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Influences of Climate on Ontario Forests MICHAEL D. FLANNIGAN AND MICHAEL G. WEBER" INTRODUCTION Climate and vegetation are intimately linked (Woodward 1987). This linkage is dynamic, because climate is always changing. Climate and its associated weather influence the structure and functioning of vegetation directly through such elements as temperature and precipitation, and indirectly through disturbance and permafrost. Climate is the total of all statistical weather information that describes the variation in weather at a given place for a specific interval of time (Greer 1996). In common usage, climate is the synthesis of weather; that is, the weather at some location averaged over a specified time period, typically 30 years, plus information on the variability and extremes of weather recorded during the same period. The factors which control the climate at any one location include variations in solar radiation due to latitude, the distribution of continents and oceans, atmospheric pressure and wind systems, ocean currents, major terrain features, proximity to waterbodies, and local features (see Trewartha and Horn [1980] for more detail). As climate changes, the corresponding weather variables change. Temperature is a good example. Traditionally, in studies and in documentation of climate, much of the focus has been on changes in the mean temperature. In terms of the impact of temperature on vegetation, however, the variability of temperature might be even more important. Specifically, extreme minimum temperatures that drop below -40°C are lethal to many tree species. In addition, unusually late frosts in spring or early summer can severely dam* Canadian Forest Service, Northern Forest Research Centn Alberta T6H 3S5 ** Canadian Forest Service, Great Lakes Forest Research Cer Ste. Marie, Ontario P6A 5M7 age seedlings. Similar principles apply to other weather variables, such as precipitation and wind: extreme drought and extreme wind speeds are capable of exerting a significant impact on vegetation. The distribution of vegetation results from the interaction of many factors, such as climate, physical geography (topography, soil nutrients, and soil drainage), the sum total of past history, disturbance (natural and anthropogenic), and competition among plants and among animals. Climate is a key determinant of species presence or absence. The objective of this chapter is to examine the influence of climate and its associated weather on the vegetation of the boreal forest and the Great Lakes-St. Lawrence forest regions (Rowe 1972), the biomes which comprise most of the commercial forest area in Ontario. We outline how climate influenced the development of Ontario's forest vegetation in the past and describe how climate accounts for present-day patterns of vegetation distribution. We discuss predictions for future vegetation change based on the use of global climate models and an assumption that the atmospheric carbon dioxide will double. We then provide a detailed description of certain direct and indirect processes by which climate affects vegetation. The direct influences described include temperature and precipitation; the indirect influences include forest pests and diseases, and the presence of permafrost in the soil. Throughout the chapter, we discuss the interaction of climate and other causes of forest change, but we conclude by considering the influence which ve getation itself exerts on climate. FILE COPY RETURN TO: PUBLICATIONS NORTHERN FORESTRY CENTRE 5320 - 122 STREET EDMONTON, ALBERTA T6H 3S5 1 0 4 Michael D. Flannigan and Michael G. Weber AN OVERVIEW OF PAST, PRESENT, AND FUTURE CLIMATIC EFFECTS ON VEGETATION Ontario is a large, floristically diverse geographic region. The province is characterized by a striking southnorth gradient in vegetation cover, from the Carolinian forest in the south, through the Great Lakes-St Lawrence forest and the boreal forest, to the forested barrenland and the tundra in the north. This pattern is caused, in part, by a north-south gradient in temperature, but there is also a northwest-southeast gradient in moisture (see Figure 2.5 in Baldwin et al. [2000, this volume] ). The climate of Ontario is diverse, as one might expect given the size of the region. The Great Lakes have a significant influence (Hare and Thomas 1974). Influences are exerted on the vegetation of Ontario at a number of different scales in space and time. Woodward (1987) provides an excellent overview of the time scales involved and the impact of effects at these different scales on vegetation. In this chapter, we will discuss changes in climate and vegetation during the Holocene period, which is the most recent geologic epoch of the Quaternary period, extending from the end of the Pleistocene, approximately 10,800 years ago, to the present. This interval represents the current interglacial period. We also address the effects