Photosynthetic performances of Quercuspannosa vary with altitude in the Hengduan Mountains, southwest China

Quercus pannosa Hand.-Mazz. is an evergreen sclerophyllous oak that is an important component of montane forests at altitudes between 2500 and 4300 m in the Hengduan Mountains. To understand photosynthetic adaptability of Q. pannosa to alpine environments, photosynthetic performances and leaf traits were investigated along altitudinal gradients (at altitudes of 3240, 3650 and 4170 m, respectively). Observed trends with increasing altitude were: (1) progressively lower carboxylation efficiency (CE), light-saturated photosynthesis (Pmax), daily average photosynthetic rate (Pday), photosynthetic pigments, leaf N content per unit area, optimum photosynthetic temperature (Topt) and (2) progressively higher leaf mass per unit area (LMA) and ratio of intercellular CO2 concentration (Ci/Ca). However, stomatal conductance (Gs) and water use efficiency (WUE) remained relatively constant along the altitudinal gradient. Lower photosynthetic rates in Q. pannosa at higher altitudes would be explained by combination of different factors: reducing CE, lower leaf N content and photosynthetic pigments content, higher LMA and Ci/Ca. In addition, Q. pannosa showed a significant mid-day depression in Pn, Gs, E and WUE due to high water vapor pressure deficits (VPD) and high temperatures. In general, the Topt of Q. pannosa was significantly lower than those of Mediterranean oaks. This species could adapt well to low temperature, but high temperature, which indicated that cold-tolerance would be an important determinant of distribution of this oak in the Hengduan Mountains.