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Geography Compass 4/7 (2010): 701–717, 10.1111/j.1749-8198.2010.00342.x
The Deciduous Forest – Boreal Forest Ecotone
David Goldblum* and Lesley S. Rigg
Department of Geography, Northern Illinois University
Abstract
Ecotones have been subject to significant attention over the past 25 years as a consensus emerged
that they might be uniquely sensitive to the effects of climate change. Most ecotone field studies
and modeling efforts have focused on transitions between forest and non-forest biomes (e.g. boreal
forest to Arctic tundra, forest to prairie, subalpine forests to alpine tundra) while little effort has
been made to evaluate or simply understand forest–forest ecotones, specifically the deciduous forest
– boreal forest ecotone. Geographical shifts and changes at this ecotone because of anthropogenic
factors are tied to the broader survival of both the boreal and deciduous forest communities as well
as global factors such as biodiversity loss and dynamics of the carbon cycle. This review summarizes
what is known about the location, controlling mechanisms, disturbance regimes, anthropogenic
impacts, and sensitivity to climate change of the deciduous forest – boreal forest ecotone.
Overview
Over the past century there have been numerous descriptive field-based studies cataloging
patterns of boundaries (ecotones) between vegetation types (biomes). These studies were
generally followed by research focused on understanding the biological processes and ⁄ or
environmental conditions creating the patterns. Recent research has addressed the temporal and spatial dynamics of ecotones, particularly in light of anthropogenic disturbances
such as climate change, logging, agriculture, and altered fire regimes. Ecotones may be
obvious (Arctic ⁄ alpine treelines or prairie ⁄ forest boundaries) or more subtle (between forested zones). Because of quantifiable geographic shifts in ecotone locations over recent
decades (e.g. Beckage et al. 2008; Parmesan 2006), centuries (e.g. Vallee and Payette
2004), and millennia (e.g. Hupy and Yansa 2009; Kullman 1995), many ecologists and
biogeographers have suggested that ecotones may be well suited to detect human impacts
on terrestrial ecosystems, including signs of anthropogenic climate change (Kupfer and
Cairns 1996; Loehle 2000; Neilson 1993; Noble 1993). This review summarizes the
current state of knowledge about an infrequently studied ecotone between forested zones,
namely the deciduous forest – boreal forest ecotone (DBE). We consider the position of
the DBE since the end of the last ice age (18,000 years BP) across the northern hemisphere, the environmental, ecological, and biological variables responsible for the transition from deciduous to boreal forest, the dynamics of the DBE during the Holocene, and
anthropogenic impacts on the DBE. Lastly, we include a discussion of the ecotone’s
future given modeled anthropogenic climate change.
Ecotones
According to Ries et al. (2004), the earliest reference to edge-related ecology was by the
influential ecologist ⁄ geographer Clements (1907) who first introduced the term ‘ecotone’,
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whereas Livingston (1903) described a distinct boundary between forest types as a ‘zone
of tension’. In both cases, according to van der Maarel (1990), these descriptions refer to
a sharp ‘stress zone’, which is distinctly different from a ‘gradient zone’ which can be
termed an ecocline, a term coined by Clements (1937) and is associated with large-scale
community change. A third type of boundary has been proposed by van der Maarel
(1990), the mosaic, to represent areas with intermixed fragments of the adjacent communities, which may be most appropriate for describing the deciduous ⁄ boreal forest transition
discussed in this article (Figure 1). We refer to boundaries between vegetation biomes as
ecotones, a term commonly used to describe boundaries and biome transitions at the global
scale (Kent et al. 1997). However, implied in our use of this term is that biome-level
boundaries may be abrupt, gradual, or composed of vegetation mosaics.
Generally, climate driven air mass activity creates global-scale biome patterns (Risser
1995). However, at progressively finer spatial scales, a hierarchy of biotic and abiotic constraints on vegetation community types becomes evident (Gosz 1993) leading to fine scale
transition zones (van der Maarel 1990). At regional and local scales, soil characteristics,
microclimatology, microtopography, competition, and population genetics ultimately
determine the exact position of the ecotone (van der Maarel 1990).
For much of the 20th century many ecologists viewed ecotones as anomalies (Fortin
et al. 2000; Yarrow and Marin 2007), but an interest in ecotones arose in the 1990s.
