The Future of the Boreal Forest

So what does the history of the boreal forest tell us about its future? Combining knowledge of forest history with future climate simulations, scientists are trying to predict what will happen to today’s forests if the Earth continues to warm at its present rate. And quite a lot hinges on the rate of change.

Says Hall, “Ice core studies from Greenland and Antarctica reveal that the Earth’s climate has varied cyclically over the past 450,000 years or so, see-sawing over the plus or minus ten degree range. Temperatures have cycled almost in lockstep with variations in atmospheric carbon dioxide.” As if the Earth is inhaling and exhaling over millennia, carbon dioxide has risen and fallen, ice sheets have advanced and retreated, and vegetation has been forced to adapt. It is this relationship of carbon dioxide and temperature that scientists refer to as the greenhouse effect.

According to Hall, we are currently in a very warm period—quite probably as warm as the Earth has been in the last 450,000 years. Warming is occurring very rapidly, especially at higher latitudes. Rapid warming in continental interiors, which are separated from the moderating influence of oceans, puts interior boreal forests at high latitudes at high risk from climate change. According to Hall, swings in the atmospheric concentrations of carbon dioxide created by wintertime exhaling of the biosphere and summertime inhaling of carbon dioxide have lengthened by 6 days since 1960. River and lake ice have been observed to freeze later and break up sooner in boreal land.

The changes predicted for the boreal ecosystem are profound. The build up of carbon dioxide could raise average global temperatures between 1.4° and 5.8°C (2.2 to 10°F) over the next century, with larger increases likely at higher latitudes. The warming temperatures will almost certainly melt the upper layers of permafrost. This permanently frozen layer of soil keeps the water table fairly close to the surface, and many boreal species have developed shallow root systems in response. Recent evidence indicates that permafrost is thawing earlier and freezing later in the year, increasing run off and drying out the soil. This drying could make the entire ecosystem more prone to fire. Finally, many species of boreal trees require a period of chilling before their buds will burst open in the spring, which ensures that new leaves will not open up before the winter is really over. If winter temperatures rise too greatly—and, in fact, models predict temperatures will rise most in the winter—this important growth cycle requirement may be lost, and species that require it might fail.
 

Model simulations performed by the scientists of the Intergovernmental Panel on Climate Change (IPCC) indicate that alpine tundra will lose ground to boreal forest spreading northward. According to their estimates, between one- and two-thirds of the current tundra will likely be replaced by boreal migrants. But as the boreal forest is gaining ground to the north, it will probably be losing ground in the south, as warmer temperatures speed up evaporation from the warming soil. If this happens, the low-moisture-requiring grasses from the prairies of southern Canada will begin to push northward, especially in the interior of the continent, and the losses to the boreal forest at the southern ecotone (transition between two biome types) are expected to exceed gains in the north.

In Western Canada, some scientists are already concerned that the expected warming and drying of the climate will drastically reduce the abundance of aspen, the primary commercial hardwood species in the southern boreal forest. Insufficient moisture could produce an open aspen parkland, where stunted aspen cluster along water courses, with grasslands in between.
 

Where forests will gain or lose ground depends on the regional characteristics of their environment, including water availability and temperature, and one other crucial factor—how fast the species can migrate. This lag time between climate change and species adaptation is something many models don’t take into account.
 

According to Hall, there are two different approaches to modeling future terrestrial environments. One kind of model, the biogeography model, begins by describing the climate conditions that support a particular biome today, and then attempts to predict where those conditions might be present in the future—coupling biology to the geography of climate. These models identify areas that ought to be suitable for certain plants and trees to colonize in the future.

These models, however, don’t provide any physical insight into how these transitions might be accomplished—or whether they could be accomplished at all. An increase in global temperatures of 2 degrees Celsius (3.6 degrees Fahrenheit) could shift the ideal growing conditions for North American tree species as far as 322 kilometers (200 miles) north of their present location. If this warming occurs over the next 100 years as predicted, trees would have to migrate about 3.2 kilometers (2 miles) every year. Just over three kilometers a year may not seem like much, but even that short a distance may be unattainable for trees that reproduce by means of wind-dispersed seeds or nuts, such as oaks. Some scientists fear that even some of the faster migrators, such as jack pine, which the fossil record has shown to spread as much as 500 meters in a year, may not be fast enough to keep pace with the rapid warming expected in the next 100 years.

The biogeography models also neglect the impacts that climate change will have on ecological factors like frequency of forest fires or insect outbreaks, both of which may amplify the impact of climate change on the forests. Mature trees can hang on for many years in less-than-optimal conditions, albeit with a major slow-down in growth. But their seedlings would be less successful. And should continued warming and drying of the boreal forests’ peaty soils increase the occurrence of fire, vast areas of forest might rapidly become open to colonization by invading grasses. And while in previous eras, plants and animals may have had a seamless canvas on which to draw the map of their migration, current human disturbances of terrestrial communities—including deforestation, urbanization, and agriculture—mean that there are few large tracts of undisturbed land through which trees and plants can gradually expand their range in response to climate changes. These land use changes may leave many forest species stranded.

