Influences of Climateon Ontario Forests

MICHAEL D. FLANNIGAN AND MICHAEL G. WEBER"

INTRODUCTIONClimate and vegetation are intimately linked (Wood-ward 1987). This linkage is dynamic, because climateis always changing. Climate and its associated weatherinfluence the structure and functioning of vegetationdirectly through such elements as temperature andprecipitation, and indirectly through disturbance andpermafrost. Climate is the total of all statistical weatherinformation that describes the variation in weather ata given place for a specific interval of time (Greer1996). In common usage, climate is the synthesis ofweather; that is, the weather at some location aver-aged over a specified time period, typically 30 years,plus information on the variability and extremes ofweather recorded during the same period.

The factors which control the climate at any onelocation include variations in solar radiation due tolatitude, the distribution of continents and oceans, at-mospheric pressure and wind systems, ocean currents,major terrain features, proximity to waterbodies, andlocal features (see Trewartha and Horn [1980] formore detail). As climate changes, the correspondingweather variables change. Temperature is a good ex-ample. Traditionally, in studies and in documentationof climate, much of the focus has been on changes inthe mean temperature. In terms of the impact of tem-perature on vegetation, however, the variability of tem-perature might be even more important. Specifically,extreme minimum temperatures that drop below -40°Care lethal to many tree species. In addition, unusuallylate frosts in spring or early summer can severely dam-

* Canadian Forest Service, Northern Forest Research CentnAlberta T6H 3S5

** Canadian Forest Service, Great Lakes Forest Research CerSte. Marie, Ontario P6A 5M7

age seedlings. Similar principles apply to other weathervariables, such as precipitation and wind: extremedrought and extreme wind speeds are capable of ex-erting a significant impact on vegetation.

The distribution of vegetation results from the in-teraction of many factors, such as climate, physicalgeography (topography, soil nutrients, and soil drain-age), the sum total of past history, disturbance (natu-ral and anthropogenic), and competition among plantsand among animals. Climate is a key determinant ofspecies presence or absence. The objective of this chap-ter is to examine the influence of climate and its asso-ciated weather on the vegetation of the boreal forestand the Great Lakes-St. Lawrence forest regions (Rowe1972), the biomes which comprise most of the com-mercial forest area in Ontario. We outline how climateinfluenced the development of Ontario's forest veg-etation in the past and describe how climate accountsfor present-day patterns of vegetation distribution. Wediscuss predictions for future vegetation change basedon the use of global climate models and an assump-tion that the atmospheric carbon dioxide will double.We then provide a detailed description of certain di-rect and indirect processes by which climate affectsvegetation. The direct influences described includetemperature and precipitation; the indirect influencesinclude forest pests and diseases, and the presence ofpermafrost in the soil. Throughout the chapter, wediscuss the interaction of climate and other causes offorest change, but we conclude by considering the in-fluence which ve getation itself exerts on climate.

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1 0 4 Michael D. Flannigan and Michael G. WeberI

AN OVERVIEW OF PAST, PRESENT, AND FUTURECLIMATIC EFFECTS ON VEGETATIONOntario is a large, floristically diverse geographic re-gion. The province is characterized by a striking south-north gradient in vegetation cover, from theCarolinian forest in the south, through the GreatLakes-St Lawrence forest and the boreal forest, to theforested barrenland and the tundra in the north. Thispattern is caused, in part, by a north-south gradientin temperature, but there is also a northwest-south-east gradient in moisture (see Figure 2.5 in Baldwinet al. [2000, this volume] ). The climate of Ontario isdiverse, as one might expect given the size of the re-gion. The Great Lakes have a significant influence(Hare and Thomas 1974). Influences are exerted onthe vegetation of Ontario at a number of differentscales in space and time. Woodward (1987) pro-vides an excellent overview of the time scales involvedand the impact of effects at these different scales onvegetation.

In this chapter, we will discuss changes in climateand vegetation during the Holocene period, which isthe most recent geologic epoch of the Quaternary pe-riod, extending from the end of the Pleistocene, ap-proximately 10,800 years ago, to the present. Thisinterval represents the current interglacial period. Wealso address the effects of climate on vegetation atspatial scales ranging from the individual forest stand,to the landscape, to the forest biome. When interpret-ing the influence of climate on vegetation, it is impor-tant to consider the climate and weather in the contextof the life cycle characteristics of individual species.For example, a late spring frost that is not lethal tomature trees of a particular species may be lethal toits seedlings. Such a frost might be harmful to the pro-duction of viable seeds and thus might limit the distri-bution or expansion of the species (e.g., Pigott andHuntley 1978; Black and Bliss 1980). The impact ofclimate on vegetation must, therefore, be examinedfor all stages of the life cycle, including germination,seedling establishment, growth to sexual maturity, andproduction of viable seed. Sensitivities to climate varyby species and also with the developmental stage.