Firstly, ecotones were seen as controlling the flux of materials between ecosystems (Risser
1995), population dynamics, and biodiversity (Naiman and Décamps 1990; Risser 1995).
Secondly, because ecotones contained species pushed to their physiological tolerance,
ecotones should be especially sensitive to environmental fluctuations, and biological
changes would be detectable (Arris and Eagleson 1994; Loehle 2000), and thus could be
bellwethers of anthropogenic impacts (Fortin et al. 2000; Neilson 1993), although this
may not apply to all ecotones (Kupfer and Cairns 1996; Noble 1993).
The deciduous forest – boreal forest ecotone
Unlike other vegetation zones on Earth, only the boreal forest biome encircles the
globe (Woodward 2003). The biomes at the southern boundary of these forests vary
Fig. 1. A generalized diagram expressing the variable nature that might exist at community (or biome) transitions
(Source: Kent et al. 1997).
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geographically. Where oceanic climates predominate, the boreal zone borders deciduous
forests; while under more xeric climates, steppe, grassland, or semi-desert adjoin boreal
forests (Breckle 2002) as is the case in large tracts of northern Asia and central North
America. Generally, there is no distinct boundary between boreal and deciduous forests;
instead, a broad transition zone exists composed of mixed stands of coniferous and
deciduous species, or a ‘macromosaic-like arrangement’ with pure stands of deciduous
trees on favorable sites and pure coniferous stands on less favorable sites found on poor
soils (Breckle 2002).
Several studies have described the complexities of locating the DBE in Asia. Sukachev
(1928) describes the DBE in European Russia, coinciding with chernozem soils, as a
broad zone associated with a general decrease in oak, maple, and lime and dominance by
Picea. Breckle (2002) describes the European DBE coinciding with the northern limit of
oak at around 60N. Given the mountainous terrain of Japan, the DBE is expressed as
elevational transitions (Ohsawa 1984, 1990; Yoshino 1978), similar to mountainous China
(Tang and Ohsawa 1997, 2002). Hou (1983), surveying China’s vegetation zones, suggests that the DBE is found only in the extreme northeastern portion of the continent;
yet it occurs as an altitudinal boundary on mountainous terrain (Pastor and Mladenoff
1992). The DBE in North America is largely intact running through the Great Lakes
from Minnesota, Wisconsin, east-central Ontario (Figure 2), and ultimately into southern
Quebec and northern Maine (Breckle 2002; Pastor and Mladenoff 1992) (Figure 3).
Additionally, similar ecotones are found along the Appalachian Mountains in eastern
North America (Beckage et al. 2008). The DBE in Eurasia is highly anthropogenically
Fig. 2. A southward view from the boreal forest towards the northern limit of the deciduous forest in Lake Superior Provincial Park, Ontario, Canada. The trees (with a slight red tinge) on the ridges of the distant hills are the
northernmost sugar maple in the region, marking the transition to boreal forest. Photo credit: David Goldblum.
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704 Deciduous – boreal forest ecotone
Fig. 3. The location of the deciduous-boreal forest ecotone in eastern North America. Vegetation zones are based
on several sources (Sources: Minnesota Department of Natural Resources, Natural Resources Canada, USDA Natural
Resources Conservation Service, Wisconsin Geological and Natural History Survey, Scott 1995; Stearns 1997;
Watkins 2006).
disturbed, making its description somewhat speculative compared with the more quantifiable and intact North American DBE (Pastor and Mladenoff 1992).
Factors determining the location of the DBE
A suite of environmental factors determine the location of the DBE across space and
time. The extent of the ecotone is not consistent globally; some regions in Europe and
North America have a much more developed mixed-forest community (Pastor and
Mladenoff 1992). In some areas the latitudinal ecotone coincides with an elevational
ecotone (i.e. parts of China), but for the purposes of this discussion the focus will be on
variables associated with changing latitude rather than elevation. At a continental scale
ecotones are strongly correlated with climate factors such as temperature, monthly precipitation, seasonality (i.e. growing degree-days), and potential evapotranspiration (Parmesan
et al. 2005; Sowell 1985; Stephensen 1998; Woodward and Williams 1987). While studies that broadly correlate vegetation–climate associations are common in the literature,
regional scale determinants may include regional water balance deficits (Stephensen
1998), ecophysiological plant response (Arris and Eagleson 1994; Stephensen 1998;
Woodward and Williams 1987), actual evapotranspiration (Stephensen 1998; Thornthwaite 1948), and extreme minimum temperature (Woodward and Williams 1987). At the
landscape scale, boreal and deciduous species within the DBE tend to establish and persist
along environmental gradients determined not by climate alone but rather by subtle variation in substrate, drainage (local watershed dynamics), physical soil properties and nutrient
availability (Pastor and Mladenoff 1992).
