六盘山半湿润区坡面华北落叶松林冠层截留的时空变化及空间尺度效应

冠层截留是森林蒸散的重要组成部分,其定量描述一直是生态水文学研究的热点问题之一,相关观测研究多是在固定样地的空间尺度上进行的。林冠截留量除在样地上受降水、气象条件和冠层结构及生长过程的动态影响外,还在坡面上受植被生长和冠层结构的坡位差异而有很大空间变化和尺度效应,需对此深入理解和精细刻画,从而为实现从样地观测结果上推到坡面的尺度转换并基于此准确评价森林水文影响来提供科学依据。在六盘山香水河小流域,选择一个水平坡长425.1m的华北落叶松人工林典型坡面,并均匀划分为空间连续的16个样地。于2015年5—10月,测定了森林冠层结构(特别是叶面积指数,LAI)的坡位差异和动态变化,并同步观测了林外降雨和林内穿透雨及树干茎流,依此计算得到每次降雨事件的冠层截留量。以"离坡顶水平距离(水平坡长)"为尺度标量,分析林冠截留量沿坡面的时空变化规律及影响因素。结果表明,林冠截留量存在沿坡变化和坡面空间尺度效应,且月份变化明显。在整个生长季,林冠截留量沿坡变化总体表现为随水平坡长增加先升高,在坡面中部达最大,之后逐渐降低,以每100m水平坡长的变化量表示的坡面尺度效应为+5.62mm/100m(坡段0~316.6m)和-2.37mm/100m(坡段316.6~425.1m);在5月,林冠截留量沿坡变化总体表现为随坡长增加而逐渐降低,坡面尺度效应为-0.26mm/100m(0~425.1m),在6—8月,林冠截留量沿坡变化总体趋势均和整个生长季相同,6月份坡面尺度效应为+1.28mm/100m(0~261.1m)和-1.78mm/100m(261.1~425.1m),7月为+0.92mm/100m(0~267.6m)和-0.88mm/100m(267.6~425.1m),8月份为+1.28mm/100m(0~211.2m)和-0.34mm/100m(211.2~425.1m);在9月、10月,林冠截留量沿坡变化表现为随坡长增加逐渐升高,坡面尺度效应分别为+2.38mm/100m和+0.81mm/100m(0~425.1m)。林冠层叶面积指数的坡位差异及其季节变化是影响林冠截留时空变化的主导因素,各月林冠截留率均与叶面积指数呈显著正相关。可基于不同坡位样地的叶面积指数与其坡面均值的关系、叶面积指数与林冠截留量的关系以及不同坡位样地的叶面积指数测定值,上推估计林冠截留的坡面均值。

Spatial-temporal Variation and Scale Effect of Canopy Interception on a Larix principis-rupprechtii Plantation Slope in Liupan Mountains, Ningxia, China

Canopy interception is a significant proportion of evapotranspiration of forest ecosystems,its quantification remains as one of hot issues in eco-hydrological studies.However,most studies were carried out just in the small scale of stand/plot.Since the canopy interception is influenced by many factors (e.g.rainfall,meteorological condition,canopy structure and growth process) at the stand/plot scale,and by the difference of forest canopy structure among slope positions,it often appears great spatial variation and scale effect at the slope scale.In order to realize the up-scaling from the measured value at plot to the whole slope,and to provide scientific basis for the accurate evaluation of forest hydrological impacts,the spatial-temporal variation and scale effect of canopy interception along the slope has to be precisely described based on deepgoing studies.In this study,a representative slope with a horizontal length of 425.1 m and covered by Larix principis-rupprechtii plantation was selected in the small watershed of Xiangshuihe of Liupan Mountains in northwest China,and it was evenly divided into 16 continuous plots.In the growing season of 2015 (from May to October),the dynamics of stand/plot canopy structure (especially the leaf area index,LAI) in different months and its difference among the 16 plots along slope position were monitored;meanwhile,the gross rainfall at open field and the throughfall and stemflow was measured to calculate the canopy interception of each plot during each rainfall event.The variation of canopy interception along slope positions and within different months and their main influencing factors were analyzed using the "horizontal distance from the slope top (horizontal slope length)" as a scale scalar.The results showed that the canopy interception presents a remarkable variation along the slope and spatial scale effect,with a remarkable difference among the months.During the whole growing season,the overall variation tendency of canopy interception with rising horizontal slope length showed firstly an increase,reaching its maximum at the middle slope,and then a decline.The scale effect was +5.62 mm/100 m within the horizontal slope length of 0～316.6 m and-2.37 mm/100 m within the horizontal slope length of 316.6～425.1 m.In May,the variation of canopy interception along rising horizontal slope length showed a decreasing tendency,with the scale effect of-0.26 mm/100 m within the horizontal slope length of 0～425.1 m.In June,July and August,the variation tendency of canopy interception along slope length was consistent with the whole growing season.In June,the scale effect was +1.28 and-1.78 mm/100 m within the horizontal slope length of 0～261.1 m and 261.1～425.1 m.In July,the scale effect was +0.92 and-0.88 mm/100 m within the horizontal slope length of 0～267.6 m and 267.6～425.1 m.In August,the scale effect was +1.28 and-0.34 mm/100 m within the horizontal slope length of 0～211.2 m and 211.2～425.1 m.In September and October,the variation of canopy interception along the slope showed an increasing tendency.The scale effect was +2.38 and +0.81 mm/100 m within the horizontal slope length of 0～425.1 m,respectively.The LAI was the main factor affecting the spatial-temporal variations of canopy interception.The correlation between canopy interception ratio and LAI was positive and significant in every month.The slope average of canopy interception could be estimated through the up-scaling from the measured LAI at certain plot,the relation between plot LAI at different slope positions and the slope average of LAI,and the relation between canopy interception and plot LAI.