Past Climate and the Establishment of OntarioForest VegetationClimate changes periodically, owing in part to anumber of changes in the earth's orbit. The eccentric-ity of the earth's revolution around the sun has a

105,000-yr cycle; there is a 41,000-year cycle in theobliquity of the earth's axis, and there is a 21,000-year cycle in the precession of the earth's axis aboutthe pole of the ecliptic (that is, the precession of theequinoxes). Milankovitch (1941) stated that the peri-odic or cyclic warming and cooling of the earth's sur-face is caused by these orbital changes. Other factorsplay a role in the natural variation of the climate aswell (see Webb 1992). Discussion in this section isrestricted to changes in climate and vegetation dur-ing the last 10,000 years. Ten thousand years ago,Ontario was still greatly influenced by the continentalice sheet, which covered much of northern Ontario.The climate warmed to a point where it was warmerthan the present day for the period from 7000 to 3000years BP. A general cooling trend has been experi-enced in the last 3000 years, in which there have beenrelatively short periods of warming such as the recentwarming period since the end of the Little Ice Age(about 1850 AD).

The vegetation in Ontario has changed dramaticallyduring the Holocene. Paleoecological evidence sug-gests that boreal tree species such as white spruce(Picea glauca) and jack pine (Pinus banksiana) wereamong the first to appear following the retreat of theglaciers. These pioneer species were quickly followedby black spruce (Picea mariana) and white birch(Betula papyrifera), and then by the poplars (Populusspp.). After the invasion of the boreal species, thewarming climate favoured the development of mixedforests of conifers and deciduous species. The predomi-nant species in these mixed forests included white pine(Pinus strobus), hemlock (Tsuga canadensis), sugarmaple (Acer saccharum) and beech (Fagus spp.)(Ritchie 1987; Liu 1990). These mixed forests spreadfarther north than the present day Great Lakes-St.Lawrence forest limit during the warm period 3000to 7000 years ago, before retreating to the present-day limits during the general cooling trend which hastaken place over the last 3000 years. The abundanceof some key species has changed considerably duringthis time. For example, hemlock showed a markeddecline around 4000 years ago, and has never re-gained its former stature. White pine has also de-creased significantly over the last 1000 years, possiblybecause of the prevalence of cooler and moister con-ditions, which favour spruce. Naturally, there is a greatdeal of regional variation according to site-specificconditions.

Influences of Climate on Ontario Forests 105

Modelling the Effects of Climate ChangeThe present climate of Ontario can be described ashumid continental, except for those areas close to Hud-son Bay that have a more maritime climate. A moredetailed description of Ontario's climate is providedby Baldwin et al. (2000, this volume). The presentvegetation of Ontario is discussed by Thompson (2000,this volume). Hills (1959, 1960) divided the provinceof Ontario into 13 site regions or ecoregions (see Fig-ure 5.1 in Perera and Baldwin [2000, this volume]),based on a qualitative description of climate, soils,topography, and vegetation communities. Rowe(1972) provides a general description of the forestgeography of Canada in terms of forest regions andforest sections. An overview of ecoregionalization ofOntario is provided by Perera and Baldwin (2000, thisvolume). The present climate of Ontario is warming(Gullett and Skinner 1992), and indications are thatthe warming will continue in the next century (Inter-governmental Panel on Climate Change [IPCC] 1996).There is consensus in the scientific community thathuman activities are responsible for recent changes inthe climate (IPCC 1996). Specifically, increases inradiatively active gases, such as carbon dioxide, meth-ane, and the chlorofluorocarbons in the atmosphereare causing a significant warming of the earth's sur-face. Significant increases in temperature are antici-pated in the next century more rapid increases thanhave occurred in the last 10,000 years. Other climaticelements are also expected to change, including pre-cipitation, wind, and cloudiness. More importantly, thevariability of the climate appears to be increasing;therefore, more extreme events such as droughts,floods, major freezing-rain storms, heat waves, andcold snaps might be in store for the next century. Allof these may do serious harm to vegetation.

