions in thisorder have significant consequences for sustained growth. Sitesof similar nutrient status without such a historical record couldwell be experiencing similar depletions of nutrients.

From studies in Quebec, foliar nutrient deficiencies in sugarmaple have been linked with cation imbalances in soil. Specifi-cally, soils associated with these stands have undergone reduc-tions in calcium and potassium saturation, increased magnesiumsaturation, and increased aluminum toxicity (Ouimet and Camiré1995). Growth and nutrient status of sugar maple seedlings declinewith increased acidity and subsequent decreased soil base satura-tion. Overall, adverse effects on ecosystem productivity are likelyto worsen with continued inputs (Ouimet et al. 1996).

Ecosystem Response to Reduced Acid Rain

At the Turkey Lakes Watershed (TLW) in central Ontario,declining acidic concentrations in precipitation from 1981 to 1993resulted in lower sulfate concentrations in soil runoff and streamwater (Fig. 1a). Annual decreases in sulfate deposition of up to31% reduced the concentrations and flux of sulfate in soil (Fosterand Hazlett 1991). These decreases have slowed the rate of acid-ification of the nutrient-poor soils associated with the hardwoodstands at TLW. Base cation concentrations in soil runoff and streamwater have been substantially reduced (Figs. 1a–d). The increasein base cation levels in soil, in response to lower base cation leach-ing, however, may be less pronounced due to declining concen-trations of base cations in precipitation across eastern NorthAmerica (Likens et al. 1996)

Interaction with Other Stresses

Forest ecosystems are exposed to a myriad of natural andanthropogenic stresses. These stresses interact with each otherand it is often the cumulative effect of more than one stress thatresults in reduced forest health. An example of interacting stresses

10

was noted earlier with the decline of sugar maple in Quebec in the1980s. The decline was triggered by extreme climatic conditionsbut exacerbated by ambient air pollution in the form of acid rain(Boutin and Robitaille 1995).

Acid rain also needs to be considered in association with otheranthropogenic stressors such as ultraviolet radiation, toxic chem-icals, and climate change to understand cause and effect relation-ships with the health of forest ecosystems.

Results of Forest Health Monitoring

During the late 1970s and throughout the 1980s, concernsregarding regional forest decline and the possible relationships

with atmospheric pollutants were expressed in the popular liter-ature, news media, and scientific publications. Much of the focuswas on the forests of central Europe, the high elevation spruce for-ests of the northeastern United States, and the deciduous forestsof eastern Canada and the United States. In Canada, discussioncentered primarily on white birch in New Brunswick and sugarmaple in southern Quebec and south-central Ontario. It was duringthis period that initiatives in forest health monitoring within Canadawere instigated.

Forests are monitored to explain the causes of observedchanges. Tree condition can vary widely from year to year, andit is through long-term monitoring that trends in forest health canbe detected. There are several forest health monitoring systemsoperated by federal and provincial governments.

11

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Figure 1. Trends in streamflow concentrations of sulfate (SO4), calcium (Ca), potassium (K),and magnesium (Mg) from basin 47 at the Turkey Lakes Watershed research site from 1981 to 1993.

Acid Rain National Early Warning System(ARNEWS)

The Canadian Forest Service (CFS) established ARNEWSin 1984 in response to prevalent concerns about the effect of acidrain on the health of Canada’s forests. Analysis of ARNEWS datastrives to detect early signs of change or damage to forest treesand soils attributable to air pollution and not to damage associatedwith natural causes or management practices. Long-term changesin vegetation and soils attributable to acid rain and other pollu-tants are monitored. Symptoms of damage from air pollution arenot obvious and frequently resemble damage from natural causes.Experience of field professionals trained to distinguish these symp-toms from abnormal climatic conditions, inherent nutrient deficien-cies, and the effects of insects and diseases is crucial to ARNEWS.

