Abstract: Quercus mongolica is the dominant tree species in the coniferous and broadleaved mixed forests in Changbai Mountains. In this paper, an investigation was made on the effects of decreased rainfall (artificially intercepting 30% of rainfall) on the leaf light-saturated photosynthetic rate, stomatal conductance, transpiration rate, and water use efficiency of Q. mongolica during its growth season (from June to September). In early growth stage (June) when the rainfall was low, there was no significant difference in light-saturated photosynthetic rate between decreased rainfall and the control, though the decreased rainfall decreased soil moisture content by 26.4%. However, the decreased rainfall increased the water use efficiency of Q. mongolica by 40.3%, due to the decrease of transpiration rate. In July-September when the rainfall was abundant, the decreased rainfall had no significant effects on the light-saturated photosynthetic rate, stomatal conductance, transpiration rate, and water use efficiency, because no difference was observed in the soil moisture content between decreased rainfall and the control. Thereby, whether in low rainfall period (June) or in abundant rainfall period (July-September), a 30% decrease of rainfall in Changbai Mountains could less affect the photosynthetic capacity of mature Q. mongolica during its whole growth season. The high photosynthetic rate in August suggested that the relatively high soil moisture content did not limit the water absorption by Q. mongolica. This study showed that Q. mongolica had a wide adaptation to soil moisture content, and could maintain its growth potential in the future climate warming and precipitation fluctuation.

Key words: Land ecosystem, Complexity, Chaos, Fractal