The use of general circulation models (GCMs) ena-bles researchers to simulate the future climate. Thereare a number of shortcomings associated with GCMs;nevertheless, most models are in agreement in pre-dicting that the greatest warming will occur at highlatitudes and in winter. Significant warming is ex-pected to occur by the middle of the next century, buttemperatures are expected to continue rising beyond2100, even if the atmospheric concentrations of green-house gases are stabilized by that time (IPCC 1996).The confidence is lower for estimates of precipitation,but many models suggest an increase in water stresson vegetation, particularly in the centre of continents.

Many researchers have addressed the topic of cli-mate in relation to vegetation using different types ofmodelling approaches. Most use a biome approach,which relates the current areal extent of biomes tocurrent climate and uses those relationships to pre-dict where the vegetation might be in the future, or atleast to identify the region most climatically suitablefor that biome. Examples of this type of model areprovided in Figure 6.1, which shows the equilibriumpotential of natural vegetation under climate changealready in progress, and Figure 6.2, which shows thepotential distribution of major biomes under predictedclimate change, defined by the Mapped Atmosphere-Plant-Soil System (MAPSS) model (Neilson 1993). Thepresent climate is provided by the climate database ofthe International Institute for Applied Systems Analy-sis (IIASA) (Leemans and Cramer 1991), while thefuture climate is derived from the difference betweenthe control run and a scenario of carbon dioxide dou-bling from the GCM of the Geophysical Fluid Dynam-ics Laboratory, termed the GFDL model (Weatheraldand Manabe 1986), with aerosols included. The pro-jected shifts in the boundaries of vegetation classesare generated by a model that simulates steady-stateleaf-area index, calculated from a sub-model of sitewater and heat balance (Neilson 1993). Figure 6.1 issimilar to Figure 3.1 in Thompson 2000 (this volume),which shows the present vegetation in Ontario (seealso Olson et al. 1983), and also to Figure 5.1 in Pereraand Baldwin (2000, this volume), except that theMAPSS model does not reflect the northern Ontariowetlands. One main difference between Figure 6.1 andFigure 3.1 (Thompson 2000, this volume) is that theCarolinian and Great Lakes-St. Lawrence forests arecombined in Figure 6.1.

Striking differences are obvious between Figures 6.1and 6.2, which show the equilibrium potential of natu-ral vegetation now and in the future. Figure 6.2 de-picts the savanna-woodland forest type as extendingover most of southern and eastern Ontario and de-picts the temperate mixed forest as moving north toJames Bay, or approximately 500 km north of itspresent-day limit. Many other models exist, based ona variety of GCMs , so many potential outcomes havebeen derived. For example, Warrick et al. (1986) usea Holdridge life-zone classification (Holdridge 1947)with the GFDL model, and suggest that the potentialvegetation would be temperate forest over all of On-tario, except for a narrow band of boreal forest along

1 06 Michael D. Flannigan and Michael G. Weber

Hudson Bay. Box (1981) relates vegetation to a num-ber of meteorological variables and uses these relation-ships to determine new patterns of vegetation underthe climate regime resulting from a doubling of at-mospheric carbon dioxide. Rizzo and Wiken (1992)apply a classification model derived from the currentecological setting to simulate the effects of climatechange from carbon dioxide doubling on Canada'secosystems. For additional information on simulatedchanges in vegetation distribution under global warm-ing see Appendix C in IPCC (1998).

There are numerous caveats to the use of modelsof this kind, in addition to the caveats associated withthe GCMs themselves. Most models use biomes andmove the vegetation as a community. We know thatthis result cannot be accurate, because vegetation isan assemblage of different species in which each spe-cies is distributed according to its own physiologicalrequirements, as constrained by competitive inter-actions (Gleason 1926). Species of vegetation moveas individuals, not as a community (Whitney 1986;Davis 1989). The issue is further complicated by dis-turbance, which plays a major role in determiningthe abundance and distribution of individual species(Flannigan 1993; Suffling 1995; Bergeron et al. 1997)and is not fully incorporated into these models. Cau-tion is also advised when interpreting results fromphysiological models because of the inherent prob-lems involved in scaling up from a leaf or a tree to astand, and eventually to a continental scale (Colemanet al. 1992). Finally, these models display regionswhere the climate is instantaneously suitable for thevarious vegetation types; however, the time requiredfor the vegetation to come into equilibrium with theprojection could take centuries, as determined by mi-gration rates, competition, and altered disturbanceregimes.

