Temperate forests and grasslands carry key ecosystem functions and provide essential services. To assess ecosystem function on carbon sequestration ability and biomass at regional and global scales, remote-sensing derived greenness has been widely used which is subject to the changes in both climate and land cover/use. Although reforestation and grassland restoration have been proposed to enhance the regional greenness in northern China, the independent contribution from climate has rarely been explored. To separate the impacts of climate from those of land cover/use change on vegetation greenness, we identified large patches of forests and grasslands in northern China according to our previous annual land cover/use mapping results, and evaluated their changes in greenness with climate. We derived the greenness of vegetation patches using the MODIS EVI datasets in 2001– 2014.
Most forest patches showed increasing EVI in the new century. For the forest patches with significant trend in EVI at p<0.1, the average trends of annual and maximum EVI are 2.7±0.11×10-3 and 2.5±0.45×10-3, respectively. On the contrary, grassland patches showed great spatial heterogeneity in the trends of EVI. Precipitation showed mostly increasing trends for both forests and grasslands. The average trends in annual and growing-season precipitation are 12.94±0.27 and 12.53±0.29 mm yr-1, respectively, for the forests with corresponding trends significant at p<0.1. The average trends in annual and growing-season precipitations are 12.06±0.14 and 9.45±0.52 mm yr-1, respectively, for the grasslands with corresponding trends significant at p<0.1. Minimum temperature increased during the growing season, but decreased during the non-growing season. Maximum temperature mostly decreased during non-growing season. However, few of the trends are significant at p<0.1. The relationships between changes in EVI and those in climate revealed that the EVI of grassland were significantly promoted by the enhancing precipitation, especially that in growing season. However, in addition to the precipitation, the elevated growing-season maximum temperature furthered the greenness in the forests.