of climate on vegetation at spatial scales ranging from the individual forest stand, to the landscape, to the forest biome. When interpreting the influence of climate on vegetation, it is important to consider the climate and weather in the context of the life cycle characteristics of individual species. For example, a late spring frost that is not lethal to mature trees of a particular species may be lethal to its seedlings. Such a frost might be harmful to the production of viable seeds and thus might limit the distribution or expansion of the species (e.g., Pigott and Huntley 1978; Black and Bliss 1980). The impact of climate on vegetation must, therefore, be examined for all stages of the life cycle, including germination, seedling establishment, growth to sexual maturity, and production of viable seed. Sensitivities to climate vary by species and also with the developmental stage. Past Climate and the Establishment of Ontario Forest Vegetation Climate changes periodically, owing in part to a number of changes in the earth's orbit. The eccentricity of the earth's revolution around the sun has a I 105,000-yr cycle; there is a 41,000-year cycle in the obliquity of the earth's axis, and there is a 21,000year cycle in the precession of the earth's axis about the pole of the ecliptic (that is, the precession of the equinoxes). Milankovitch (1941) stated that the periodic or cyclic warming and cooling of the earth's surface is caused by these orbital changes. Other factors play a role in the natural variation of the climate as well (see Webb 1992). Discussion in this section is restricted to changes in climate and vegetation during the last 10,000 years. Ten thousand years ago, Ontario was still greatly influenced by the continental ice sheet, which covered much of northern Ontario. The climate warmed to a point where it was warmer than the present day for the period from 7000 to 3000 years BP. A general cooling trend has been experienced in the last 3000 years, in which there have been relatively short periods of warming such as the recent warming period since the end of the Little Ice Age (about 1850 AD). The vegetation in Ontario has changed dramatically during the Holocene. Paleoecological evidence suggests that boreal tree species such as white spruce (Picea glauca) and jack pine (Pinus banksiana) were among the first to appear following the retreat of the glaciers. These pioneer species were quickly followed by black spruce (Picea mariana) and white birch (Betula papyrifera), and then by the poplars (Populus spp.). After the invasion of the boreal species, the warming climate favoured the development of mixed forests of conifers and deciduous species. The predominant species in these mixed forests included white pine (Pinus strobus), hemlock (Tsuga canadensis), sugar maple (Acer saccharum) and beech (Fagus spp.) (Ritchie 1987; Liu 1990). These mixed forests spread farther north than the present day Great Lakes-St. Lawrence forest limit during the warm period 3000 to 7000 years ago, before retreating to the presentday limits during the general cooling trend which has taken place over the last 3000 years. The abundance of some key species has changed considerably during this time. For example, hemlock showed a marked decline around 4000 years ago, and has never regained its former stature. White pine has also decreased significantly over the last 1000 years, possibly because of the prevalence of cooler and moister conditions, which favour spruce. Naturally, there is a great deal of regional variation according to site-specific conditions. Influences of Climate on Ontario Forests 105 Modelling the Effects of Climate Change The present climate of Ontario can be described as humid continental, except for those areas close to Hudson Bay that have a more maritime climate. A more detailed description of Ontario's climate is provided by Baldwin et al. (2000, this volume). The present vegetation of Ontario is discussed by Thompson (2000, this volume). Hills (1959, 1960) divided the province of Ontario into 13 site regions or ecoregions (see Figure 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 forest geography of Canada in terms of forest regions and forest sections. An overview of ecoregionalization of Ontario is provided by Perera and Baldwin (2000, this volume). The present climate of Ontario is warming (Gullett and Skinner 1992), and indications are that the warming will continue in the next century (Intergovernmental Panel on Climate Change [IPCC] 1996). There is consensus in the scientific community that human activities are responsible for recent changes in the climate (IPCC 1996). Specifically, increases in radiatively active gases, such as carbon dioxide, methane, and the chlorofluorocarbons in the atmosphere are causing a significant warming of the earth's surface. Significant increases in temperature are anticipated in the next century more rapid increases than have