It is not surprising, then, that some scientists consider the biogeography models to be overly optimistic about the fate of northern forests. It’s one thing to say that a spruce could find suitable habitat in far northern Canada in 100 years, and quite another to explain how it will actually get there. Hall describes a second kind of vegetation model as a process model. These models simulate a scenario in which a dying tree creates a gap in a forest canopy. Based on what is known about the variety of plants available for reseeding, their individual reproductive and physical characteristics, and the changing climate, the models try to predict what species will fill that gap.

These models, while predicting some increases in forest productivity in certain locations and for certain species, overall “suggest that large numbers of areas may no longer be able to support forests, particularly if the climate becomes drier,” according to Environmental Protection Agency reports. Of course, there’s no such thing as a perfect model. The trouble with gap or process models is that because they require detailed information about the regional conditions, so far, they have only been applied to individual stands of trees, not to an entire ecosystem at once.

According to Hall, the integration of the two approaches along with climate models into a single coupled model has become a prime objective for many researchers in the field, but without those coupled models we are a long way from completely understanding how the boreal forest will change in most global-warming scenarios. These changes have implications not just for the forest itself, but for human and other animal populations that depend on it—for timber, for habitat, and for food.

Concludes Hall, “In the last 150 years, the amount of atmospheric carbon dioxide is higher than anything we have seen in the last 450,000 years. The Earth has taken up increasingly more carbon dioxide as we have put it out. So far it appears to be sopping up about half of the 6 gigatons we put in each year from burning fossil fuels. But we don’t know how long that can continue.” If that natural cleansing is reversed by climate change—such as by the predicted dramatic losses in the extent of the boreal forest—our scientific models are currently at a loss to predict what the outcome might be.

In the meantime, Davis says, our best shot at preserving biodiversity may be to create large forest reserves at all latitudes. “We need to provide reserves that would not only allow species to migrate in response to climate change,” she says, “but that are large enough to contain a genetically diverse group of trees.” She explains: “If a reserve is large enough and spread out over a broad latitude range, you increase the likelihood that even within a single species of tree there exists a range of genetic backgrounds already uniquely adapted to the different growing conditions found in the north versus the south.” This type of genetic variation, sometimes called ecotypic differentiation, increases the chance that an individual species of tree will be able to genetically adapt to changing climate. But Davis also says we may have to face the fact that many tree species may not be able to keep pace with expected rates of warming, and may simply go extinct. “It’s quite sad, really,” says Davis, “but starting in about mid-[21st] century, we are probably going to see vast areas of forests disappear, especially at the southern limits of the current range.” The loss of the current boreal forest would have radical commercial, biological, and climatological consequences, some of which we can’t even yet imagine.

Selected References
Jackson, S.T., Webb, R.S., Anderson, K.H., Overpeck, J.T., Webb, III, T. , Williams, J.W., and Hansen, B.S. (2000) Vegetation and environment in eastern North America during the last glacial maximum. Quaternary Science Reviews 19:489-508.

IPCC Special Report on The Regional Impacts of Climate Change: An Assessment of Vulnerability . Accessed June 1, 2002. http://www.grida.no/climate/ipcc/regional/

Prentice, I.C, Bartlein, P.J., and Webb, T. (1991). Vegetation and Climate Change in North America since the last glacial maximum. Ecology. 72(6). 2038-2056.

United States Geological Survey. How does climate change influence Alaska’s Vegetation? Insights from the fossil record. Accessed online June 1, 2002. http://greenwood.cr.usgs.gov/pub/fact-sheets/
fs-0071-97/

United States Environmental Protection Agency. The impacts of global warming on forests. Accessed online June 1, 2002.

W. D. Carroll, Peter R. Kapeluck, Richard A. Harper, David H. Van Lear. (2001). Historical Overview of the Southern Forest Landscape and Associated Resources. In History: Southern Forest Resource Assessment (Draft report.) USDA Forest Service. Accessed online June 1, 2002. http://www.srs.fs.fed.us/sustain/report/histry/

Hogg, E. H. and Hurdle, P.A. (1995). The aspen parkland in western Canada: a dry-climate analogue for the future boreal forest? Water Soil and Air Pollution, 82: 391-400.

Davis, M.B., and Shaw, R.G. (2001) Range Shifts and Adaptive Responses to Quaternary Climate Change. Science 292: 673-679.

Additional Resources and Reading
Williams J.W., Shuman B.N., and Webb T. (2001) Dissimilarity analyses of late-Quaternary vegetation and climate in eastern North America . Ecology, 82 (12): 3346-3362

Shafer, S.L., Bartlein, P.J., and Thompson, R.S. (2001) Potential Changes in the Distributions of Western North America Tree and Shrub Taxa under Future Climate Scenarios. Ecosystems, 4:200-215.

 Changes Since the Last Ice Age