With global warming trees are and will be exposed to a considerable change in climate within their life. How trees respond to an increase in temperature, specifically in their nitrogen (N) allocation to leaves, we studied by experimental warming. In tomakomai experimental forest, hokkaido, 4 adult Quercus crispula trees are warmed up to 5 degrees higher soil temperature since 2007. In the top of the canopy the photosynthetic capacity, leaf respiration, leaf mass and N content were determined monthly during the growing season from 2007 to 2009. When the soil was warmed the leaf respiration was lower in spring than in control trees indicating an earlier development. Also leaf nitrogen content tended to be lower and leaves used nitrogen more efficient (photosynthetic capacity per unit nitrogen, PNUE) than in control trees. In August 2009 the higher PNUE was ascribed to a higher Rubisco per unit nitrogen. A higher Rubisco per unit N was at the cost of a lower allocation of N to cell walls, however this was independent of experimental warming. Although canopy leaves are not directly exposed to soil warming small effects on leaf characteristics were shown indicating the importance of interaction between soil and aboveground temperature for the leaf canopy.