中文摘要：

以黄土丘陵区峁坡和沟坡刺槐林为研究群落,在2002年5、7和9月观测了0～250cm深度土壤水分（Wc）变化和植物气体交换日动态。峁坡Wc明显低于沟坡,在生长季的大多数时间里80cm以下深度的Wc接近凋萎系数。沟坡刺槐林的净光合速率（Pn）大于同一生长期的峁坡刺槐林,而蒸腾速率（Tr）的差别不显著,表明光合速率对Wc的胁迫更敏感。在Wc为9.35%时,刺槐的水分利用效率达到最高。通过研究Pn与胞间CO2浓度（Ci）的比值及气孔限制因子（Lg）随Wc的变化,进一步分析了水分胁迫下刺槐的气孔限制和非气孔（生物化学）限制。结果表明,Wc降到8%左右时,刺槐的Pn主要受气孔限制,而在5%～8%时,主要受非气孔限制,即这时水分胁迫已经引起了刺槐叶肉细胞光合能力的下降。总之,沟坡刺槐受水分胁迫较小,而峁坡刺槐生长受到严重的水分胁迫。

英文摘要：

Two black locust forests,growing on the convex slope and concave slope respectively in Hilly Loess Plateau,were selected as research communities. The soil water content （Wc） and diurnal gas exchange were observed in May,July and September of 2002. The forest soil water content on the convex slope,which was close to the wilting coefficient under 80 cm depth during the most growing season,was obviously worse than that on the concave slope. The net photosynthesis rate（Pn）of the forest on the concave slope was high than that on the convex slope,while the transpiration rates （Tr） did not show obvious difference. It means that the photosynthesis may be more sensitive than the transpiration to soil water stress. The highest water use efficiency （WUE） was observed at 9.35% soil water content. The stomatal and non-stomatal （biochemical） limitation were also analyzed based on the ratio of net photosynthesis rate to intercellular CO2 concentration （Pn/Ci） and the stomatal limitation factor （Lg）. It shows that the photosynthetic depression occurs in response to stomatal closure while the water content declines to about 8%,while the decrease in Pn at Wc between 8% and 5% is attributable to non-stomatal limitation. In summary,concave slope is considered unsuitable for planting black locust forest,because black locust community grows under severer water stress on concave slope than that on convex slope.