To understand links between plant diversity and ecological/evolutionary backgrounds, and their implications for ecosystem functions, ecophysiological approaches are essential. In this talk, I will introduce our ongoing effort on trait-based approach for forest ecosystems in Japan. We built the trait database of woody species which spans >300 species over 26 sites from boreal to subtropical climates. Among communities, there were clear temperature gradients in traits. For example, leaf mass per area (LMA) increased and leaf N concentration decreased with increase in temperature. Such systematic changes in traits were linked with net primary production (NPP). NPP itself declined with decreasing temperature, but its reduction was not so much as expected for the case with a constant trait value across temperature. Lower LMA at cold regions increased leaf area for a given leaf mass, resulting into no reduction in light interception efficiency at stand levels. Lower LMA at cold regions in theory reduces the amount of photosynthetic machinery, but leaf N content per unit area were maintained due to higher N concentration, which should mitigate declines of photosynthetic rate with lower temperature. These results suggest that plant diversification across temperature contributes to stability of ecosystem functions.