occurred in the last 10,000 years. Other climatic elements are also expected to change, including precipitation, wind, and cloudiness. More importantly, the variability of the climate appears to be increasing; therefore, more extreme events such as droughts, floods, major freezing-rain storms, heat waves, and cold snaps might be in store for the next century. All of these may do serious harm to vegetation. The use of general circulation models (GCMs) enables researchers to simulate the future climate. There are a number of shortcomings associated with GCMs; nevertheless, most models are in agreement in predicting that the greatest warming will occur at high latitudes and in winter. Significant warming is expected to occur by the middle of the next century, but temperatures are expected to continue rising beyond 2100, even if the atmospheric concentrations of greenhouse gases are stabilized by that time (IPCC 1996). The confidence is lower for estimates of precipitation, but many models suggest an increase in water stress on vegetation, particularly in the centre of continents. Many researchers have addressed the topic of climate in relation to vegetation using different types of modelling approaches. Most use a biome approach, which relates the current areal extent of biomes to current climate and uses those relationships to predict where the vegetation might be in the future, or at least to identify the region most climatically suitable for that biome. Examples of this type of model are provided in Figure 6.1, which shows the equilibrium potential of natural vegetation under climate change already in progress, and Figure 6.2, which shows the potential distribution of major biomes under predicted climate change, defined by the Mapped AtmospherePlant-Soil System (MAPSS) model (Neilson 1993). The present climate is provided by the climate database of the International Institute for Applied Systems Analysis (IIASA) (Leemans and Cramer 1991), while the future climate is derived from the difference between the control run and a scenario of carbon dioxide doubling from the GCM of the Geophysical Fluid Dynamics Laboratory, termed the GFDL model (Weatherald and Manabe 1986), with aerosols included. The projected shifts in the boundaries of vegetation classes are generated by a model that simulates steady-state leaf-area index, calculated from a sub-model of site water and heat balance (Neilson 1993). Figure 6.1 is similar to Figure 3.1 in Thompson 2000 (this volume), which shows the present vegetation in Ontario (see also Olson et al. 1983), and also to Figure 5.1 in Perera and Baldwin (2000, this volume), except that the MAPSS model does not reflect the northern Ontario wetlands. One main difference between Figure 6.1 and Figure 3.1 (Thompson 2000, this volume) is that the Carolinian and Great Lakes-St. Lawrence forests are combined in Figure 6.1. Striking differences are obvious between Figures 6.1 and 6.2, which show the equilibrium potential of natural vegetation now and in the future. Figure 6.2 depicts the savanna-woodland forest type as extending over most of southern and eastern Ontario and depicts the temperate mixed forest as moving north to James Bay, or approximately 500 km north of its present-day limit. Many other models exist, based on a variety of GCMs , so many potential outcomes have been derived. For example, Warrick et al. (1986) use a Holdridge life-zone classification (Holdridge 1947) with the GFDL model, and suggest that the potential vegetation would be temperate forest over all of Ontario, 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 number of meteorological variables and uses these relationships to determine new patterns of vegetation under the climate regime resulting from a doubling of atmospheric carbon dioxide. Rizzo and Wiken (1992) apply a classification model derived from the current ecological setting to simulate the effects of climate change from carbon dioxide doubling on Canada's ecosystems. For additional information on simulated changes in vegetation distribution under global warming see Appendix C in IPCC (1998). There are numerous caveats to the use of models of this kind, in addition to the caveats associated with the GCMs themselves. Most models use biomes and move the vegetation as a community. We know that this result cannot be accurate, because vegetation is an assemblage of different species in which each species is distributed according to its own physiological requirements, as constrained by competitive interactions (Gleason 1926). Species of vegetation move as individuals, not as a community (Whitney 1986; Davis 1989). The issue is further complicated by disturbance, which plays a major role in determining the abundance and distribution of individual species (Flannigan 1993; Suffling 1995; Bergeron et al. 1997) and is not fully incorporated into these models. Caution is also advised when interpreting results from physiological models because