Complex Biophysical Climate Effects of Changing Temperate Forests

Changes of forest cover regulate climate system directly through the alteration of water vapor, energy, and momentum exchange between land surface and the atmosphere. These land-based biophysical effects vary with locations and seasons and cause regional cooling or warming, which enhances or diminishes the climatic benefits of forest carbon drawdown in different cases. Biophysical climate effects of forest conversion exhibit the largest uncertainty in the mid-latitudes. The sign and magnitude of biophysical effect in temperate zones are still under hot debate. Over the past two decades, most of our understandings on how forest affects climate through biophysical processes came from sensitivity analysis of climate modeling, by comparing paired model simulations of forest and short vegetation covers. However, much remains unknown in the real world due to the complicated process and uncertainty in magnitude, especially in the temperate bioclimate regions. Here we reviewed complex results and debates from model simulation, field measurements, and satellite observation, and then applied satellite-based observation to investigate the biophysical climate response to potential forest conversion in China, especially on the spatial and temporal patterns and underlying mechanisms. We evaluated the differences of land surface temperature (ΔLST) between adjacent forest and cropland, in terms of the latitudinal and seasonal patterns. Compared to cropland, the temperate forest to the south of 40°N showed the cooling effect of -0.61 ± 0.02°C (95% confidence interval, and hereafter), and it presented the warming effect of 0.48 ± 0.06°C to the north of 48°N (the transition zone was between 40°N and 48°N). Seasonal analysis further demonstrated that the cooling effects of temperate forest in China in spring (March, April, May), summer (June, July, August), and autumn (September, October, November) were -0.53 ± 0.02°C, -0.55 ± 0.02°C, and -0.30 ± 0.02°C, respectively, while the forest caused the warming effect of 0.10 ± 0.04°C in winter (December, January, February). However, the biophysical climate response to forest conversion in temperate regions was complex and showed highly spatial and temporal heterogeneity. We further assessed the role of two major biophysical processes, i.e., albedo and evapotranspiration (ET), in shaping land surface temperature from surface energy budget perspective. Results showed that the latitudinal, seasonal, and spatiotemporal patterns of ΔLST were determined by the net effect of ET-induced latent heat changes and albedo-induced solar radiation absorption changes.