Download Influences of Climate on Ontario Forests

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.
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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. Thanks also go to Mike Wotton, who helped prepare those figures for this chapter.
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