Although there is a large investment of nitrogen (N) in photosynthetic proteins the photosynthetic capacity per unit N (PNUE) may change with environment and have evolved in large interspecific variation. We questioned whether the PNUE changed with latitude and why. We assumed that ecotypes from low annual temperature (T) would invest relatively more nitrogen in photosynthesis to perform at low T, and a shorter growth season. Fagus crenata 20 year old trees of 9 ecotypes from south to north Japan were grown in a common garden in Tomakomai, Hokkaido. The PNUE decreased with an increase in annual T as a result of the increase in leaf N content. The LMA and photosynthetic capacity did not change suggesting that leaf N was invested in others than photosynthetic proteins. If higher N investment in cell walls decreased the PNUE, we would assume an increase in N resorption proficiency (senesced leaf N content). The PNUE was weakly negatively related to N resorption proficiency. Thus ecotypes of low annual T had a higher PNUE because they tended to invest less in cell wall N but other factors might also play a role. The higher investment in cell walls at high annual T might protect the leaf against other co-factors such as drought or herbivory.