Forest Greening Increases Land Surface Albedo During the Main Growing Period Between 2002 and 2019 in China

摘要

This study is the first to find the unexpected increases in albedo due to forest greening,which has important implications in research of land-surface energy-balance and the forest-climate feedback.We investigated interannual changes of vegetation and albedo for forests and grassland during the main growing period,May to September,between 2002 and 2019 in China using the MODIS-satellite vegetation index and albedo products.These results indicate significant greening (P_(MK) < 0.01) in all types of forests as well as grasslands in China.However,forests had an increasing trend of shortwave albedo (P_(MK) < 0.01) for evergreen forests,deciduous broadleaf forests,and mixed forests.Grasslands showed a decreasing albedo trend (P_(MK) < 0.01) over the same period.Corroborating statistical analyses revealed a positive correlation (P < 0.01) between Moderate Resolution Imaging Spectroradiometer (MODIS) enhanced vegetation index (EVI) and albedo for forests on interannual scales.However,EVI and albedo for grasslands had a negative correlation.Forest greening increased the albedo during the main growing period.This contrasts previous results indicating that albedo decreases as annual forest-cover increases.Developing and analyzing a simple vegetation-coverage-based albedo model showed that increasing forest coverage (i.e.,forest greening) had increased area of more brightly leafy canopy and decreased area of darker gaps in the forest canopy.In conjunction,these produced the increased overall albedo observed by satellite.Contrastingly,grassland greening and more precipitation produced decreased albedo.Hence,the greening of forests and grasslands,representing dense and scarce vegetation respectively,has contrasting albedo responses.