六盘山华北落叶松人工林土壤水分空间异质性的降雨前后变化及其影响因素

Variation and Main Influencing Factors of Spatial Heterogeneity of Soil Moisture of Larix principis-rupprechtii Plantation Before and After Raining in Liupan Mountains of Ningxia,China

为了阐明林冠叶面积指数、枯落物厚度等植被因子及降雨量影响下的林地土壤水分空间异质性变化规律,在宁夏六盘山香水河小流域选择了2个35a华北落叶松典型坡面(东南坡A、南坡B),遵循空间格局分析的小支撑、多样点设计原则,结合所选坡面特征,利用线状样带取样法,测定了坡面上各样点的表层(0—20cm)和亚表层(20—40cm)土壤水分、林冠叶面积指数、枯落物层厚度、坡度及离开最近一棵树主干的距离,采用经典统计结合地统计学方法,分析了降雨前后土壤水分空间变异特征;利用Pearson相关分析法分析了降雨前后坡面土壤水分与主要环境因子的数量关系;经Kriging插值得到了实验坡面降雨前后的土壤水分空间分布图。结果表明:本研究选择的2个坡面上表层和亚表层土壤水分均存在显著的空间自相关性,呈现明显的斑块状分布格局。A、B坡面雨前表层土壤水分的块基比分别为0.20和0.16,有强烈的空间自相关性;亚表层的块基比均为0.50,具有中等的空间自相关性;表层土壤水分变程分别为27.6m和23.7m,亚表层土壤水分变程为54.9m和186.0m,空间自相关性存在于较大的范围内。降雨后A、B坡面的表层土壤水分的块基比分别为0.13和0.11,亚表层的块基比为0.28和0.20;表层土壤水分变程分别为17.7m和17.3m,亚表层土壤水分变程为31.4 m和42.6 m,空间自相关性存在的范围变小。Pearson相关分析表明,A坡面土壤水分的空间变化在雨前主要受样点离开树木的距离和坡度的影响,雨后则主要受样点上方叶面积指数和枯落物厚度的影响。由此可见,此次11mm降水使2个坡面上土壤水分的空间异质性增加、空间自相关性增强,且乔木林冠层和枯落物层对降雨的截持和再分配是雨后土壤水分空间异质性增大的主要原因。

Two typical slopes of 35-year-old Larix principis-rupprechtii plantation（southeast slope A and south slope B）were chosen to be studied in order to elucidate the spatial characteristics of soil moisture under the influence of rainfall and vegetation factors such as canopy leaf area index and litter layer thickness in forest land.Following the principles and methodology of spatial pattern analysis in geostatistics,and took the characteristics of the selected slopes into consideration,the linear sampling method were chosen to measure the moisture of surface（0—20cm）and subsurface（20—40cm）soil,canopy leaf area index,litter layer thickness,slope degree and distance from the nearest tree trunk.The spatial variation characteristics of soil moisture before and after raining were analyzed combining with classical statistics and geostatistics methods,the relationship between soil moisture and the main environmental factors before and after raining were analyzed using the Pearson correlation analysis,the spatial distribution of soil moisture before and after raining on the slopes was obtained by Kriging interpolation.Result indicated that there was significant spatial autocorrelation of soil moisture of surface and subsurface layer on both selected slopes.In addition,the soil moisture presented an obvious patchy distribution pattern.Before raining,structural ratio of surface soil water content were 0.20 and 0.16 respectively on slope A and B,indicated that soil moisture of surface layer had a strong level of spatial autocorrelation selected slopes.Meanwhile,the structural ratio of subsurface soil water content was 0.50 on both slope,soil moisture of subsurface layer indicated medium level of spatial autocorrelation on both chosen slopes.The range of the autocorrelation functions of surface soil moisture on slope A and B was from 27.6mto 23.7m,and the range of subsurface soil moisture was from 54.9mto 186.0m respectively,manifested that spatial autocorrelation of the soil moisture existed in a large range.After raining,the structural ratio of surface soil water content were 0.13 and 0.11 on slope A and B,the ratio of subsurface were 0.28 and 0.20 respectively,the range of the autocorrelation functions of surface soil moisture was from 17.7mto 17.3m,and the range of subsurface was from 31.4mto 42.6mrespectively.Those results indicated a shrinking trend of the range of spatial autocorrelation after rainfall.Pearson correlation analysis revealed that before rainfall,the distance from the nearest tree trunk and litter layer thickness made the main impact factors of the spatial variation of soil moisture on slope A.However,canopy leaf area index and litter layer thickness became more significant variables after raining.So,this precipitation process（11mm of rainfall amounts）increased the spatial heterogeneity and autocorrelation of soil moisture,in addition,the interception and redistribution of rainfall caused by the tree canopy and litter layers was the main reason of the increase of the soil moisture spatial heterogeneity in the study area.