Assessments of ARNEWS data indicate that there is no large-scale decline in the health of Canadian forests that can be directlyattributed to atmospheric pollution (Hall and Addison 1991; Hall1995a, 1995b, 1996). However, on a regional scale damage fromair pollution is evident for birch in the Bay of Fundy area of NewBrunswick, certain high elevation forests, and some red oak, redpine, and sugar maple forests on acid-sensitive soils in Ontarioand Quebec. Investigations into these declines are continuing. Airpollution may have a greater effect on forest stands weakened byother stressors such as climate extremes and insect defoliation.

Analyses of soils from ARNEWS plots were completed in1985 and 1990, to examine changes in soil properties in relationto wet sulfate or wet nitrate deposition. Generally, no consistentchanges in pH occurred over the 5-yr period. There were reduc-tions in concentrations of calcium and magnesium in the forestfloor in areas of relatively higher acidic deposition. Trend analysisof soil chemistry within ARNEWS is ongoing.

North American Maple Project (NAMP)

In 1988, the North American Maple Project (NAMP) estab-lished a network of 166 monitoring sites in the northeastern UnitedStates and Canada covering most of the range of sugar maple.NAMP, jointly managed by the Canadian Forest Service and theUnited States Forest Service, currently comprises 233 plots. InCanada, 62 plots are monitored across three ecozones (BorealShield, 16 plots; Mixedwood Plains, 18 plots; and Atlantic Mar-itime, 28 plots), in both unmanaged stands and those managedfor maple syrup production.

The objectives of the NAMP program are to determine:

i) the rate of annual change in sugar maple condition;

ii) whether the rate of change in condition differs with the levelof sulfate and nitrate wet deposition, between a sugar bushand undisturbed forest, and for various levels of initial standconditions; and

iii) the possible causes of decline.

From data compiled between 1989 and 1995, crown mortal-ity was higher on plots in the Boreal Shield ecozone than in eitherthe Mixedwood Plains or Atlantic Maritime ecozones (Bowersand Hopkin 1997). Moreover, McLaughlin et al. (1997) reportedmaple decline at locations on the Boreal Shield where shallowacid soils dominate. Some researchers have argued that these siteson the Precambrian Shield are poorly buffered against sulfate andnitrate and lower deposition levels can have a greater effect thanon deeper and better buffered sites (Arp et al.1996). However, inearlier work, Basham (1973) had concluded that sugar maple inOntario, within the Boreal Shield ecozone, is generally slowergrowing and contains more defects due to poorer site conditions.

Results from NAMP also indicate slightly higher levels ofdieback in stands actively managed for sap compared with naturalstands. This may be a factor of frequent incursions in sugar bushescausing more stress on the trees compared with unmanaged stands.This additional stress may originate from soil compaction, treewounding, frequent light thinnings, and tapping. Interactions withair pollutants cannot be discounted.

Quebec Monitoring Networks

The Réseau de surveillance des écosystèmes forestiers(RESEF, network for the monitoring of forest ecosystems) wasestablished in 1988 to collect information on climate, nutrient sta-tus of ecosystems, precipitation, and air quality. Linkages existwith other networks operated by the departments of Agricultureand of Environment and Wildlife of Quebec, and the Canadian Airand Precipitation Monitoring Network (CAPMON) of Environ-ment Canada. A total of 31 sites exist covering various ecologicalregions of Quebec. The network compiles data on the effects ofnatural and anthropogenic stresses on forest ecosystems, partic-ularly on their biological diversity and growth (Gagnon et al. 1994).Analyses of data have not been published yet.

Forest Health Monitoring in Ontario

The hardwood forest of south-central Ontario receives the high-est rate of acid rain and ground-level ozone in eastern Canada. Inaddition, much of the forest grows on naturally acidic, nutrient-poor, shallow soils characteristic of the Canadian Shield. Thiscombination of high pollution loading and poorly buffered, acid-sensitive soils makes a large portion of the hardwood forest inOntario particularly susceptible to deterioration by acid rain.