of the inherent problems involved in scaling up from a leaf or a tree to a stand, and eventually to a continental scale (Coleman et al. 1992). Finally, these models display regions where the climate is instantaneously suitable for the various vegetation types; however, the time required for the vegetation to come into equilibrium with the projection could take centuries, as determined by migration rates, competition, and altered disturbance regimes. The Impact of Climate Change on Ontario's Vegetation As we have already seen in Figures 6.1 and 6.2, models suggest that the climate suitable for the major biomes in Ontario will shift northwards by 500 km or more by the end of the next century. Paleoecological studies have shown, however, that maximum rates of migration are much less than would be required for the vegetation to keep pace with projected climate change (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 migration over a recently deglaciated landscape. The existing forests in the transition zone between forest and grassland will not necessarily be rapidly replaced by grassland. Another factor which might slow down the anticipated vegetation transition is a decrease in disturbance regimes that might be associated with climate warming. For example, Bergeron and Archambault (1993) have shown for a region near Lake Abitibi in Quebec that the fire frequency has decreased since the end of the Little Ice Age despite temperature increases of more than 1°C over the same period, because of increased precipitation frequency. Modelling results from Flannigan et al. (1998) suggest that fire weather severity will decrease in portions of eastern Canada with a doubling of atmospheric carbon dioxide, because increased precipitation in the warmer climate will more than compensate for the increase in temperature. Decreased disturbance in the Claybelt region of Ontario might lead to an increased abundance of balsam fir (Abies balsamea) and cedar (Thuja occidentalis) because of their shade tolerance. These species would be difficult to replace with southern competitors, not only because of their shade tolerance, but also because decreased disturbance rates would mean smaller and fewer areas for the southern competitors to exploit. In regions where disturbances from fire, insect pests, and disease increase, the transition of the vegetation assemblages to the adjacent types may be accelerated (Suffling 1995). The vegetation changes associated with the new climate may lead, moreover, to new assemblages of species (Martin 1993). Competition may be a key factor in defining the vegetation composition. Bonan and Sirois (1992) have suggested that the southern limit of black spruce is dictated by competition rather than climate, as black spruce is at its optimum climate for growth at its present-day southern limit. Thompson et al. (1998) present an overview of possible changes to Ontario's forested landscapes as a result of climate change. Increases in climate variability under a new climate could have major impacts on the vegetation of Ontario (Mearns et al. 1989; Solomon and Leemans 1997). Models have suggested that synoptic storm frequency would decrease in the long term, but that there would be an increase in the overall intensity of disturbances (Lambert 1995). In the next century, there may thus be fewer storms, but more extreme Influences of Climate on Ontario Forests 107 p Land Cover Types Boreal Conifer Forest Temperate Mixed Forest it Temperate Evergreen Forest . . ; El Shrub/Woodland "'Savanna/Woodland Grasslands Arid Lands Taiga/Tundra Tundra 0 300 600 Kilometres Figure 6.1 The distribution of major biome types as simulated under current climate change by the Mapped Atmosphere-Plant-Soil System (MAPSS) model. (Adapted from Neilson and Drapek 1998) weather (for example, extreme wind speeds or very heavy precipitation causing flooding). Research has also suggested that the persistence of blocking ridges in the upper atmosphere will increase in a climate scenario of doubled carbon dioxide (Lupo et al. 1997). This factor could have significant impact on forest fires, as these upper ridges are associated with dry and warm conditions at the earth's surface that are conducive to forest fires. Extreme environmental conditions caused by prolonged drought, floods, extreme heat, extreme cold, and the increased occurrence of severe winds, can be expected to have a negative influence on forest health. These environmental stresses predispose individual plants, species, and ecosystems to secondary stressors, such as outbreaks of insect infestation and disease. Research has shown that resistance to drought increases with increased carbon dioxide (Townend 1993). Recent research has also suggested that increased carbon dioxide may lead to increased tolerance of cold temperatures (Boese et al. 1997). The anticipated changes in climate will have significant impacts on physiological processes and the cycling