An isoline separating relatively warm from relatively cool wintertime forest surface temperatures for the southeastern United States

a r t i c l e i n f o Forest-oriented climate mitigation policies promote forestation as a means to increase uptake of atmospheric carbon to counteract global warming. Some have pointed out that a carbon-centric forest policy may be overstated because it discounts biophysical aspects of the influence of forests on climate. In extra-tropical regions, many climate models have shown that forests tend to be warmer than grasslands and croplands because forest albedos tend to be lower than non-forest albedos. A lower forest albedo results in higher absorption of solar radiation and increased sensible warming that is not offset by the cooling effects of carbon uptake in extra-tropical regions. However, comparison of forest warming potential in the context of climate models is based on a coarse classification system of tropical, temperate, and boreal. There is considerable variation in climate within the broad latitudinal zonation of tropical, temperate, and boreal, and the relationship between biophysical (albedo) and biogeochemical (carbon uptake) mechanisms may not be constant within these broad zones. We compared wintertime forest and non-forest surface temperatures for the southeastern United States and found that forest surface temperatures shifted from being warmer than non-forest surface temperatures north of approximately 36°N to cooler south of 36°N. Our results suggest that the biophysical aspects of forests' influence on climate reinforce the biogeochemical aspects of forests' influence on climate south of 36°N. South of 36°N, both biophysical and biogeochemical properties of forests appear to support forestation as a climate mitigation policy. We also provide some quantitative evidence that evergreen forests tend to have cooler wintertime surface temperatures than deciduous forests that may be attributable to greater evapotranspiration rates. There is uncertainty regarding the influence of extra-tropical forests on climate (Bala et al., 2007; Bonan, 2008). Most studies examining the role of mid-and high-latitude forests have found that removal of forests at these latitudes would promote cooling, with relatively few finding that such forest removal would promote warming (Wickham et al., 2013). Although many factors influence forest versus non-forest surface temperatures (e.g., surface roughness, frictional resistance to transpira-tion), the competing influences of albedo and carbon uptake appear to be the main factors producing the uncertainty (Bala et al. Forests tend to be dark and therefore absorb more of the sun's radiation than surrounding fields. In the absence of photosynthetic activity (i.e., mid-and high-latitude winters) the lower albedo of forests leads to warming that …