What?s in Store for Trees in North America with Climate Change?
Here I provide some brief insights as to what might be in store for trees in North America with climate change. The story actually involves a large effort to map out the potential range limits for thousands of plant species across North America. If climate change progresses as rapidly as some of the Intergovernmental Panel on Climate Change (IPCC) global climate models suggest, forest ecosystems in North America could face unprecedented pressures.
The first paper, "Beyond Traditional Hardiness Zones: Using Climate Envelopes to Map Plant Range Limits", tells the overall story of how we are mapping where things could grow now and in the future. The mapping is being done for thousands of species and provides an alternative to traditional hardiness zone mapping that gardeners are familiar with. It is an ongoing effort that involves the collation of plant observation data from dozens of agencies and also the public from across both the US and Canada. To date, we have approximately 2,000,000 observations for over 4,000 species and potential distribution models for over 1500 species. We link the longitude and latitude coordinates of the observations to new “spatially continuous” climate models that cover the United States and Canada. Basically this allows us to derive and map the climate tolerances of individual species across both Canada and the US. On our website, we present three types of results: 1) Models for “Gardeners”, which make use of only temperature related variables in the mapping process (the idea being that gardeners can add water and hence less of a reliance on naturally occurring precipitation, 2) Models for “Ecologists”, which also include precipitation variables in potential distribution models, and 3) Maps under various climate change scenarios.
As our plant distribution database continues to grow, we are able to examine the effects of rapid climate change on plants at large spatial scales. In the second paper, "Potential Impacts of Climate Change on the Distribution of North American Trees" (McKenney et al 2007b), we examine how much climate habitats might shift in the future for 130 tree species. Since the dispersal of plant species into future climate habitats is a difficult thing to predict, we present findings for 2 scenarios; a scenario which represents the notion that species are able to fully disperse into their future climate habitat and a second (worst-case?) situation in which plants are unable to disperse quickly enough and thus survive only in areas that overlap with their current range. Although neither of these outcomes is highly likely, this approach helps to bound the problem. The metrics that we present for each species are the predicted change in climate habitat size and change in the latitude of the species potential range. Details on the methodology can be seen in the paper(s). Maps for each species and GCM scenario can be viewed at the plant hardiness web site and by searching for the species of interest and select the desired model.
Under the full dispersal scenario there was an average decrease in climate habitat size of about 12%. Of the 130 species, 72 showed a decrease in future climate habitat size. Under the no dispersal scenario future climate habitat of course sharply decreased in size-- by 58% on average and all species showed declines. How far north could species move? Under the full dispersal scenario, the shift northward was about 6.4 degrees latitude (approximately 700 km) on average and all species showed northward shift. This would mean that, if the IPCC General Circulation Models are correct, trees would have to migrate at a rate of about 10km/year in order to keep pace with the shifting climate. Thisis a lot faster than natural migration is thought to be able to occur (10-100 m/year). Under the no dispersal scenario, the northward shift was about 3.0 degrees latitude (approximately 330 km) on average and all species again showed northward shift.
Our results suggest that tree species climate habitat “richness” or potential growing space may increase in Northern parts of the continent but may decrease in southern areas quite substantially. In fact it appears that the climate could change so drastically to be completely outside the realm of what many species are used to. For example if the General Circulation Models are correct, sugar maple, other hardwoods, etc. could be grown around Hudson Bay, and some conifers even further north by the mid to later part of the century.
McKenney, D.W., Pedlar, J., Hutchinson, M., Lawrence, K., Campbell, K. 2007. Potential impacts of climate change on the distribution of North American trees. Bioscience. 57: 939-948
McKenney, D.W., Pedlar, J., Lawrence, K., Campbell, K., Hutchinson, M. 2007. Beyond traditional hardiness zones: using climate envelopes to map plant range limits. Bioscience. 57:929-938