Temperature
Climate is frequently identified as an important variable in determining the location of
the DBE (Prentice et al. 1992). Arris and Eagleson (1989) discuss the coincidence of the
DBE with the )40 C average annual minimum temperature isotherm (Figure 4). Trees
common in the deciduous forest experience cellular damage with temperatures below
)40 C, whereas boreal and northern tree species tolerate colder extremes through deep
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Fig. 4. The observed northern limit of the deciduous forest (short dashed line) in North America and the )40 C
average annual minimum temperature isotherm (long dashed line) (Source: Arris and Eagleson 1989).
super-cooling to avoid intercellular freezing (Lee et al. 2005; Lenihan and Neilson 1993;
Sakai 1975; Sakai and Weiser 1973; Woodward and Williams 1987). In China, Liu et al.
(1998) found that accumulated seasonal warmth for months over 5 C (warmth index),
not necessarily minimum temperatures, explained the location of the DBE. Additionally,
growing season length, growing degree-days (GDD), and frost-free period have been
offered as factors in determining the location of the DBE (Arris and Eagleson 1994; Kupfer and Cairns 1996; Lenihan and Neilson 1993; Neilson 1995; Pastor and Mladenoff
1992; Prentice et al. 1992). Kupfer and Cairns (1996) describe a growing season length
threshold (generally four months), and Prentice et al. (1992) delineate a 1200 GDD
threshold, leading to a transition to conifers because of the greater water and growing
season length requirements of the more photosynthetically efficient, but thermally sensitive deciduous leaves. While these constraints explain the northern limit of the deciduous
forest, competitive limitations have been proposed as accounting for the southern limit of
the boreal forest as a longer growing season, coupled with higher angiosperm photosynthesis rates effectively leads to competitive exclusion of conifers (Arris and Eagleson 1994)
and dominance by deciduous species in the deciduous forest biome.
Soil
Soil formation is coincident with vegetation development but responsive to the underlying substrate, climate, topography, and time (Jenny 1994). Throughout the DBE the
dominant soils are spodosols and are relatively young, having formed as the most recent
deglaciation, with sparse mineral soil (Kellman 2004). In North America, these soils are
developing in some locations on the Canadian Shield, and in other locations on relatively
unweathered glacial deposits or till or outwash (Kellman 2004; Pastor and Mladenoff
1992). Spodosols throughout the DBE in both North America and Europe (Podosols) are
characterized by a sandy texture, generally low nutrient status, low pH, and organic
matter accumulation (Elgersma and Dhillion 2002; Kellman 2004). The location of the
DBE is responsive to climate, but changes in the edaphic conditions within the ecotone
play a role in the distribution of species because of changing soil nutrient status and pH
(Barras and Kellman 1998; Demers et al. 1998; Elgersma and Dhillion 2002; Messaoud
et al. 2007; Pastor and Mladenoff 1992). The connection between edaphic conditions
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706 Deciduous – boreal forest ecotone
and climate is paramount, with reduced soil temperatures slowing the rates of decomposition causing an increased accumulation of organic matter and increased acidity (Messaoud
et al. 2007).
Feedbacks between litter quality (Barras and Kellman 1998) and nutrient availability
(particularly soil nitrogen) have been found to strongly impact the location of boreal
versus deciduous stands within the ecotone (Pastor and Mladenoff 1992). Nitrogen is the
critical limiting nutrient (Fonara and Tilman 2008; Reich and Oleksyn 2004) in both the
deciduous and boreal forest biomes (Pastor and Mladenoff 1992; Reich et al. 1995). The
feedback between soil quality (including microbial communities), litter quality, and
species presence ⁄ absence is crucial within the DBE. Soil moisture may also be a factor in
explaining the location of the DBE (Weishampel et al. 1999). Hogg (1994) found that
the southern limit of several boreal forest conifers coincided with zero isoline of annual
precipitation minus potential evaporation.
