Sunday, November 2, 2008

Climate Change and Virginia’s Forests – Part 1

While there is much debate and uncertainty about the extent and causes of climate change to be expected over this century, there is good evidence from pollen records that tree species do shift with climate. The pollen records also indicate that such shifts are neither smooth nor uniform among species.

A very useful modeling exercise has been recently updated by the US Forest Service. The new Climate Change Tree Atlas examines current distributions and modeled future-climate habitats for individual tree species and forest types. The maps below indicate a shift in climate potential for forest types in the Eastern US. They suggest a decline in Virginia for the potential area of oak-hickory forest type and an increase in potential area of oak-pine, loblolly/shortleaf pine, and oak/gum/cypress.

Figure 1 - Current forest types (from FIA data)

Figure 2 - Predicted Forest-Types (CGM3-High)

Maps from: Prasad, A. M., L. R. Iverson., S. Matthews., M. Peters. 2007-ongoing. A Climate Change Atlas for 134 Forest Tree Species of the Eastern United States [database]. Northern Research Station, USDA Forest Service, Delaware, Ohio.
Long-lived trees can persist in areas where they are no longer the most suited. Also, species tend to move individually rather than as existing species groups, potentially creating new forest types in both the short and long-terms. Studies have indicated that some species existed in disequilibrium with climate conditions for hundreds of years after the last retreat of the continental glaciers. As species persist in areas where they are no longer most suited, they may be subject to elevated stress and experience higher rates of mortality.

Species composition is not solely a function of climate, but is also a result of disturbance regimes. Disturbances arise from both natural and human causes. In Virginia, Native American use of fire influenced species composition for centuries before European settlement. Since European settlement, land clearing for agriculture, industrial logging, and fire use and fire exclusion have help create our current species composition. Global trade has introduced exotic diseases, pests and plants which have influenced the species mix. American chestnut and eastern hemlock have been greatly reduced; ash and oak are now threatened by recently introduced species.

Remember that the Climate Change Tree Atlas models indicate importance value potential under predicted climate scenarios and were developed using current species distributions and a combination of predicted climate, current climate, topography, soil, and land use parameters. The models do not specifically incorporate disturbance regimes or any changes in disturbance regimes (such as fire) arising from climate change or changes in human management of forests.

The work done for the Climate Change Tree Atlas has also produced predictions of importance values by species and these have been summarized by state, including Virginia. In my next post I hope to take a more detailed look at the species-specific changes and how these may affect future forest composition and health in Virginia. I hope to factor in, at least in a very subjective way, the interaction of changing disturbance regimes, exotic species and climate change in determining future forest composition and health.

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