The Impact of Climate Change onOntario's VegetationAs we have already seen in Figures 6.1 and 6.2, mod-els suggest that the climate suitable for the majorbiomes in Ontario will shift northwards by 500 km ormore by the end of the next century. Paleoecologicalstudies have shown, however, that maximum rates ofmigration are much less than would be required forthe vegetation to keep pace with projected climatechange (Prentice et al. 1991; Webb and Bartlein 1992).These maximum rates are, if anything, greater than

can be expected in the future, as they represent mi-gration over a recently deglaciated landscape. The ex-isting forests in the transition zone between forest andgrassland will not necessarily be rapidly replaced bygrassland.

Another factor which might slow down the antici-pated vegetation transition is a decrease in disturbanceregimes that might be associated with climate warm-ing. For example, Bergeron and Archambault (1993)have shown for a region near Lake Abitibi in Quebecthat the fire frequency has decreased since the end ofthe Little Ice Age despite temperature increases ofmore than 1°C over the same period, because of in-creased precipitation frequency. Modelling results fromFlannigan et al. (1998) suggest that fire weather se-verity will decrease in portions of eastern Canada witha doubling of atmospheric carbon dioxide, becauseincreased precipitation in the warmer climate willmore than compensate for the increase in tempera-ture. Decreased disturbance in the Claybelt region ofOntario might lead to an increased abundance of bal-sam fir (Abies balsamea) and cedar (Thuja occidentalis)because of their shade tolerance. These species wouldbe difficult to replace with southern competitors, notonly because of their shade tolerance, but also becausedecreased disturbance rates would mean smaller andfewer areas for the southern competitors to exploit.In regions where disturbances from fire, insect pests,and disease increase, the transition of the vegetationassemblages to the adjacent types may be accelerated(Suffling 1995). The vegetation changes associatedwith the new climate may lead, moreover, to new as-semblages of species (Martin 1993). Competition maybe a key factor in defining the vegetation composi-tion. Bonan and Sirois (1992) have suggested that thesouthern limit of black spruce is dictated by competi-tion rather than climate, as black spruce is at its opti-mum climate for growth at its present-day southernlimit. Thompson et al. (1998) present an overview ofpossible changes to Ontario's forested landscapes as aresult of climate change.

Increases in climate variability under a new climatecould have major impacts on the vegetation of On-tario (Mearns et al. 1989; Solomon and Leemans1997). Models have suggested that synoptic stormfrequency would decrease in the long term, but thatthere would be an increase in the overall intensity ofdisturbances (Lambert 1995). In the next century,there may thus be fewer storms, but more extreme

p

Land Cover Types

Boreal Conifer Forest

Temperate Mixed Forest

it Temperate Evergreen Forest

El Shrub/Woodland

"'Savanna/WoodlandGrasslands

Arid Lands

Taiga/Tundra

Tundra

0 300 600

Kilometres

. . ;

Influences of Climate on Ontario Forests 107

Figure 6.1 The distribution of major biome types as simulated under currentclimate change by the Mapped Atmosphere-Plant-Soil System (MAPSS) model.(Adapted from Neilson and Drapek 1998)

weather (for example, extreme wind speeds or veryheavy precipitation causing flooding). Research hasalso suggested that the persistence of blocking ridgesin the upper atmosphere will increase in a climatescenario of doubled carbon dioxide (Lupo et al. 1997).This factor could have significant impact on forestfires, as these upper ridges are associated with dryand warm conditions at the earth's surface that areconducive to forest fires. Extreme environmental con-ditions caused by prolonged drought, floods, extremeheat, extreme cold, and the increased occurrence ofsevere winds, can be expected to have a negative in-fluence on forest health. These environmental stressespredispose individual plants, species, and ecosystemsto secondary stressors, such as outbreaks of insectinfestation and disease. Research has shown that re-sistance to drought increases with increased carbondioxide (Townend 1993). Recent research has alsosuggested that increased carbon dioxide may lead toincreased tolerance of cold temperatures (Boese etal. 1997).

