Wednesday, December 10, 2008

Climate Change and Virginia Forests – Part 2

In a previous post I reviewed the projected impact of climate change on forest types within Virginia. The Climate Change Tree Atlas also looks at changes in potential habitats for individual species under different climate change scenarios. In this post I will examine the major species changes predicted by these models.

Looking at importance values (an average of basal area and tree counts) from current FIA data, the ten most “important” species in Virginia are in descending order:

1. red maple
2. yellow-poplar
3. loblolly pine
4. chestnut oak
5. post oak
6. white oak
7. Virginia pine
8. flowering dogwood
9. sweetgum
10. blackgum

Under the highest level of change predicted from three different models, the Climate Change Tree Atlas, predicts the ten “important” species would be as follows, as least in terms of potential habitat:

1. loblolly pine
2. sweetgum
3. shortleaf pine
4. red maple
5. white oak
6. winged elm
7. flowering dogwood
8. blackgum
9. eastern redcedar
10. black oak

Three major species have large predicted reductions in habitat potential: red maple, yellow-poplar and chestnut oak. This is in apparent contradiction with recent trends; over the last 60 years of forest inventory data collection these three species have increased greatly in volume. Red maple and yellow-poplar have benefited from the exclusion of wildfires and chestnut oak from the demise of chestnut and the decline in some of the red oaks.

Not surprisingly, some species with limited potential habitat currently, such as balsam fir and red spruce, are predicted to have no future potential habitat.

The model used in the Climate Change Tree Atlas uses current climate and current species distributions to model potential species habitat, and then maps the potential habitat into the future using climate predictions. Implicit are several assumptions. First, that current species distribution is at equilibrium with current climate. Second, that species will move quickly in response to new conditions. Third, that species will not adapt within place to changing climate.

The first of these assumptions is tenuous at best. Our forests are still responding to major changes that have occurred over the last couple of centuries, including: agricultural clearing, industrial logging, fire regime change, chestnut blight, and other invasive species.

The second assumption is widely recognized as problematic given that trees are long-lived and migrate slowly. Furthermore, forest fragmentation is recognized by many as an impediment to migration.

Third, the genetic diversity of many forest species is not considered in these models. The major forest tree species have wide geographic range which indicates a genetic potential to adapt to a wide range of climate. There will be some degree of selection pressure and adaption which will allow some species to show greater persistence in place.

Despite all this uncertainty, climate change on the level predicted would likely have significant impact of our forests, even if there is a large degree of uncertainty as to just what these impacts will be. Climate change effects on fire hazard and fire regimes are a large unknown but could have large impacts on forest conditions and species importance. As forest species move or adapt to changing conditions, mortality and disturbance rates may increase in many areas. Changes in storm patterns, including ice storms and tropical cyclones could change disturbance regimes and interact with other climate-related changes.

For those interested in further reading, a recent article by Botkin et al reviews some of the difficulties in predicting biodiversity changes resulting from climate change models.