中文摘要：

确定土壤水分的植被承载力是我国北方半干旱地区合理调控土壤水分和植被生长关系、科学恢复林草植被的核心问题。我国北方半干旱地区的土壤水分主要来自大气降水,作为土壤水分限制型生态系统的坡面植被,植物种类和植被密度等结构特征与降水量紧密相关。根据六盘山石质山区的特点,基于水量平衡原理,建立了阴坡华北落叶松（Larix principis-rupprechti）和阳坡草地的土壤水分植被承载力数学模型,即由4～10月的生长季降雨量（P）计算得到可承载的叶面积指数（Leaf area index,LAI）公式：LAItree=exp（（0.7731×P–186.12）/146.46）和LAIgrass=exp（（0.5112×P–345.93）/227.89）,并提出了考虑坡面水分再分配影响的不同坡位的土壤水分植被承载力计算方法。阴坡华北落叶松的植被承载力（用LAI表示）从坡顶的1.45升高到坡中的4.83,然后稳定在3.0～3.3。对于阳坡草地,土壤水分可承载的LAI从坡顶的0.37上升到坡中的0.46,然后在0.41～0.47之间变动,LAI的计算值与实测值较为接近,计算结果比较合理。

英文摘要：

Aims Soil water is the main limiting factor for vegetation,and it mainly comes from pre-cipitation in the Diediegou small watershed in the Liupan Mountains,northwestern China,where the structure of slope vegetation is usually closely related with the amount of precipitation. Our objectives were to develop a method for estimating vegetation carrying capacity （VCC） on sites based on water balance and to evaluate variation of VCC along typical slopes,for guiding the restoration and manage-ment of vegetation on similar slopes. Methods We established nine plots along two typical slopes of natural grassland and larch （Larix principis-rupprechti） plantation,measured leaf area index （LAI） every 15 days and continuously moni-tored all water fluxes and soil water dynamics. After analyzing plot water balance,we determined the relationships among evapotranspiration （ET）,LAI and precipitation to estimate VCC based on growing season precipitation and calculated the variation of VCC along slopes.Important findings Our mathematical models for determining VCC （expressed as the maximal LAI） based on precipitation amount （P,mm） in growing season （April–October） were LAItree=exp （（0.773 1×P–186.12）/146.46） for the larch stand on the shady slope and LAIgrass=exp （（0.511 2× P–345.93）/227.89） for the grassland on the sunny slope. The VCC of larch stands on the shady slope in-creased from upper slope （1.45） to middle slope （4.83） and then stabilized between 3.0 and 3.3. For grassland on the sunny slope,VCC increased from upper slope （0.37） to middle slope （0.46） and then stabilized between 0.41 and 0.47. Measured values of LAI were close to those of simulated VCC,sup-porting the rationality of our method and the result. Using LAI as an index expressing VCC,which is mainly determined by soil water in dryland regions,is a new method for calculating VCC for slopes where precipitation water is redistributed in the forms of runoff and interflow.