The anticipated changes in climate will have sig-nificant impacts on physiological processes and the

cycling of nutrients. The global atmospheric concen-tration of carbon dioxide has risen from pre-industriallevels of 280 parts per million by volume (ppmv) to360 ppmv in 1994 (Amthor 1995). Plants and eco-systems are closely coupled with nitrogen and car-bon cycles, which might be altered by the elevatedcarbon dioxide and by climate change. The nitrogenand carbon cycles are closely linked (Reynolds et al.1996) through decomposition and litter quality. Tem-perature increases will greatly influence decomposi-tion and nutrient cycling (Anderson 1992).Historically, the boreal forest has been presumed tobe a carbon sink in the global carbon budget. Thiscarbon sink likely will be reduced under climatechange (Kurz and Apps 1993; Kurz et al. 1995), ormay even become a carbon source. Increased tem-peratures will lead to an increase in soil temperatureand an associated increase in the active layer overpermafrost. Improved soil drainage as a result of soilwarming, especially at northern latitudes, is an im-portant consideration, because of the implications fororganic layer drying, and hence fire severity(Anderson 1992).

Land Cover Types

Boreal Conifer Forest

Temperate Mixed Forest

Temperate Evergreen Forest

Shrub/Woodland

Savanna/Woodland

q Grasslands

IN Arid Lands

11 Taiga/Tundra

Tundra

0 300 6001n 1n 1

Kilometres

•108 Michael D. Flannigan and Michael G. Weber

Figure 6.2 The potential distribution of major biomes as simulated under the GeophysicalFluid Dynamics Laboratory (GFDL) Global Climate Model, with aerosols included, by MAPSS.(Adapted from Neilson and Drapek 1998)

PROCESSES OF CLIMATE INFLUENCE ONVEGETATIONWeather variables such as temperature, precipitation,and wind have a direct influence on vegetation in termsof growth, mortality, species abundance, and compo-sition. Weather also exerts an indirect influence onvegetation through such factors as forest fires, pestand disease outbreaks, and the presence or absenceof permafrost.

Direct Effects of Climate

Influences of Temperature on VegetationVarious aspects of temperature can have a significantimpact on vegetation. These include winter minimumtemperature, frost during the growing season, andwarmth during the growing season.

Winter minimum temperatures are important in de-termining the distribution of tree species. Many stud-ies suggest that the poleward limit of a tree species iscontrolled by the minimum winter temperature that

is regularly experienced (Sakai and Weiser 1973;George et al. 1974; Sakai 1978; Larcher and Bauer1981; Woodward 1987; and Arris and Eagleson 1989).In Ontario, this is probably true for most, if not all, ofthe non-boreal, deciduous tree species. Most decidu-ous species cannot tolerate temperatures below -30°Cto -40°C, the limit of the strategy which they use tosurvive freezing temperatures. There are three stand-ard strategies that plants use to survive freezing tem-peratures: deep supercooling, extracellular freezing,and extraorgan freezing (Sakai and Larcher 1987;Woodward and Williams 1987). Deep supercoolingallows water in the plant cells to remain liquid de-spite temperatures well below 0°C, owing to a lack ofice nucleation sites. As long as ice does not form withinthe cell, there is no mechanical damage. Typically, thecoldest temperature that plants can survive using deepsupercooling for pure water is about -40°C. Survivalat lower temperatures (to about -55°C) using deepsupercooling is possible only in the presence of highconcentrations of solutes in the cell water (Gusta et

• Influences of Climate on Ontario Forests 109

al. 1983). Most deciduous tree species, except thebirches (Betula spp.) and poplars, use deep super-cooling and typically cannot tolerate temperaturesbelow -40°C.

Extracellular and extraorgan freezing occurs afterthe migration of water out of the plant cells or or-gans and into intercellular spaces, where freezing canusually occur without damage. The intercellularspaces are usually large enough to accommodate theinflux of water and the expansion associated withthe phase change from liquid to solid, without dam-age to the surrounding cells and organs. The survivalof plants using extracellular and extraorgan strate-gies is limited by the extent to which the plant canwithstand extreme dehydration of the cell or organcaused by the outward migration of water from cells(Sakai 1979), which results in desiccation. The borealconifers use extracellular or extraorgan strategies andcan survive temperatures of -70° to -80°C, which iscolder than anything experienced in Ontario, andthese conifers are not limited by extreme minimumtemperatures.

