Abstract: Aimed to understand the causes of the boundary formation of Quercus wutaishanica forest, four sites were chosen along a precipitation and thermal gradient from Qinling Mountain to Loess Plateau of northern Shaanxi Province, i.e., north slope of Qinling Mountain (Jiwozi)and southern (Chenjiashan), central (Huaishuzhuang), and northern site (Xiasiwan) of Loess Plateau, with the gas exchange, chlorophyll fluorescence parameters, stomatal density, and stomatal size of Q. wutaishanica leaves measured. From the south to the north along the precipitation and thermal gradient, weather varied from cool and humid to warm and semi-arid, and the maximum net photosynthetic rate (P_{n max}) and light saturation point (L_sp) of Q. wutaishanica leaves reduced obviously from 13.34 to 6.99 μmol CO₂·m^-2·s^-1 and from 1489.9 to 1055.6 μmol·m^-2·s^-1, dark respiration rate (R_d) and light compensation point (L_cp) increased evidently from 0.313 to 1.080 μmol CO₂·m^-2·s^-1 and from 7.49 to 31.96 μmol·m^-2·s^-1, the minimum fluorescence (F_o) and coefficient of non-photochemical quenching (NPQ) displayed an increasing trend, and actual photochemical quantum yield of PSⅡ (Φ_PSⅡ), apparent electron transport rates (ETR), and coefficient of photochemical quenching (q_P) reduced obviously. These results demonstrated that the photosynthetic capacity of Q. wutaishanica declined gradually with decreasing precipitation, but the stomatal density and stomatal size showed no obvious difference among the study sites except for the southern site of Loess Plateau. It was speculated that the depressed net photosynthetic rate of Q. wutaishanica leaves induced by the declined Φ_PSⅡ and ETR could be one of the main causes for the formation of northern boundary of Q. wutaishanica forest.

Key words: Quercus wutaishanica, light response curve, gas exchange, chlorophyll fluorescence parameters, stomatal density, Loess Plateau of northern Shaanxi Province