Topography
Maycock and Curtis (1960), who surveyed the forests around Lake Superior to quantify
the forest composition associated with the DBE, identified a mosaic of stands dominated
by conifers distinguishable from stands dominated by deciduous hardwoods. Where the
mediating influence of Lake Superior accentuates topographically induced changes in
climatic conditions at the hillslope scale, sugar maple dominated stands are found on
ridges, whereas conifers dominate low-lying waterlogged sites (Barras and Kellman 1998;
Boucher et al. 2009; Goldblum and Rigg 2002; Pastor and Mladenoff 1992). Similarly,
Hayes et al. (2007) found that the DBE in the Appalachian Mountains varied based on
topographic position, and Messaoud et al. (2007) found that species distributions at the
DBE in Quebec were also governed in part by topographic position. So, while the general
location of the DBE may be climatically controlled, fine scale environmental heterogeneity
creates pockets of stands of one forest type or another, as well as accounts for the broad
transition between the two biomes in North America (Arris and Eagleson 1994).
Fire regimes
Similar to topography, fire may modify the climatically mediated location of the ecotone
by altering competitive interactions and resource availability, yet the role fire plays in
modifying the location of the DBE is poorly understood (Pastor and Mladenoff 1992).
The flammability and role of fire varies dramatically between the boreal and deciduous
forest biomes (Heinselman 1973; Runkle 1990) with fire return intervals increasing as
one moves southward across the DBE from 50 to 80 years in the boreal forest to
>300 years in the deciduous forests of North America (Pastor and Mladenoff 1992).
Despite the contrast in return intervals, Bergeron et al. (2004) determined that the transition between the mixed and coniferous forests in the ecotone cannot be simply explained
by a difference in fire frequency over the past three centuries, but rather is due to fire
size and severity, with small fires favoring deciduous dominance and larger, intense fires
favoring boreal communities. In central Sweden Axelsson et al. (2002) describe that
anthropogenic modification of the fire regime dramatically altered the location and presence of deciduous species within the boreal forest matrix. Similarly, Clark and Royall
(1995) demonstrate changes to forest composition in the Canadian DBE associated with
Native American burning, sufficient to tip dominance between biome types. Time
since fire, as is true elsewhere affects species composition, but in the ecotone these
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compositional changes may lead to a transition from deciduous forest to boreal forest, or
vice versa (Gauthier et al. 2000), a phenomenon not often associated with successional
processes outside the DBE.
Historical legacies
While environmental variables are extremely important in determining species distributions, much remains unexplained at the local scale. Messaoud et al. (2007) identify site
history as explaining a significant portion of the variation in the distribution of a common
ecotone species (balsam fir) at the North American DBE. Specifically, Messaoud et al.
(2007) identify initial site colonization patterns and accidental elimination and replacement as important agents modifying species distributions, altering the subtle location of
the ecotone. Further, Bergeron et al. (2004) describe lags in changes to disturbances
regimes and positive feedbacks between forest structure and fire regimes that may confound the association of forest type with environmental conditions.
Anthropogenic and natural disturbances
Ecotones reflect ongoing competitive tensions between species living at the extremes of
their range, and whereas large-scale climate fluctuations may be critical in determining
the broad establishment of ecotones, subtle environmental changes may tip the balance in
favor of one species (or one biome) or another. External factors that may affect the competitive relationship between ecotonal species include fire, treefall, species-specific insect
outbreaks, and the ability of one species to create microenvironments that inhibit the
establishment of potential competitors (Barras and Kellman 1998; Wilson and Agnew
1992). This has occurred over the past centuries and millennia as climate fluctuations and
human impacts on disturbance regimes (Colombaroli et al. 2008; Miettinen et al. 2002)
have given certain species a slight competitive advantage reflected in the establishment
and survival of ecotonal species (Noble 1993).