During the growing season, tree species, and in par-ticular seedlings, are not frost-hardy, so that tempera-tures of -2°C to -5°C can be lethal. Growing-seasonfrost can also damage reproductive structures. Femaleconifer flowers and conelets are particularly suscepti-ble to frost damage in early spring, which can limitseed production (Schooley et al. 1986). Frost can alsodamage other parts of the tree, including the stem,bud, and root collar, and can cause leaf and needledamage. If the initial damage from temperatures be-low freezing is not lethal, then the damaged areas of-ten become sites of infection by canker and otherdiseases, or become susceptible to insect attack(Hiratsuka and Zalasky 1993). Growing-season frostcan be critical in plantations, especially where topog-raphy creates low-lying areas (Stathers 1989). Stud-ies of frost hardiness have been conducted on manyconiferous species found in Ontario (Glerum et al.1966; Glerum 1973; Joyce 1987). Results suggest thatthere is little difference in frost-hardiness betweenthose conifer species (Glerum 1973).

Growing-season warmth can also be an importantdeterminant in vegetation distribution. For example,Black and Bliss (1980) found that the northern limitof black spruce was determined by the summerwarmth required for seed germination. Pigott andHuntley (1978) found that insufficient warmth at the

northern limit of small-leaf linden (Tilia cordata)during the flowering period resulted in non-viableseed. For most tree species, there is a critical tempera-ture that needs to be exceeded for growth to begin.The growing-degree concept was developed from thefact that many grasses require temperatures of 5°Cor higher for growth to occur. For some trees, such asred pine (Pinus resinosa), the critical temperature forinitiating and maintaining growth is 10°C; therefore,if summer mean annual temperatures did not exceed10°C, it would be unlikely that red pine could remainestablished in such a climate. Summer warmth is criti-cal in plantations, where site treatments such asmounding have serious micro-meteorological impli-cations on the local thermal regime (Spittlehouse andStathers 1990). McCaughey et al. (1997) provide agood overview of the weather and climate associatedwith Canadian forests.

Influences of Precipitation on VegetationThe lack of precipitation, if prolonged, results indrought that can damage or kill trees. Drought can berestricted to one growing season or may persist forseveral growing seasons. If drought is severe enough,leaf abscission will occur. Summer drought is differ-ent from winter desiccation. Drought is caused by in-adequate soil moisture; whereas desiccation occurswhen soil moisture is unavailable because the groundis frozen. Drought-stressed trees are prone to attacksfrom insects and diseases. On the other hand, if theprecipitation is too heavy, flooding can occur and causeextensive damage in low-lying areas.

Freezing rain and heavy snow can accumulate onthe vegetation to such an extent that the added weighton the foliage and branches causes physical damage.The build-up of snow and ice is influenced by standdensity and the shape of the crown. This is a commoncause of damage in plantations (Powers and Oliver1970). The amount of damage can be significant; thereare reports of more than 20 percent of stems brokenin a stand (Van Cleve and Zasada 1970). When treesare laden with a coating of ice, they are more proneto windthrow. A severe freezing rain event in January1998 damaged millions of trees in eastern Ontario andsouthern Quebec. Hail also can cause extensive dam-age to vegetation (Riley 1953; Laut and Elliot 1966).Seedlings and saplings are especially prone to dam-age; whereas mature trees typically sustain only mi-nor damage. As with other types of physical damage,

110 Michael D. Flannigan and Michael G. Weber •

the parts of the trees damaged are potential sites ofinfection by pathogens.

Indirect Effects of Climate

Climatic Aspects of the Influence of Insectsand Disease on VegetationClimate and weather play a major role in the life cycleof many forest insects, some of which have a majorinfluence on forest productivity (Fleming and Volney1995). Additionally, climate and weather can be im-portant in disease contraction and spread. If climatechanges, as the GCMs suggest, the greatest impact ofclimate change on the structure and function of theboreal forest will be mediated through changes in dis-turbance regimes such as insect outbreaks and fire.Discussion of a large number of insect defoliators isbeyond the scope of this chapter, so the sprucebudworm (Choristoneura fumiferana) is chosen as arepresentative species. Fleming et al. (2000, this vol-ume) provide a detailed description of the effects ofvarious insect pests and forest diseases on Ontario'sforest landscapes.

Fleming (1996) reviewed the possible influences ofclimate change on defoliating insects in North Ameri-ca's boreal forests and outlined the interrelationshipsamong climate, vegetation, and insect populations.The direct influence of climate on vegetation may havea secondary impact on insect populations. Climate in-fluences the synchrony of host plant phenology withspruce budworm development as well as the synchronywith natural invertebrate enemies. Finally, weather el-ements such as drought and late-spring frost may havea direct impact on spruce budworm populations; infact, Cerezke and Volney (1995) suggest that late-spring frosts coincide with the collapse of the sprucebudworm outbreak. Spruce budworm is only one ex-ample of the many types of insects that influence theforest, but that work does highlight the complex inter-actions and feedbacks among vegetation host, climate/weather, and natural enemies. As the climate andweather change, non-linear and perhaps unexpectedinteractions may have devastating effects, allowinginsects to become an additional agent of acceleratedchange in the forests.

