What to plant and where to plant it: Modeling the biophysical effects of forests on climate using the Community Earth System Model
Forests influence local climate through biophysical processes such as albedo and evapotranspiration. It is well established that boreal forests have a warming effect by reducing albedo and tropical forests have a cooling effect by increasing evapotranspiration. However the influence of temperate forests on climate is less certain. Previous model experiments have shown the average effect of temperate forests as a whole to be a weak warming effect. However, some non-model studies that examine these effects at the sub-biome level show a latitudinal gradient of effects presumably due to differences in climate while other studies show variation in biophysical effects due to forest type that is independent of climate.
Using the Community Earth System Model, we explore spatial variation in the direction and magnitude of the climate response to changing forest cover in temperate regions. To examine the sensitivity of climate to the forest-type we ran simulations using three land-cover compositions: 100% needle-leaf evergreen trees, 100% broad-leaf deciduous trees and 100% grass cover. To understand the effect of latitude, we then separated grid cells into 12 clusters based on pre-land-use change precipitation and temperature and repeated the land-cover composition simulations for each cluster independently.
Overall, we found that broad-leaf deciduous trees offer the least local warming effects on climate and, given equal carbon uptake, would be more beneficial for climate protection. However, we did find a gradient in the strength of the biophysical effects due to background climate and albedo differences. In the southeastern United States, broad-leaf deciduous trees provide a local cooling effect. This regional effect may be even more obvious when land-use changes take place at the regional level.
In North America, needle-leaf evergreen trees are on average warmer than corresponding grassland/cropland throughout the year, but this difference is smallest in the late fall. In addition, the annul average of broad-leaf trees is warmer than grassland/cropland. However, in late summer to early autumn, broad-leaf trees are on average cooler than grasslands. Our results help better understand how land-cover change in the temperate region.