Paleo-records and mitochondrial DNA chronologies place humans in Europe 40–
60,000 years BP and North America approximately 20,000–18,000 years BP (Forster
2004). Coincident with the onset of ice sheet retreat in North America and Europe
(18,000 years BP), human populations spread from their glacial maximum refugia (Forster 2004), forever changing the forests of these two continents. Paleontological, archeological and paleobotanical evidence supports the notion that a combination of climate
factors and human hunting were responsible for the extinction of Pleistocene megafauna
(Barnosky et al. 2004) that were integrated into the forest ecosystems present at the time
(Donlan et al. 2006). Those flora–fauna biological interactions are absent today.
While early human populations most likely played a key role in altering forest structure
of the DBE, it is the movements, more recently, of colonial and post-colonial Europeans
who have had the greatest impact on the DBE globally (Delcourt and Delcourt 1987). In
both Europe and North America, the boreal forest remains largely intact, whereas the
deciduous forest has been extensively utilized for many centuries for farming and habitation (Pastor and Mladenoff 1992). This is not to say the boreal forest, especially at its
southern margins is not highly managed for timber and other activities, but in comparison, the history of human land-use within this ecotone, is more pronounced on the
deciduous side of the DBE.
The DBE has shifted as glaciers have advanced and retreated during the past 2–3
million years across North America and Eurasia (Brubaker 1988; Delcourt and Delcourt
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1987; Hewitt 2000), and the DBE has been shown to migrate with Holocene climate
fluctuations (Hupy and Yansa 2009) as well. In the past few decades there has been
extensive research examining the climatic conditions during and since the last glacial
maximum (Davis 1983;2000; MacDonald 2003; McLachlan et al. 2005; Williams et al.
2007). Ice cores from Greenland and Antarctica provide the most detailed information
today for reconstructing past climates using the relative abundances of oxygen isotopes
(MacDonald 2003). Reconstructed proxy temperatures suggest that 140,000 years BP was
as warm as today, but that by 100,000 years BP the temperature had cooled by 6 C.
Temperatures began to warm approximately 10,000 years BP, with the last glacial maximum occurring approximately 20,000 years BP, coinciding with a period of particularly
cold temperatures (MacDonald 2003). The ice sheets at their maximum extended far into
central Europe and northern North America.
When Europeans began to colonize North America, the forest was not untouched by
human influence; the area today that is largely farmland, urban centers, and secondary
forest, was at the time dominated by deciduous and boreal forest (Davis 1983), but was
inhabited by Native Americans. Based on the period immediately prior to European presence, there is evidence from fire-scarred trees that Native Americans were a source of
frequent fires in the North American Great Lakes region (Loope and Anderton 1988),
although their impacts may have been localized (Drobyshev et al. 2008). More broadly,
throughout the deciduous forests of eastern North America, Native American activity
(i.e. burning and agriculture) selected for disturbance-tolerant trees leading to forest communities dominated by those species (Black et al. 2006). Profound impacts, such as
Native American contribution to Holocene migration of forest tree species, are likely to
have occurred at some level (MacDougall 2003), but are challenging to quantify.
Fire is the dominant disturbance in boreal forests and windthrow is the dominant
disturbance in hardwood forest communities (Pastor and Mladenoff 1992; Runkle 1990).
While treefall gaps may occur in the boreal forest (Drobyshev 2001), the deciduous
forests of eastern North America are rarely subject to fire except near the DBE (Runkle
1990) where little difference exists between fire regimes in the two forest types (Bergeron
et al. 2004). The contrasting disturbance regimes of the two biomes may have a significant impact on the spatial pattern of the two communities within the DBE. Barras and
Kellman (1998) demonstrated that small-scale micro-site factors (e.g. litter depth, moss
cover) affect the ability of both boreal and deciduous species to establish within the DBE.
Furthermore, as climate change progresses, disturbance regimes (Krawchuk et al. 2009)
and pathogen dynamics (Logan et al. 2003) will likely be altered in both forests.
In some areas, for approximately 4000 years (McLauchlan 2003) temperate deciduous
forest composition in eastern North America has been profoundly altered by Native
American agriculture. Native Americans were practicing swidden agriculture and arboriculture in portions of the eastern United States partially accounting for shifts in hardwood
species composition (Black et al. 2006) associated with forest thinning, clearance, and
removal of undesirable trees. Foster et al. (1998) found that tree species distributions in
the deciduous forests of Massachusetts are no longer tied to broad climatic gradients, a
condition attributable to the effects of post-colonization agricultural practices.