In 1985, the Ontario Ministry of the Environment and Ener-gy initiated a forest health survey of the tolerant hardwood forestsin Ontario. The objective is to provide a province-wide data baseon the visual condition of the predominantly sugar maple forest.By periodically reevaluating the same trees and comparing theresults with earlier data, the intent is to identify trends in changingtree condition and allow comparisons between geographic areasacross the province. The survey is based on plots containing over

12

14 000 trees across the range of sugar maple. The main visualsymptoms of decline are pale green or yellowed foliage, smallleaves, and dieback of the fine twig structure followed by the deathof main branches.

Acid rain and ground level ozone, the two main regional airpollutants in Ontario, are highest in the southwestern portion ofthe province and decrease to the north and northwest. Analysis todate reveals a significant relationship between dieback and soiltype. Plots on acidic, sandy soils have more dieback than plots onneutral-to-slightly-alkaline, loamy soils with carbonates in the sur-face horizons.

It is not surprising that trees in the northern edge of theirrange and growing on sites with impoverished soils are in poorercondition then southern trees under less climatic stress and onbetter sites. However, the soils of much of the northern hardwoodforest are sensitive to acid deposition, and pollutant levels are highenough to cause accelerated soil acidification. A statistically sig-nificant relationship exists between tree condition and soil pH,base saturation, and available aluminum on acid-sensitive sites.This significance is apparent even though the forest has been sev-erely affected by natural insect and weather stresses. Air pollutantsmay have altered the chemical cycles in the soil, thus predisposingthe forest to devastation by natural stresses.

The average annual tree mortality over the 10-yr survey periodfrom all the plots was 1.2%. Results are consistent with other stud-ies that confirm an average annual mortality in tolerant hardwoodstands of approximately 1%. The first 10 yr of monitoring indi-cate that the hardwood forest in Ontario, on average, is healthyand stable. Annual fluctuations in tree condition are small andlargely explainable in terms of known anthropogenic and naturalstresses. Regional analyses, however, suggest deterioration in afew areas. Continued monitoring is necessary to determine whetherthese regional trends are induced by air pollution or are longer-term fluctuations in natural forest cycles.

Turkey Lakes WatershedStudies carried out at Turkey Lakes Watershed from 1980

to 1996 indicate that atmospheric sulfur and nitrogen depositionproduce an accelerated leaching of calcium, magnesium, and potas-sium from soils. Since the soils are well-buffered they have notbeen depleted of the nutrients necessary to support healthy treegrowth. Simulated long-term forest productivity projections sug-gest that when the cumulative loss of nutrients is taken into account,forest productivity will decline if sulfate and nitrate deposition con-tinue (Oja and Arp 1997).

Critical Loads/Levels

The term “critical load” is defined as the “highest deposition ofacidifying compounds that will not cause chemical changes lead-

ing to long-term harmful effects on the overall structure or func-tion of the aquatic and terrestrial ecosystem.” According to Nilssonand Grennfelt (1988), critical loads are quantitative estimates ofan exposure to one or more pollutants below which significantlyharmful effects on specified sensitive elements of the environmentdo not occur. Development of critical loads for atmospheric sulfurand nitrogen deposition allows the determination of whether or notcurrent or anticipated sulfur and nitrogen deposition loads exceedsuch values (Arp et al. 1996).

In contrast, the “target load” is a political decision about riskand accepting change. The target load may be set above or belowthe critical load. The current Canadian objective to reduce acid rainis set at a target load of 20 kg/(ha•yr) of sulfate in precipitation.This target was derived in the early 1980s from limited data basedmainly on the loss of sport fish which occurs when pH in streamsand lakes drops below pH 5.3. At that time, the assumption wasthat a reduction of atmospheric sulfur deposition to 20 kg/(ha•yr)would protect moderately acid-sensitive waters. Sensitive ter-restrial ecosystems were considered only in the sense of theirbuffering capacity to prevent leaching of nitrates and sulfates toreceiving waters. In addition, not enough information was avail-able to set target loads for protecting highly acid-sensitive areas.Thus, there exists a need to specify critical levels of wet and drysulfate and nitrate deposition for forest ecosystems both in the shortterm (years to several decades) and in the long term (several com-plete stand cycles).

