Most of the world’s terrestrial biome types can be found in China. To systematically investigate species composition and structure of China’s forest communities, we launched a long-term project consisting forest vegetation surveys across China’s mountains in the mid 1990s. Over the study period, we have conducted vegetation surveys for 65 mountains and collected vegetation data from about 1500 forest plots, using consistent sampling protocols. In this paper we first introduce the aims, protocols, and major research themes of the project, and then describe the major characteristics of forest communities and their geographic patterns and climatic controls. As latitude increased, diameter at breast height (DBH) and height of trees increased, while individual density of trees and woody species richness decreased. Total basal area (TBA) of trees and species richness of herbs did not vary with latitude. Contemporary climate seems to drive these patterns: temperature was the leading factor for DBH, precipitation was most important for tree height and individual density, actual evapotranspiration (a surrogate of productivity) determined woody (trees and shrubs) species richness, and rainfall was the major controller of the herb species richness. The species–abundance relationship showed that species dominance (measured by the number of individuals per species) declined significantly from boreal forests to evergreen broadleaf forests from north to south. Our results are in line with the idea that productivity drives woody species richness. Similarly, we find that biomass (measured as TBA) is invariant along the environmental gradients. However, individual density varies dramatically, in contrast to the assumptions underlying the metabolic theory of ecology.