The forests of the DBE in North America have been logged extensively for
100 + years (Boucher et al. 2009; Friedman and Reich 2005) and until recently little was
known about how logging activity might alter forest composition of the post-logging forests. The general post-logging pattern for much of the DBE in Ontario and the Great
Lakes states is for shade-intolerant hardwoods to replace boreal species in areas that have
been clear cut (Jackson et al. 2000; Schulte et al. 2007) and previously well-established
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topographic and altitudinal segregation of conifers (lowlands) and hardwoods (upper
slopes) to be erased (Boucher et al. 2009). Further, Schulte et al. (2007) document a
trend towards structural simplification, lower species diversity, and functional diversity of
post-logging forests in the North American DBE compared with the pre-European settlement forest. In some cases, community types (species assemblages) that did not exist
before logging are now extremely common (Friedman and Reich 2005).
Forest decline associated with air pollution is often a concern within the deciduous forest, particularly within and near the DBE (Gawel et al. 1996; Jones 2006; McLaughlin
1998; McLaughlin and Percy 1999; Houle et al. 2007; Watmough 2002). The extent of
forest decline occurring within the DBE is most pronounced in Eastern Europe (e.g.
Godbold et al. 1988; Percy and Ferretti 2004) but also in eastern North America (e.g.
Houle et al. 2007; Watmough 2002).
Within both the boreal and deciduous zones, pollution is a key factor affecting forest
change, as exposure to subtle, but long-term, pollutants such as acid deposition, weaken
trees leaving them susceptible to disease, insect outbreaks, and extremes in weather conditions (Drohan et al. 2002; Duchesne et al. 2002; Watmough 2002; Watmough et al.
1998). In North America, studies of sugar maple forests south of the DBE have noted
changes in elemental concentration of Ca, Al, Mg, Mn, and K, over time in the woody
tissue of trees growing in regions experiencing forest decline (e.g. Watmough 2002). In
Europe, spruce seedling root growth has been found to be dramatically reduced in the
presence soil soluble Al (Godbold et al. 1988). In the northeast of North America, fine
root production in both conifer and hardwood stands was found to decline with cation
leaching (particularly Ca) as a result of acid precipitation (Park et al. 2008). The longterm impacts of pollutants on forested communities within the DBE include forest
decline, because of either nutrient deficiencies or Al ⁄ trace metal toxicity, and dieback of
species particularly sensitive to changes in soil environmental chemistry or species growing in marginal soils (Bondietti et al. 1989; Jones 2006; Kogelmann and Sharpe 2006).
DBE dynamics through the holocene
Climate changes over at least the past 20,000 years resulted in massive biome shifts in
terms of geographic location and spatial extent (Amundson and Wright 1979; Davis
1983; Williams et al. 2004, 2007). At the height of the most recent glaciation much of
the area in North America, Europe, and Asia currently occupied by the boreal forest and
the northern deciduous forest was ice covered. In Europe, the extent of the shift was
more pronounced than in North America with only pockets of boreal and deciduous species surviving in small populations in protected locations (MacDonald 2003). In North
America the fragmentation, restriction, and marginalization of many of the boreal and
deciduous forest species resulted in ephemeral biomes that do not exist today (Williams
et al. 2004) and the location of what might be considered the DBE is difficult to identify
before 10,000 years BP (Webb 1988). As climates warmed in the post-glacial maximum
period, tree species migrated northward out of their southerly refugia (although see
McLachlan et al. 2005) and shifted into geographic locations and ranges currently associated with the boreal and deciduous forests (Webb 1988).
Pollen records in North America show that spruce initially colonized the post-glacial
landscapes across the northern United States by approximately 12,500 years BP reaching
the current DBE by 10,000 years BP (Davis 1983; Jacobsen et al. 1987). Maple and birch
species, currently associated with the DBE reached their current northerly limit
approximately 6000 and 7–10,000 years BP, respectively (Davis 1983; Webb et al. 1983).
