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5.2 Climate Change Methodology: Future Predictions

The effects of climate change on southern forest productivity and hydrology across a range of climate and site conditions were assessed with the well-validated, physiologically based forest process model PnET-II (McNulty and others 2000). PnET-II used four monthly climate variables (minimum air temperature, maximum air temperature, precipitation, and solar radiation), forest type-specific vegetation parameters, and site-specific soil water holding capacity to predict forest growth and drainage across the South at a 0.5 by 0.5 degree (approximately 30 by 30 mile) spatial resolution. Atmospheric CO₂ increases were incorporated into PnET-II by entering the relationship between water use efficiency (WUE) and CO₂ level. PnET-II results for pine and hardwood forest types have been validated for the South (McNulty and others 2000).

Impacts of climate change on forest area, distribution, and biodiversity were studied with biogeography models. This type of model uses resource and ecophysiological constraints such as available soil water and minimum winter temperatures to simulate climate change impacts on forest ecosystems at regional, continental, and global scales (Bachelet and Neilson 2000). The biogeography models used here predict the dominance of different plant species under different climatic and environmental scenarios. The several biogeography models used for this Assessment included the Mapped Atmosphere Plant Soil System (MAPSS), BIOME3, and MC1 (Bachelet and Neilson 2000; Bachelet and others 2001). Input datasets include latitude, mean monthly temperature, windspeed, solar radiation, and soil properties such as texture and depth. All of these models project vegetation responses to changes in CO₂, but through different mechanisms.