Existing guidelines (RMCC 1990) to protect surface waterssuggest a range of target loads of 8 kg/ (ha•yr) in some parts ofAtlantic Canada, to more than 20 kg/ (ha•yr) of sulfate in precip-itation in some of the less acid-sensitive regions of Ontario andQuebec. The validity of these target loads in protecting sensitiveforest ecosystems needs to be determined along with actual air pol-lutant loadings for policy actions on pollution abatement strategies.

The determination of sulfur and nitrogen loads is critical tothe management of forest ecosystems. The pulp and paper indus-try needs to know the levels of wet and dry sulfate and nitrate depo-sition that lead to unacceptable long-term effects on soils and forestgrowth. Similarly, the electric utility industry and regulatory agen-cies need such data to ascertain and prevent harmful levels ofindustrial sulfur and nitrogen emissions.

Case Study—ARNEWS Network

A recent study has determined critical loads and relatedexceedances or nonexceedances for ARNEWS plots under cur-rent atmospheric deposition rates, from Newfoundland to Alberta(M.H. Moayeri and P.A. Arp 1997, unpublished data, Assessingcritical soil acidification load effects for ARNEWS sites: prelim-inary results, Univ. New Brunswick, Fredericton, N.B.). ARNEWSsites in British Columbia will be incorporated into this study asit evolves. Focus on ARNEWS plots offers several advantages for

13

critical load assessments: plots are georeferenced, vegetation andsoil within plots are monitored through common protocols, andthe monitoring effort is long-term having begun in 1985.

Critical rates of soil acidification and related exceedanceswere calculated from the atmospheric acid deposition loads (wetsulfur, nitrogen, calcium, magnesium, and potassium), and thecapability of soil and vegetation to neutralize the incoming acidity.An aluminum/ base cation ratio of 0.15 eq/eq was the criterion forsoil acidification. At this level, both top soils and subsoils are con-sidered protected against acidification and aluminum mobilization(M.H. Moayeri and P.A. Arp 1997, unpublished data, cited pre-viously). Exceedances were calculated by subtracting net acid neu-tralization rate from atmospheric wet nitrogen and sulfur depositionrates.

Model-calculated values for current soil acidification exceed-ances are presented in Figures 2 and 3. Isolines drawn aroundthese plots divide the regions into (a) areas of low and high soilsensitivity to acidification, as indicated by the critical load num-bers. Areas with >500 eq/(ha•yr) are less sensitive than areas with<500 eq/(ha•yr); (b) areas of exceedance and nonexceedance(negative numbers); and into areas of high and moderate exceed-ance, with the division line at 500 eq/(ha•yr).

Critical acidic loads of <500 eq/(ha•yr) are prevalent in north-ern Ontario, Quebec, Alberta, Saskatchewan, and Manitoba becauseof the presence of an acidic soil substrate (Fig. 2). Areas in south-ern Alberta, Saskatchewan, Manitoba, Ontario, and Quebec, andmany parts in the Maritime provinces have high critical acidic loads

because of calcareous soils. Acid-sensitive areas tend to occur wherethe soils are derived from acid igneous or acid sedimentary bedrock.

Areas of nonexceedance characterize Alberta, Saskatchewan,and Manitoba due to low rates of acidic deposition (Fig. 3). Like-wise, exceedance levels are generally negative in most of NovaScotia, where acidic deposition tends to be moderate (exceptionsoccur along the Bay of Fundy coast and southwestern Nova Scotia).High exceedance areas are found in the midlatitudes of Ontarioand Quebec where acidic deposition is relatively high, and crit-ical values are relatively low.