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By 7000 years BP, within the Great Lakes region of North America, beech had migrated
into the oak ⁄ hickory forests of Lower Michigan, and hemlock, beech and white pine
formed extensive stands throughout southern Ontario (Davis 1983). Around this period,
white pine was replacing jack pine to the north along with northern white cedar, birch
species and alder (Anderson 1995), forming many of the currently established species associations coincident with the mixed forests of the contemporary DBE. Using pollen analysis, Anderson (1995) suggests that regional warming and reduced precipitation with
increased fire activity, between 5000 and 7000 years BP, resulted in a northward shift in
the DBE by 140 km in the Great Lakes region of North America. In Europe, Barnekow
et al. (2008) studied the past 10,000 years of forest change in northeastern Sweden, using
pollen, and found that an initial northward expansion of species associated with warmer
climates (oak, elm, and linden) was mediated by a cooler period, around 3200 years BP,
when spruce, pine, and birch become established beyond the current DBE. Nearer to the
present, during the late Holocene, in response to the Little Ice Age (500 to 150 years
BP) and Medieval Warm Period (1000 to 700 years BP), Hupy and Yansa (2009) document northward and southward shifts in the DBE in response to small temperature
changes of 1–2 C.
Pollen evidence suggests that most species migrated rapidly across the continents under
post-glacial climates, at rates of between 100 and 1000 meters ⁄ year (Anderson 1995;
Davis 1983; Jacobsen et al. 1987; McLachlan et al. 2005). More recently, the use of
molecular indicators has shown that certain species which currently reside in the DBE,
such as red maple, may have survived during the last glacial maximum in limited populations within 500 km of the Laurentide ice sheet (McLachlan et al. 2005). Ultimately, the
close proximity of species to the ice sheets means that the post-glacial migration rates
were slower than the pollen records suggest; likely less than 100 meters ⁄ year. (McLachlan
et al. 2005). A key feature of vegetation migration in relation to climate change was the
individualistic nature of the response (Brubaker 1988; Williams et al. 2004) of the species.
For example, Williams et al. (2004, 2007) postulates that biomes emerge and vanish as
different species shift through space and time, temporally intersecting and forming communities. The implications of this suggestion are that forest biomes, and therefore the
boundaries and ecotones where they meet, will change in terms of their species composition and abundance depending on individual species tolerances and competitive abilities
given the suite of biotic and abiotic factors to which they are responding. As the climate
of the DBE changes in the near future, the region of the current DBEs around the world
may be moving into periods of novel climates and therefore the present-day coexistence
of species within biomes will become segregated. Changes in climatic conditions may
lead to positive climate change feedbacks as carbon is released due to forest dieback (King
and Neilson 1992).
The DBE and climate change
While spatial resolution from general circulation models (GCM) has improved over the
past two decades, the problem remains that many of the climatic variables responsible for
the location of the DBE are at the synoptic scale (Pastor and Mladenoff 1992) and the
location may also be tied closely to disturbance regimes, neither of which are readily
extracted from GCM output. Further, vegetation responds to climate at a range of spatial
and temporal scales (Tang and Bartlein 2008), so what may be an ecological control at
the global or regional scale (i.e. )40 C average annual minimum temperature isotherm)
will likely differ dramatically from local scale controls on species distributions. Even the
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Deciduous – boreal forest ecotone
711
finest spatial GCM resolutions of 0.5 (lat ⁄ long) are too coarsely scaled for identifying the
local location of the ecotone. Nonetheless, efforts have been made to model biome-level
changes at both regional (e.g. Bachelet and Neilson 2000; Frelich and Reich 2009; Koca
et al. 2006; Prentice et al. 1991; Sykes et al. 1996; Xu et al. 2007) and global scales (e.g.
Cramer et al. 2001; Scholze et al. 2006).
High-latitude vegetation, given its sensitivity to temperature, may exhibit the largest
response to climate change of all the world’s biomes (Pastor and Post 1988; Prentice et al.
