Nitrogen supply influences photosynthetic acclimation of yellow birch (Betula costata Trautv.) to the combination of elevated CO2 and warmer temperature

Rising CO 2 , global warming, and N deposition create challenging environmental conditions to vegetation. Since elevated CO 2 and rising temperature are coupled with each other, it is important to understand their combined effects on plants. We investigated the growth and photosynthetic responses of yellow birch to five levels of nitrogen supply under the current (cCT: current CO 2 and temperature) and the predicted future CO 2 and temperature conditions (fCT: elevated CO 2 , current + 4℃ temperature). The results show that fCT and high N supply increased seedling growth but fCT reduced photosynthetic capacity (e.g., maximum rate of Rubisco carboxylation- V cmax , maximum rate of photosynthetic electron transport- J max )) and foliar N concentration. However, the magnitude of the fCT effect declined with increases in N supply. Furthermore, the fCT treatment significantly reduced the J max / V cmax ratio, indicating a possible shift of N allocation from J max to V cmax in the photosynthetic machinery. This result suggests that the photosynthesis of yellow birch may be more limited by electron transport under the predicted future climate condition. Both low N supply and fCT significantly increased photosynthetic N use efficiency ( PNUE ) and there was a negative relationship between PNUE and photosynthetic capacity. In general, yellow birch grew better under fCT than cCT, particularly above-ground growth.