Although only negative exceedances exemplify ARNEWSplots in northern and western Canada, these areas are not immuneto soil acidification. Areas of local impact exist and are not includedwithin the current analysis (that is, downwind from smelters, andurban centers).

Areas with high exceedances are prone to forest decline, asobserved for Ontario (Arp et al. 1996; Hopkin and Dumond 1996)and for Quebec (Ouimet et al. 1996). M.H. Moayeri and P.A. Arp(1997, unpublished data, cited previously) found that defoliationlevels correlated partly with calculated exceedance levels, crit-ical loads for soil acidification, degree of insect damage, and abi-otic factors. With respect to defoliation, both the exceedance andcritical load calculations factor strongly and positively across theARNEWS plots. Preliminary analyses indicate that a 500 eq/(ha•yr)exceedance is associated with an annual productivity loss of 10%.In terms of mortality, a 1000 eq/(ha•yr) exceedance contributesto an 8% ± 2% increase in mortality over a period of 11 years.

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Figure 2. Critical acid deposition loads, eq/(ha•yr), for individual ARNEWS plots.

These exceedance numbers are underestimates since sulfurand nitrogen inputs from dry deposition have not been included.Fog inputs near the Great Lakes also aggravate the situation. Towhat extent exceedances of critical loads are deleterious to for-est ecosystems is not known. Species shifts to nitrogen-loving florawould probably occur particularly in nutrient-poor ecosystems,such as peat bogs and heathlands.

Case Study—Ontario

Atmospheric sulfur and nitrogen deposition varies acrosssouthern Ontario. Highest rates occur in the southernmost parts;lowest rates occur in the northwest. Geological substrates of south-ernmost Ontario are limestone and dolomite. Soils derived fromthese substrates are well-buffered and therefore have high criti-cal loads for soil acidification (Fig. 2). Farther to the north is theCanadian Shield which is primarily granitic igneous bedrock. Soilsderived from this substrate are generally poorly buffered, and there-fore have low critical loads (Fig. 2). Superposing current atmos-pheric deposition rates on the potential for soil acidification asdetermined by location and by critical load calculations indicatesthat the ability of the soils to buffer incoming acidity is exceededimmediately north of the limestone/dolomite areas by at least500 eq/(ha•yr) in some cases (Fig. 3) (Arp et al. 1996). Compar-ing forest decline symptoms (for example, exceedances for sugar

maple stands) leads to a significant correlation: decline symptomsincrease with calculated soil exceedances (Fig. 4).

Within the Canadian Shield, to reduce the greatest exceed-ance by about one half, sulfate deposition rates would have to bereduced by 12 kg/(ha•yr). To curb both soil acidification and poten-tial nitrogen eutrophication, a strategy is needed for controllingboth sulfur and nitrogen deposition. The need for control of nitro-gen is particularly strong in southern Ontario because nitrogeneutrophication rather than soil acidification is likely to become amajor concern on well-buffered soils.

Mean dieback levels of sugar maple, as measured by theNAMP, have been compared to critical loads maps (Arp et al.1996). Areas of critical load exceedances consistently have higherlevels of dieback than areas of no exceedance. Other factors suchas climate and ground-level ozone may also play a role.

Critical Levels for Vegetation

Levels to protect vegetation are usually developed for a fewsensitive species or plant processes and applied to mapping ata regional scale to do risk assessment. The UN-ECE has estab-lished critical levels for sulfur dioxide, nitrous oxides, ammonia,and associated ions in cloud/rain (Ashmore and Wilson 1995).These levels were based largely on measurements of leaf sur-faces (Cape 1993). The level is currently set at 0.3 mmol (H+)

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Figure 3. Exceedances of critical acid deposition, eq/(ha•yr), for individual ARNEWS plots. Boldline divides regions into positive, >100 eq/(ha•yr), and negative exceedance areas. Shaded areashows strongest exceedance area, >500 eq/(ha•yr).