of nutrients. The global atmospheric concentration of carbon dioxide has risen from pre-industrial levels of 280 parts per million by volume (ppmv) to 360 ppmv in 1994 (Amthor 1995). Plants and ecosystems are closely coupled with nitrogen and carbon cycles, which might be altered by the elevated carbon dioxide and by climate change. The nitrogen and carbon cycles are closely linked (Reynolds et al. 1996) through decomposition and litter quality. Temperature increases will greatly influence decomposition and nutrient cycling (Anderson 1992). Historically, the boreal forest has been presumed to be a carbon sink in the global carbon budget. This carbon sink likely will be reduced under climate change (Kurz and Apps 1993; Kurz et al. 1995), or may even become a carbon source. Increased temperatures will lead to an increase in soil temperature and an associated increase in the active layer over permafrost. Improved soil drainage as a result of soil warming, especially at northern latitudes, is an important consideration, because of the implications for organic layer drying, and hence fire severity (Anderson 1992). • 108 Michael D. Flannigan and Michael G. Weber 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 0 300 600 1n 1n 1 Kilometres Figure 6.2 The potential distribution of major biomes as simulated under the Geophysical Fluid Dynamics Laboratory (GFDL) Global Climate Model, with aerosols included, by MAPSS. (Adapted from Neilson and Drapek 1998) PROCESSES OF CLIMATE INFLUENCE ON VEGETATION Weather variables such as temperature, precipitation, and wind have a direct influence on vegetation in terms of growth, mortality, species abundance, and composition. Weather also exerts an indirect influence on vegetation through such factors as forest fires, pest and disease outbreaks, and the presence or absence of permafrost. Direct Effects of Climate Influences of Temperature on Vegetation Various aspects of temperature can have a significant impact on vegetation. These include winter minimum temperature, frost during the growing season, and warmth during the growing season. Winter minimum temperatures are important in determining the distribution of tree species. Many studies suggest that the poleward limit of a tree species is controlled by the minimum winter temperature that is regularly experienced (Sakai and Weiser 1973; George et al. 1974; Sakai 1978; Larcher and Bauer 1981; Woodward 1987; and Arris and Eagleson 1989). In Ontario, this is probably true for most, if not all, of the non-boreal, deciduous tree species. Most deciduous species cannot tolerate temperatures below -30°C to -40°C, the limit of the strategy which they use to survive freezing temperatures. There are three standard strategies that plants use to survive freezing temperatures: deep supercooling, extracellular freezing, and extraorgan freezing (Sakai and Larcher 1987; Woodward and Williams 1987). Deep supercooling allows water in the plant cells to remain liquid despite temperatures well below 0°C, owing to a lack of ice nucleation sites. As long as ice does not form within the cell, there is no mechanical damage. Typically, the coldest temperature that plants can survive using deep supercooling for pure water is about -40°C. Survival at lower temperatures (to about -55°C) using deep supercooling is possible only in the presence of high concentrations of solutes in the cell water (Gusta et • Influences of Climate on Ontario Forests 109 al. 1983). Most deciduous tree species, except the birches (Betula spp.) and poplars, use deep supercooling and typically cannot tolerate temperatures below -40°C. Extracellular and extraorgan freezing occurs after the migration of water out of the plant cells or organs and into intercellular spaces, where freezing can usually occur without damage. The intercellular spaces are usually large enough to accommodate the influx of water and the expansion associated with the phase change from liquid to solid, without damage to the surrounding cells and organs. The survival of plants using extracellular and extraorgan strategies is limited by the extent to which the plant can withstand extreme dehydration of the cell or organ caused by the outward migration of water from cells (Sakai 1979), which results in desiccation. The boreal conifers use extracellular or extraorgan strategies and can survive temperatures of -70° to -80°C, which is colder than anything experienced in Ontario, and these conifers are not limited by extreme minimum temperatures. During the growing season, tree species, and in particular seedlings, are not frost-hardy, so that temperatures of -2°C to -5°C can be lethal. Growing-season frost can also damage reproductive structures. Female conifer flowers and conelets are particularly susceptible to frost damage in early spring, which can limit seed production (Schooley et al. 1986). Frost can also damage other parts of the tree, including the stem, bud, and root collar, and