Climate directly influences vegetation, its pathogens,and its insects, including pathogen vectors. The rela-tionships between weather and tree diseases have beenstudied for many years (Hepting 1963). So-called "for-

est declines" (Manion 1981) may be a result of aninteraction between climate and disease. For exam-ple, red spruce (Picea rubens) decline consists of aninteraction between winter injury and air pollution,which allows pathogenic fungi such as Cytospora sp.,Fames sp., Artnillaria sp., needle-cast diseases, rust dis-eases, and several other butt-rot and stem-rot fungi(Johnson 1992) to injure or kill the tree. Coakley(1988) suggests that a change in climatic conditionsor a change in climatic variability may alter plant dis-ease development by affecting the following factors:(1) the speed of pathogen development; (2) the geo-graphical range of the host, pathogen, or vector, espe-cially at the boundaries of their respective distributions;and (3) control of the disease. Predicting the impactof climate change on forest diseases is made more com-plicated by the need to take into account the interac-tions among climate, pathogens, and insect vectors ofthe pathogens, but it is clear that, with warming, thepotential for rapid outbreaks of forest disease acrossOntario is a real threat.

Influences of Permafrost on VegetationIn some northern Ontario forested landscapes, perma-frost is an important agent, exerting control over for-est ecosystem structure and function. Although ofconcern only locally, permafrost is a terrain featurethat may be of concern to ecosystem managers chargedwith maintaining the integrity of Ontario's northern-most areas. The terrain sensitivity of landscapes under-lain by permafrost must be considered in planning bothcommercial and non-commercial northern develop-ment activities, such as the construction of roads, set-tlements, or fire-guards. According to Brown (1973),continuous permafrost underlies only a narrow, tree-less band along the Hudson Bay coast of northernOntario. Discontinuous permafrost, consisting of scat-tered islands of permanently frozen ground, each afew square metres to several hectares in size, occursmainly in peatlands. Other areas where discontinu-ous permafrost may be encountered are on north-fac-ing slopes of east-west oriented valleys, or alongisolated patches of forested stream-banks, where in-creased shading reduces summer thaw and wintersnow cover (Brown 1973).

The southern limit of discontinuous permafrost inOntario lies at about latitude 51°N, to 52°N aroundJames Bay and coincides with the mean annual airtemperature isotherm of -10°C. The area occupied by

• Influences of Climate on Ontario Forests 111

discontinuous permafrost, also known as the HudsonBay Lowland physiographic region, contains the north-ern limits of all boreal forest tree species in Ontarioand is characterized by a fire-dominated disturbanceregime.

The impact of potential climate change on the north-ern Ontario boreal forest of the Hudson Bay Lowlandmay be envisaged from simulation studies carried outfor other parts of the North American boreal forest,where permafrost and fire interact to dominate forestecosystem structure and function. An example hasbeen provided by Bonan et al. (1990) for interiorAlaska. Their simulations assumed climate changescenarios of warming by 1°C, 3°C, and 5°C, factoriallycoupled with increases of 120 percent, 140 percent,and 160 percent in monthly precipitation values. Toemphasize the importance of site conditions in re-sponse to expected climate change, the simulationswere performed for two contrasting forest types:a black spruce (Picea tnariana) forest growing on apermafrost-dominated, poorly drained, north-facingslope, and a forest of white spruce, paper birch, andaspen located on a well-drained, permafrost-free,south-facing slope. According to these simulations, theeffects of climatic warming on ecosystem structure andfunction in the northern boreal forest may not be somuch a direct response to increased air temperatureas to increased potential evapotranspiration demands.Analysis of their simulation results also revealed theimportance of the forest floor organic layers in con-trolling ecosystem response to climatic warming. Forexample, the thick forest floor layer of 20 cm to 30cm typical of many black spruce forests in interiorAlaska and elsewhere is the major factor responsiblefor cold, wet soil conditions which restrict nutrientavailability and tree growth (Weber and Van Cleve1981, 1984).