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for forests in cloud at least 10% of the time. The difficulty of mea-suring cloud water over large areas is overcome by modeling ofparticulate sulfate air concentrations and combining such datawith climatological data or cloud occurrence (Cape 1993). In thisway, critical levels may be mapped for forest areas that are atgreatest risk from acid rain.

Currently in Canada, some high elevation forests, particu-larly in the east, are blanketed in clouds over 30% of the time. Aswell, areas of white birch and red spruce decline along the easternseaboard are typically inundated by acidic coastal fogs for longperiods. Critical levels of acidity in both these situations may beexceeded over extended periods.

International Linkages

International involvement and collaboration remain inte-gral to Canada’s Science and Technology program. This report pro-vides the scientific underpinning for assessing the effects of acid

rain on Canadian forests and builds on international efforts aimedat abatement of transboundary air pollution. Acid rain, the prin-cipal bilateral air quality issue, is the focus of research under theUS/Canada Air Quality Accord. Both countries are making sub-stantial progress in implementing their respective acid rain controlprograms. Canada continues to play a constructive role in promot-ing the activities of the International Cooperative Programme onthe Assessment and Monitoring of Air Pollution Effects on Forests(ICP Forests) of UN-ECE under the Convention on Long RangeTransport of Atmospheric Pollutants (LRTAP).

Fundamental research directed to effects of air pollution formsa scientific basis for development of indicators of forest sustain-ability under the Criteria and Indicators Initiative of the CanadianCouncil of Forest Ministers and the parallel international “Mon-treal Process.” Development of the critical loads and estimationsof exceedances of these loads enhances Canada’s ability to mea-sure and evaluate the effects of pollution on forested ecosystems.Linking this approach to dynamic models also enhances predic-tive ability, minimizes uncertainties, and contributes to interna-tional efforts on mitigation of the effects of acid rain.

Research and Information Needs

Several research needs arise from this assessment:

• Early warning systems that provide valuable information toresource managers on impending conditions require improve-ment. Specifically, thresholds and exceedance values for sen-sitive forest ecosystems are needed. Effective strategies tocombat pollutants must be based on meaningful dose–responserelationships.

• There is urgent need to determine the effects of air pollutants onforest soils and foliage and to make direct links between ambientlevels of pollution and foliar damage. Again, establishment oftarget loadings, thresholds, and exceedance values, and deter-mination of risks are needed. The assumption that the currenttarget loading of 20 kg/(ha•yr) of wet sulfate deposition providesreasonable protection for forest ecosystems is invalid. This levelwas derived for moderately sensitive aquatic systems and the levelof risk it poses to terrestrial systems requires further resolution.

• Current negotiations and limits within the UN-ECE LongRange Transport of Atmospheric Pollutants (LRTAP) Conven-tion are based on calculation of deposition in excess of criticalloads for forests soils. It is necessary to apply these principles toCanadian forests. Canadian efforts are constrained by the avail-ability of data and modeling efforts. Current networks provideintensive data for only a small number of sites and need to beexpanded to provide a comprehensive database.

• The establishments of critical and target loads along with exceed-ance values and determination of risk are needed. The assump-

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Exceedance eq/(ha•yr)600 800

Figure 4. Comparison of observed percent decline and mortalityfor sugar maple (Acer saccharum L.) in southern Ontario with cal-culated exceedances of soil acidification.

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tion that 20 kg/(ha•yr) of wet sulfate deposition is a reasonabletarget load is suspect; this level was derived for aquatic ecosys-tems and is not directly applicable to terrestrial ecosystems.

• Current critical load assessment needs to be extended to includea greater range of tree species and a much broader geographi-cal area. In addition, research must be extended to examine theeffects of exceedances on forest decline and tree mortality.