can cause leaf and needle damage. If the initial damage from temperatures below freezing is not lethal, then the damaged areas often become sites of infection by canker and other diseases, or become susceptible to insect attack (Hiratsuka and Zalasky 1993). Growing-season frost can be critical in plantations, especially where topography creates low-lying areas (Stathers 1989). Studies of frost hardiness have been conducted on many coniferous species found in Ontario (Glerum et al. 1966; Glerum 1973; Joyce 1987). Results suggest that there is little difference in frost-hardiness between those conifer species (Glerum 1973). Growing-season warmth can also be an important determinant in vegetation distribution. For example, Black and Bliss (1980) found that the northern limit of black spruce was determined by the summer warmth required for seed germination. Pigott and Huntley (1978) found that insufficient warmth at the northern limit of small-leaf linden (Tilia cordata) during the flowering period resulted in non-viable seed. For most tree species, there is a critical temperature that needs to be exceeded for growth to begin. The growing-degree concept was developed from the fact that many grasses require temperatures of 5°C or higher for growth to occur. For some trees, such as red pine (Pinus resinosa), the critical temperature for initiating and maintaining growth is 10°C; therefore, if summer mean annual temperatures did not exceed 10°C, it would be unlikely that red pine could remain established in such a climate. Summer warmth is critical in plantations, where site treatments such as mounding have serious micro-meteorological implications on the local thermal regime (Spittlehouse and Stathers 1990). McCaughey et al. (1997) provide a good overview of the weather and climate associated with Canadian forests. Influences of Precipitation on Vegetation The lack of precipitation, if prolonged, results in drought that can damage or kill trees. Drought can be restricted to one growing season or may persist for several growing seasons. If drought is severe enough, leaf abscission will occur. Summer drought is different from winter desiccation. Drought is caused by inadequate soil moisture; whereas desiccation occurs when soil moisture is unavailable because the ground is frozen. Drought-stressed trees are prone to attacks from insects and diseases. On the other hand, if the precipitation is too heavy, flooding can occur and cause extensive damage in low-lying areas. Freezing rain and heavy snow can accumulate on the vegetation to such an extent that the added weight on the foliage and branches causes physical damage. The build-up of snow and ice is influenced by stand density and the shape of the crown. This is a common cause of damage in plantations (Powers and Oliver 1970). The amount of damage can be significant; there are reports of more than 20 percent of stems broken in a stand (Van Cleve and Zasada 1970). When trees are laden with a coating of ice, they are more prone to windthrow. A severe freezing rain event in January 1998 damaged millions of trees in eastern Ontario and southern Quebec. Hail also can cause extensive damage to vegetation (Riley 1953; Laut and Elliot 1966). Seedlings and saplings are especially prone to damage; whereas mature trees typically sustain only minor 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 of infection by pathogens. Indirect Effects of Climate Climatic Aspects of the Influence of Insects and Disease on Vegetation Climate and weather play a major role in the life cycle of many forest insects, some of which have a major influence on forest productivity (Fleming and Volney 1995). Additionally, climate and weather can be important in disease contraction and spread. If climate changes, as the GCMs suggest, the greatest impact of climate change on the structure and function of the boreal forest will be mediated through changes in disturbance regimes such as insect outbreaks and fire. Discussion of a large number of insect defoliators is beyond the scope of this chapter, so the spruce budworm (Choristoneura fumiferana) is chosen as a representative species. Fleming et al. (2000, this volume) provide a detailed description of the effects of various insect pests and forest diseases on Ontario's forest landscapes. Fleming (1996) reviewed the possible influences of climate change on defoliating insects in North America's boreal forests and outlined the interrelationships among climate, vegetation, and insect populations. The direct influence of climate on vegetation may have a secondary impact on insect populations. Climate influences the synchrony of host plant phenology with spruce budworm development as well as the synchrony with natural invertebrate enemies. Finally, weather elements such as drought and late-spring frost may have a direct impact on spruce budworm populations; in fact, Cerezke and Volney (1995) suggest that latespring frosts coincide