In the absence of fire, the short-term response ofthese permafrost-dominated sites to climate warmingwas a decrease in the depth of the active soil layer(that is, the layer of soil lying above the permafrostthat thaws out annually in response to summer warm-ing). This decrease occurred from a drying of the for-est floor, which impeded the conduction of heat intodeeper soil layers. In the long term, however, withrecurrent forest fires, the drier organic layers wereconducive to increased fire severity, and thus to theremoval of greater amounts of forest floor material.As a result, the depth of the active layer increased,

and soil drainage further improved (Bonan 1989;Bonan et al. 1990). The complete elimination of shal-low, discontinuous permafrost would be a possiblescenario under these conditions. The final outcome ofthis simulation run was the fire-caused conversion ofthe low-productivity black spruce forests to mixed for-ests of spruce and hardwood growing on warmer soils.In contrast, on the well-drained, south-facing spruceand hardwood forest sites, increased potential waterloss in the warmer climate reduced soil moisture andresulted in the site-conversion of these stands to dryaspen forests. The greatest simulated reduction in soilmoisture resulted in steppe-like vegetation and anelimination of the tree overstory on these sites. Bonanet al. (1990) thus highlighted the sensitivity of diver-gent forest ecosystems to water balance and to its inter-action with the fire regime under climate change(Weber and Flannigan 1997).

The Influence of Vegetation on ClimateThe link between climate and vegetation is wellknown, but the reverse link is not as well known. Thelink between vegetation and climate is found at allscales, from microscales to the global scale. At smallerspatial scales, differences in temperature, wind, andrelative humidity would be expected to exist betweenan agricultural field and an adjacent forest stand, be-cause of differences in the energy budget between thetwo areas. At larger scales, for example, across theentire boreal forest biome, the influence of vegeta-tion on the climate can be significant. From using theGCM of the United Kingdom Meteorological Office(UKMO), Thomas and Rowntree (1992) suggest that,in the absence of boreal forests, northern hemispheretemperatures would be 2.8°C cooler and precipitationwould decrease. These changes would result from thedifference in albedo between the forest and non-for-est vegetation, especially in winter, as the albedo ofsnow is particularly high. (Albedo is the amount ofelectromagnetic radiation reflected by a body relativeto the amount incident upon it, and is commonly ex-pressed as a percentage [Greer 1996].) Also using aGCM, Bonan et al. (1992) suggest that, if tundra orbare ground replaced the boreal forest, the climate ofthe entire northern hemisphere would be significantlycooler, and that latent heat flux and atmospheric mois-ture would increase. The warming effect of the borealforest consists of masking the high reflectance of snowover vast areas of the northern hemisphere. Other

112 Michael D. Flannigan and Michael G. Weber a

researchers (Otterman et al. 1984; Crowley and Baum1997) confirm that vegetation does play a significantrole in regional to global temperature and precipita-tion patterns. Foley et al. (1994) argue that the inter-action of vegetation with climate was operating duringthe Holocene and gave rise to large positive feedbackbetween the climate and the boreal forests, whichresulted in warmer temperatures in the northernhemisphere.

SUMMARYClimate and vegetation interact across the range ofspatial and temporal scales in a complex fashion. Cli-mate determines the suite of species that is availableto colonize the landscape. The actual vegetationpresent over the landscape is the result of many fac-tors among which climate is of primary importance.Climate exerts direct control over vegetation througheither beneficial or deleterious effects of temperature,precipitation, and wind, and indirect control throughclimatic influences on fire and insect disturbances,disease, and soil properties such as permafrost, which,in turn, influence vegetation.

Across the province of Ontario, there are large north-south and northwest-southeast climatic gradients intemperature and precipitation, respectively, which giverise to a great diversity of vegetation types. As cli-mate changes, so does the vegetation, although at aslower pace. Should the climate continue to warm,dramatic change in the forests of Ontario can be ex-pected, especially if the climate changes as rapidly asthe global climate models suggest. The interactionbetween climate change and disturbance regimes hasthe potential to overshadow the importance of thedirect effects of global warming on species distribu-tion, migration, substitution, and extinction. Distur-bance could thus be the most effective agent of change,and the rate and magnitude of disturbance-inducedchanges to the forested landscape of Ontario couldgreatly exceed anything caused by atmospheric warm-ing alone.

ACKNOWLEDGEMENTSWe thank Ron Neilson and Ray Drapek for providing Figures 6.1 and6.2. Thanks also go to Mike Wotton, who helped prepare those fig-ures for this chapter.

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