1991). For the DBE in Scandinavia (Prentice et al. 1991), East Asia (He et al. 2005;
Zhang et al. 2009) and North America (Frelich and Reich 2009; Solomon 1986) a northward movement in deciduous forest is modeled to occur at the expense of the southern
boreal forest. Pastor and Post (1988) and Xu et al. (2007) found the nature of the forest
change to be conditional on soil moisture, with boreal forests being competitively
replaced only if soil moisture was adequate. However, these transitions may take several
centuries given lags due to ecological processes (Solomon 1986; Sykes and Prentice
1996). Given species-specific responses to past climate changes, the nature of the northward expansion of the deciduous forest is unlikely to be simple because it will be mediated by climatic conditions, changes in competitive interactions, invasive species,
herbivore distributions, and disturbance regimes, as well as exogenous factors such as acid
precipitation, CO2 fertilization, and altered disease and pathogen dynamics (Bergeron
et al. 2004; Frelich and Reich 2009; Loehle 2000; Price and Apps 1996; Solomon 1986;
Sykes and Prentice 1996; Sykes et al. 1996). A general concern is that plant migration
rates may be inadequate to track rapid anthropogenic climate change potentially leading
to depauperate forest ecotone communities (Solomon 1986; Solomon and Kirilenko
1997), a probable scenario if migration rates described by McLachlan et al. (2005) are
accurate.
Nearly all the research on potential climate change driven dynamics of the DBE is
based on forest simulations. However, a few field-based empirical studies have been
conducted, although at fine spatial scales and generally not at the DBE. Some studies
(e.g. Bronson et al. 2009; Edwards and Norby 1999; Farnsworth et al. 1995; Gunderson
et al. 2000; Norby et al. 2003; Wan et al. 2004) have manipulated air and ⁄ or soil temperature in field environments in one of the two forest types to assess the impact that
warmer climates might have on plant communities. Similarly, field experiments enhancing CO2 levels (free-air CO2 experiments: FACE experiments) have been conducted in
both boreal and deciduous forests (Nowak et al. 2004), or in boreal forests containing
deciduous species (Rasmussen et al. 2002), but none have been conducted at the DBE.
Research at the ecotone is somewhat limited. Goldblum and Rigg (2005) employed past
growth rates derived from tree rings to predict future growth rates of the dominant tree
species at the ecotone in Ontario, and Goldblum and Rigg (2002) described the stand
structure and demography at the ecotone, also in Ontario, but little else has been
published. Given the substantial body of research focused on the impact of climate
change on forest communities around the world, there continues to be a need to pursue
field-based research on climate change impacts in ecotone areas, including the DBE.
Short Biographies
David Goldblum’s research focuses on the role of disturbances on natural plant communities, most recently focusing on the impact of anthropogenic climate change; he has
authored or co-authored papers in these areas for Dendrochronologia, Physical Geography,
Canadian Journal of Forest Research, Bulletin of the Torrey Botanical Society, Journal of Vegetation
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712 Deciduous – boreal forest ecotone
Science, Journal of Biogeography, and Plant Ecology. Current research involves a field experiment simulating climate change in the forest at the deciduous forest – boreal forest ecotone
in Ontario, Canada and studying the dynamics of herbaceous plants in the understories of
boreal and deciduous forests. Before coming to Northern Illinois University, where he
presently teaches, Goldblum taught at the University of Wisconsin – Whitewater, Northern
Michigan University, and University of Melbourne. He holds a BS in Geography from
UCLA and a MS and PhD in Geography from the University of Colorado.
Lesley Rigg’s research is currently focused on the population dynamics of the boreal
forest ⁄ deciduous forest boundary in Lake Superior (Ontario, Canada) and potential species range shifts associated with climate change. Ongoing studies include the population
ecology of tree species growing in mixed angiosperm ⁄ conifer communities associated
with ultramafic soils, in New Caledonia (South Pacific) and the regeneration status of
oak ⁄ hickory woodlands in Northern Illinois. She has authored or co-authored papers in
these areas for Biotropica, Physical Geography, Austral Ecology and Canadian Journal of Forest
Research. Rigg currently is the Chair of the Biogeography Specialty Group of the Association of American Geographers and is active in the area of women in science. Rigg holds
a BA in Physical Geography from York University in Toronto, Canada, and a master’s
degree in Geography from the University of Colorado, Boulder. She completed her PhD
work in Australia at the University of Melbourne before joining the faculty at Northern
Illinois University where she currently holds the position of Associate Professor.
Note
* Corresponding address: D. Goldblum, Department of Geography, Northern Illinois University, DeKalb, IL
60115, USA. E-mail: [email protected]
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