with the collapse of the spruce budworm outbreak. Spruce budworm is only one example of the many types of insects that influence the forest, but that work does highlight the complex interactions and feedbacks among vegetation host, climate/ weather, and natural enemies. As the climate and weather change, non-linear and perhaps unexpected interactions may have devastating effects, allowing insects to become an additional agent of accelerated change in the forests. Climate directly influences vegetation, its pathogens, and its insects, including pathogen vectors. The relationships between weather and tree diseases have been studied for many years (Hepting 1963). So-called "for- • est declines" (Manion 1981) may be a result of an interaction between climate and disease. For example, red spruce (Picea rubens) decline consists of an interaction between winter injury and air pollution, which allows pathogenic fungi such as Cytospora sp., Fames sp., Artnillaria sp., needle-cast diseases, rust diseases, 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 conditions or a change in climatic variability may alter plant disease development by affecting the following factors: (1) the speed of pathogen development; (2) the geographical range of the host, pathogen, or vector, especially at the boundaries of their respective distributions; and (3) control of the disease. Predicting the impact of climate change on forest diseases is made more complicated by the need to take into account the interactions among climate, pathogens, and insect vectors of the pathogens, but it is clear that, with warming, the potential for rapid outbreaks of forest disease across Ontario is a real threat. Influences of Permafrost on Vegetation In some northern Ontario forested landscapes, permafrost is an important agent, exerting control over forest ecosystem structure and function. Although of concern only locally, permafrost is a terrain feature that may be of concern to ecosystem managers charged with maintaining the integrity of Ontario's northernmost areas. The terrain sensitivity of landscapes underlain by permafrost must be considered in planning both commercial and non-commercial northern development activities, such as the construction of roads, settlements, or fire-guards. According to Brown (1973), continuous permafrost underlies only a narrow, treeless band along the Hudson Bay coast of northern Ontario. Discontinuous permafrost, consisting of scattered islands of permanently frozen ground, each a few square metres to several hectares in size, occurs mainly in peatlands. Other areas where discontinuous permafrost may be encountered are on north-facing slopes of east-west oriented valleys, or along isolated patches of forested stream-banks, where increased shading reduces summer thaw and winter snow cover (Brown 1973). The southern limit of discontinuous permafrost in Ontario lies at about latitude 51°N, to 52°N around James Bay and coincides with the mean annual air temperature isotherm of -10°C. The area occupied by • Influences of Climate on Ontario Forests 111 discontinuous permafrost, also known as the Hudson Bay Lowland physiographic region, contains the northern limits of all boreal forest tree species in Ontario and is characterized by a fire-dominated disturbance regime. The impact of potential climate change on the northern Ontario boreal forest of the Hudson Bay Lowland may be envisaged from simulation studies carried out for other parts of the North American boreal forest, where permafrost and fire interact to dominate forest ecosystem structure and function. An example has been provided by Bonan et al. (1990) for interior Alaska. Their simulations assumed climate change scenarios of warming by 1°C, 3°C, and 5°C, factorially coupled with increases of 120 percent, 140 percent, and 160 percent in monthly precipitation values. To emphasize the importance of site conditions in response to expected climate change, the simulations were performed for two contrasting forest types: a black spruce (Picea tnariana) forest growing on a permafrost-dominated, poorly drained, north-facing slope, and a forest of white spruce, paper birch, and aspen located on a well-drained, permafrost-free, south-facing slope. According to these simulations, the effects of climatic warming on ecosystem structure and function in the northern boreal forest may not be so much a direct response to increased air temperature as to increased potential evapotranspiration demands. Analysis of their simulation results also revealed the importance of the forest floor organic layers in controlling ecosystem response to climatic warming. For example, the thick forest floor layer of 20 cm to 30 cm typical of many black spruce forests in interior Alaska and elsewhere is the major factor responsible for cold, wet soil conditions which restrict nutrient availability and tree growth (Weber and Van Cleve 1981, 1984). In the absence of fire, the short-term