Forests influence climate through both the biochemical and biophysical processes, and the impacts of the latter on local climate may be much larger than the former. However, the biophysical effects of afforestation in arid regions have received little attention compared with afforestation in the tropic, temperate and boreal zones. In this study, we combined in situ eddy covariance flux measurements from a neighboring pairs of forested and background desert sites with the decomposed temperature metric (DTM) method to characterize the impacts of arid forests on surface temperature (Ts). A clear-sky, one-dimensional planetary boundary layer (PBL) model was used to estimate the impacts of afforestation on state of regional climate. We showed that despite absorbing more net radiation (35.4 W m-2) the riparian forests tended to cool Ts (-1.28 °C) on annual basis, but with a significant seasonality. Specifically, afforestation may lead to a net cooling effect from March to September and a slightly warming effect in other months. The DTM method revealed that evapotranspiration played a dominant role in cooling surface temperature, while surface albedo (α) and incoming longwave radiation (L↓) acted together to increase forest surface temperature. From June to September, a shallower, cooler and wetter boundary layer was developed over the forest due to high plant transpiration. In other months, the PBL was slightly deeper and warmer over the forest than that over the desert. Therefore, the riparian forests were important in moderating warming trends in arid regions.
Keywords: Biophysical processes; Planetary boundary layer; Riparian forest, arid regions; Surface temperature.
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