• There is a strong need for continued monitoring, research onmethods, detection of symptoms, and cause and effect linkages.Increased understanding of basic mechanisms is required tounderstand the behavior and effects of air pollution.

• Long-term chronic nutrient depletion is likely to continue andaffect both forest function and structure. Research into the nutri-tional dynamics, status, and stability of the forests should beextended. Specifically, protection against leaching of nutrientsis needed as is research to determine acceptable aluminum andnitrate concentrations for Canadian tree species and soils.

• Greater focus overall needs to be placed on wet and dry formsof nitrogen deposition. As the sources are more diffuse, controlof nitrogen emissions is more difficult. Moreover, regional depo-sition of ammonia and other forms of dry nitrogen will proba-bly increase with the development of more intensive agriculturalfertilization.

• New threats (ozone, UV-B, heavy metals, trichloroacetic andother acids) to forested ecosystems continue to emerge. Theseissues need to be addressed through the formulation of envi-ronmental objectives, guidelines, and legislation. Interactionsbetween air pollution and other influences must be better under-stood. Improved and heightened cooperation among all stake-holders is needed.

• Tropospheric ozone occurs in much of eastern Canadian forestswhere acidic deposition is high. Research on interactions betweenboth pollutants and forest ecosystem processes is required.

• Assessment of the relationship between air pollution, forest health,and economic ramifications is needed. Potential consequencesof reduced forest health to Canada’s competitive advantage inforeign trade and the protection of international market sharemay be substantial. Canada ranks first in the world in the exportvalue of forest products and the forest sector is a key driver ofthe overall Canadian economy.

Conclusions

This report discusses progress in research addressing theeffects of acid rain on forest ecosystems since 1990, the year ofthe last national assessment of acid rain research. A better under-standing of above- and below-ground ecosystem linkages asso-ciated with acidic deposition exists. Critical loads for certain forest

soils have been postulated through modeling, and research on crit-ical loads for vegetation has begun. It is now possible to make reli-able predictions on the effects of control strategies on tree growthand soil quality. Findings can also be linked to long-term strate-gies for forest management and sustainability. Uncertainty doesexist and many gaps in knowledge remain but enough is knownto provide some direction for the future.

• Current target loads of acidic deposition, 20 kg wet sulfate/(ha•yr), are too high to protect sensitive forest ecosystems.Projections show that forest decline is 30–40 % higher for for-est stands for regions with exceedances 300–500 eq/(ha•yr)than for less sensitive regions.

• In Quebec, foliar deficiencies in sugar maple stands are asso-ciated with cation imbalances in soil. In addition, growth andnutrient status of sugar maple seedlings decline as soil acidityincreases, and soil base saturation decreases. These effects arelikely to continue or worsen with continued inputs.

• In certain sugar maple forests within Ontario, acid rain at ambientlevels has accelerated the loss of base cations from soils. Growthof sugar maple is sensitive to increased levels of soil nitrate. Asnitrogen deposition increases, a nitrogen/base cation imbalancein this species develops resulting in reduced tree vigor.

• In New Brunswick, areas of acidic fog are characterized by wide-spread birch decline which is expected to increase in severityand extent as current deposition levels continue. As well, redspruce decline in the Gulf of Maine/Bay of Fundy area is attrib-utable to the frequent presence of acidic coastal fog.

• Data from the ARNEWS plot network of CFS indicate that sev-eral of these plots are characterized by forest decline linked toair pollution. These sites, within the Boreal Shield, MixedwoodPlains, and Atlantic Maritime ecozones are influenced by highlevels of acidic deposition acting singly or in combination withother stressors such as insect defoliation and extreme climaticevents that amplify the effects of pollutant exposure.

• In the northeastern United States (Hubbard Brook Experimen-tal Forest), long-term data reveal that depletion of base cationsfrom the forest soil has been occurring over a 30-yr period andis continuing. Depletion of soil nutrients over such a long periodresults in reduced forest productivity. Although no such long-term data exist within Canada, similar ecosystems under similaracid rain scenarios do occur.