response of these permafrost-dominated sites to climate warming was a decrease in the depth of the active soil layer (that is, the layer of soil lying above the permafrost that thaws out annually in response to summer warming). This decrease occurred from a drying of the forest floor, which impeded the conduction of heat into deeper soil layers. In the long term, however, with recurrent forest fires, the drier organic layers were conducive to increased fire severity, and thus to the removal 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 shallow, discontinuous permafrost would be a possible scenario under these conditions. The final outcome of this simulation run was the fire-caused conversion of the low-productivity black spruce forests to mixed forests of spruce and hardwood growing on warmer soils. In contrast, on the well-drained, south-facing spruce and hardwood forest sites, increased potential water loss in the warmer climate reduced soil moisture and resulted in the site-conversion of these stands to dry aspen forests. The greatest simulated reduction in soil moisture resulted in steppe-like vegetation and an elimination of the tree overstory on these sites. Bonan et al. (1990) thus highlighted the sensitivity of divergent forest ecosystems to water balance and to its interaction with the fire regime under climate change (Weber and Flannigan 1997). The Influence of Vegetation on Climate The link between climate and vegetation is well known, but the reverse link is not as well known. The link between vegetation and climate is found at all scales, from microscales to the global scale. At smaller spatial scales, differences in temperature, wind, and relative humidity would be expected to exist between an agricultural field and an adjacent forest stand, because of differences in the energy budget between the two areas. At larger scales, for example, across the entire boreal forest biome, the influence of vegetation on the climate can be significant. From using the GCM of the United Kingdom Meteorological Office (UKMO), Thomas and Rowntree (1992) suggest that, in the absence of boreal forests, northern hemisphere temperatures would be 2.8°C cooler and precipitation would decrease. These changes would result from the difference in albedo between the forest and non-forest vegetation, especially in winter, as the albedo of snow is particularly high. (Albedo is the amount of electromagnetic radiation reflected by a body relative to the amount incident upon it, and is commonly expressed as a percentage [Greer 1996].) Also using a GCM, Bonan et al. (1992) suggest that, if tundra or bare ground replaced the boreal forest, the climate of the entire northern hemisphere would be significantly cooler, and that latent heat flux and atmospheric moisture would increase. The warming effect of the boreal forest consists of masking the high reflectance of snow over vast areas of the northern hemisphere. Other a 112 Michael D. Flannigan and Michael G. Weber researchers (Otterman et al. 1984; Crowley and Baum 1997) confirm that vegetation does play a significant role in regional to global temperature and precipitation patterns. Foley et al. (1994) argue that the interaction of vegetation with climate was operating during the Holocene and gave rise to large positive feedback between the climate and the boreal forests, which resulted in warmer temperatures in the northern hemisphere. SUMMARY Climate and vegetation interact across the range of spatial and temporal scales in a complex fashion. Climate determines the suite of species that is available to colonize the landscape. The actual vegetation present over the landscape is the result of many factors among which climate is of primary importance. Climate exerts direct control over vegetation through either beneficial or deleterious effects of temperature, precipitation, and wind, and indirect control through climatic 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 northsouth and northwest-southeast climatic gradients in temperature and precipitation, respectively, which give rise to a great diversity of vegetation types. As climate changes, so does the vegetation, although at a slower pace. Should the climate continue to warm, dramatic change in the forests of Ontario can be expected, especially if the climate changes as rapidly as the global climate models suggest. The interaction between climate change and disturbance regimes has the potential to overshadow the importance of the direct effects of global warming on species distribution, migration, substitution, and extinction. Disturbance could thus be the most effective agent of change, and the rate and magnitude of disturbance-induced changes to the forested landscape of Ontario could greatly exceed anything caused by atmospheric warming alone. ACKNOWLEDGEMENTS We thank Ron Neilson and Ray Drapek for providing Figures 6.1 and 6.2. 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