中文摘要：

受立地环境条件的坡位差异影响,叶面积指数（LAI）在坡面这个基本空间单元内往往具有很大空间（坡位、坡长）和时间（季节）变化,因而存在着坡面空间尺度效应及其季节变化,需对此加以深入研究、准确理解和精细刻画,从而为准确描述森林的结构、生长和生态水文功能提供科学基础。在六盘山香水河小流域选取了一个34年生华北落叶松（Larix gmelinii var.principis-rupprechtii）人工林典型坡面,均匀划分为空间连续的16个样地,于2015年5–10月测定了各样地林冠层LAI,分析其坡位变化及季节差异,并以LAI的顺坡滑动平均值在水平坡长增加100 m时的变化值（LAI/100 m）表示坡面尺度效应。研究表明,林冠层LAI具有明显的坡位变化和尺度效应,且月份变化明显。在5月份,LAI随离坡顶距离增加（坡位下降）而逐渐减小,坡面尺度效应是降低0.02/100 m。在6、7和8月份,LAI均从坡顶向下逐渐增大,在坡中部达最大后又逐渐减小,坡面尺度效应在离开坡顶的0–244.2 m和244.2–425.1 m范围内分别为升高0.15/100 m、0.16/100 m、0.18/100 m及降低0.09/100 m、0.08/100 m、0.07/100 m;在9和10月份,LAI均为随离坡顶距离增加而逐渐增大,坡面尺度效应分别为升高0.03/100 m和0.09/100 m。主要影响因素的季节变化导致了上述冠层LAI的坡位差异和尺度效应的季节变化。在5月份,主导因素是地形遮挡引起的辐射和温度的坡位差异;在6–8月份,主导因素是土壤含水量限制;在9–10月份,地形条件（海拔（坡位）、坡度）、气象条件、土壤含水量及土壤水文性质（孔隙度、持水量）等因子共同影响林冠层LAI坡位变化。对生长季各月,拟合了不同坡位样地林冠层LAI与坡面均值的比值随水平坡长增加的非线性关系,可藉此由任意坡位样地的LAI测定值估算坡面均值,以节省野外测定时间和工作量。

英文摘要：

Aims Leaf area index（LAI） is an important canopy structure parameter characterizing ecological and hydrological processes,such as forest growth,canopy interception and transpiration.Forest LAI is limited by both light and soil water availability,thus may vary with slope position and seasonality.This study is aimed at the spatiotemporal variation of LAI and its relationship with environmental variables.Methods A 34-years-old Larix gmelinii var.principis-rupprechtii planted forest situated on a typical slope located in a small watershed of Xiangshuihe within Liupan Mountains was selected for LAI observations.Sixteen plots along a 30 m wide transect along the slope was surveyed from May to October of 2015 to measure the monthly canopy LAI.Important findings It showed there was a remarkable difference of LAI among slope positions.The LAI in May decreased toward downslope direction with a scale effect of –0.02/100 m.Whereas for the period from June to August,LAI showed a nonlinear variation along slope positions：increasing from to top slope downward,reaching its maximum at the middle slope,and then decreasing to the slope foot.The scale effect of LAI was ＋0.15/100,＋0.16/100,and ＋0.18/100 m in the slope range（downward positive） of 0–244.2 m,but –0.09/100,–0.08/100,and –0.07/100 m in the slope range of 244.2–425.1 m for June,July and August,respectively.The LAI increased toward downslope in September and October,with a slope scale effect of ＋0.03/100 m and ＋0.09/100 m,respectively.The seasonal variation of LAI-slope relationship showed a shift from the light and temperature control in the early growing season,to the soil water resources control in the mid growing season,and then to an integrated control of many factors in the late growing season.In the early growing season when soil moisture and nutrients were abundant,terrain shading limited the leaf growth in middle and downslope.From early to the mid growing season,the soil moisture on the slope was quickly depleted due to fast evapotr