• Model calculations on plots of the Réseau de Surveillance desEcosystèmes Forestiers (RESEF) indicate that of the 31 plots,19 receive atmospheric acidic inputs in excess of their criticalloads (67% and 42% of the deciduous and coniferous plots,respectively). In upland forests of the Canadian Shield of South-ern Ontario, atmospheric deposition also exceeded critical loads.The critical levels approach is useful in the derivation of policy for

18

air pollution control. Levels to protect vegetation are being devel-oped for several species, representing a range of sensitivities.

• Mapping of critical loads for Canadian forests at risk from acidrain is not currently available. Further definition of dose–responserelationships is needed along with data on atmospheric anionconcentrations for forests at greatest risk.

Continued scientific research is essential to assess the effec-tiveness of control strategies, to develop and implement a newgeneration of predictive models required to refine critical load/levelapproaches that can be fully integrated into forest health moni-toring programs in Canada, and to provide reliable measures offorest sustainability. The maintenance of forest ecosystem healthis essential to the sustainability of Canada’s forests and the overallwell-being of the country.

Acknowledgments

The authors would like to thank the following individuals fortheir contribution and critical review of the document:

NRCan/CFS Review

E. AllenNatural Resources Canada, Canadian Forest ServicePacific Forestry Centre506 West Burnside RoadVictoria, British Columbia V8Z 1M5

J. BrandtNatural Resources Canada, Canadian Forest ServiceNorthern Forestry Centre5320-122 StreetEdmonton, Alberta T6H 3S5

R. BoutinNatural Resources Canada, Canadian Forest ServiceLaurentian Forestry Centre1055 du P.E.P.S. StreetSainte-Foy, Quebec G1V 4C7

P. DesRochersNatural Resources Canada, Canadian Forest ServiceLaurentian Forestry Centre1055 du P.E.P.S. StreetSainte-Foy, Quebec G1V 4C7

A. HopkinNatural Resources Canada, Canadian Forest ServiceGreat Lakes Forestry Centre1219 Queen St. East, P.O. Box 490Sault Ste. Marie, Ontario P6A 5M7

Ed HurleyNatural Resources Canada, Canadian Forest ServiceAtlantic Forestry CentreHugh John Fleming Forestry CentreP.O. Box 4000Fredericton, New Brunswick E3B 5P7

Bruce PendrelNatural Resources Canada, Canadian Forest ServiceAtlantic Forestry CentreHugh John Fleming Forestry CentreP.O. Box 4000Fredericton, New Brunswick E3B 5P7

Tom SternerNatural Resources Canada, Canadian Forest Service Atlantic Forestry CentreHugh John Fleming Forestry CentreP.O. Box 4000Fredericton, New Brunswick E3B 5P7

External Review

Neil CapeInstitute of Terrestrial EcologyDusit EstatePenicuik, Lothian, ScotlandUnited Kingdom EH26 0QB

Guy FenechAtmospheric Environment ServiceEnvironment Canada4905 Dufferin St.Downsview, Ontario M3H 5T4

Jean-Pierre MartelCanadian Pulp and Paper AssociationSun Life Building, 19th Floor1155 Metcalfe StreetMontreal, Quebec H3B 4T6

David McLaughlinPhytotoxicology Section, Air Resources BranchMinistry of the Environment and Energy7510 Farmhouse Ct.Brampton, Ontario L6T 5N1

Rock OuimetMinistère des ressourcesDirection de la recherche2700 EinsteinSainte-Foy, Quebec G1P 3W8

M. Germain ParéMinistère des ForêtsService de la recherche appliquée2700 rue EinsteinSainte-Foy, Quebec G1P 3W8

Wally SzumyloINCO LtdCopper Cliff, Ontario P0M 1N0

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