The leaf nitrogen (N) content within a single plant canopy follows the availability of light. The optimization theory explains that each leaf maximizes its daily carbon gain per unit N and therefore the leaf N content is higher at higher irradiance. However, the daily carbon gain changes not only with irradiance but also with temperature. We hypothesized that a higher temperature would lead to a lower leaf N at a certain light availability maximizing the daily carbon gain per unit N. To test this hypothesis we determined the light availability, photosynthetic rate and leaf N of individual leaves in the lower and upper canopy of Quercus crispula trees monthly from June to October 2010. The branches the leaves were on had experienced 5 º C higher than control branch temperature through experimental warming with electric heating cables. Leaf N increased with light availability and tended to be lower for warm branches in June and July. The relatively warm year of 2010 might have resulted in no difference between the leaf N in the other months. Interestingly the warm 2010 resulted in a high acorn production and in the colder 2008 branch warming increased acorn production significantly that could both be a consequence of a surplus of nitrogen not “necessary” to maximize the daily carbon gain per unit N.