Gas exchange and resource utilization in two alpine oaks at different altitudes in the Hengduan Mountains

The evergreen sclerophyllous broadleaf forests composed of alpine oaks in the Hengduan Mountains of southwest China are mainly distributed at altitudes of 1700-4800 m. Gas exchange and related leaf traits of Quercus guyavifolia H. Lév. and Quercus pannosa Hand.-Mazz. were measured at eight sites along an altitudinal gradient to understand their physiological adaptabilities. Both Q. guyavifolia and Q. pannosa showed a significant midday depression in the photosynthesis rate (P_N) due to a high vapor pressure deficit and high temperature around noon. The optimum temperature for photosynthesis for the two oak species ranged from 17 to 23°C depending on altitude. When the temperature exceeded 25°C, P_N decreased sharply. However, P _N was not depressed by high irradiance. As altitude increased, the leaf dry mass per unit area of the two oaks increased but the chlorophyll content decreased, while the maximum values of daily mean photosynthesis rate (P_d), maximum photosynthesis rate (P_max), P _d/P_max, water-use efficiency, and photosynthetic nitrogen use efficiency occurred at altitudes of 3240-3610 m. Although the photosynthetic capacities of the two oaks were higher in August than in May, altitudinal trends did not change with season. The altitudinal range from 3240 to 3610 m would be optimal for the growth and development of these two alpine oaks in the Hengduan Mountains.