| The Sanjiang Plain is the largest agricultural reclamation area and the biggest marsh area in China.The regional vegetation coverage in this area is vital to local ecological systems,and vegetation growth is affected by natural and anthropogenic factors.The accurate estimation of regional foliage clumping index(CI)and the quantitative explorations of its spatio-temporal variation characteristics and driving factors are of great significance for the land surface models and the retrieval of other vegetation structures based on remote sensing data.However,most existing ecological models and the retrieval of other vegetation structures do not consider the spatial and temporal variations of CI,and few studies have focused on detecting driving factors that influence the spatial differentiation of CI.This study compared the field measurement estimation methods of foliage CI based on field measurement data in the Sanjiang Plain.The applicability of the Chinese Academy of Sciences Clumping Index(CAS CI)to the Sanjiang Plain was validated with field measurement data and Beijing Normal University Clumping Index(BNU CI).This study investigated the spatial and temporal variation characteristics of foliage CI in the Sanjiang Plain from 2001 to 2019,analysing the correlation between CI and leaf area index(LAI)through multiple methods(such as Theil-Sen trend analysis,the Mann-Kendall test,and the correlation coefficient)based on CAS CI and GLASS LAI(Global Land Surface Satellite Leaf Area Index).The driving factors of the spatial differentiation of CI were also investigated based on the geographical detector model with natural data and anthropogenic data.The main study contents and the results are as follows:1)The positive correlation between LX method and LXW method was the strongest(> 0.85).Both of them were in line with “The CI in the growing season was the smallest value in a year,and the CI in the early and late growing season was lager”.They also decreased with the increasing zenith angle,and the revised effects of these two methods on effective LAI were similar.Moreover,the consistencies of them with the ratio method based on digital hemispherical photography(DHP)were also similar.The ratio methods based on different optical instruments(DHP,LAI-2200,and Accu PAR)showed strong correlations(> 0.7),and the seasonal variations of them three were similar.The seasonal variation of CI directly measured by LAI-2200 was not obvious,which had the worst revised effect on effective LAI,and was inconsistent with the ratio method based on LAI-2200.The similarities and differences between different field measurement estimation methods were comprehensively reflected by comparing nine clumping indexes,which provided a reference for the subsequent study of the comparison of estimation methods.2)The direct and cross validation of CAS CI in the Sanjiang Plain further showed that CAS CI had good accuracy and could be applied to ground basic study in the Sanjiang Plain.The amplitude and pace of seasonal variation of the field measurement data were consistent with the CAS CI,and there was spatio-temporal consistency between CAS CI and BNU CI.3)The interannual variation of foliage CI was not obvious in the Sanjiang Plain from2001 to 2019,but the seasonal variation was obvious with the characteristics of “the minimum CI in summer and the maximum CI in winter”.75% of the study area showed the slight variation trend,and the slight degradation was the main variation trend of foliage CI from 2001 to 2019.The spatial distribution of CI in the Sanjiang Plain was similar to the spatial distribution of land use type,and the CI decreased slightly with increases in elevation.Moreover,the CI of cultivated land was greater than that of forestland.The correlation between the CI time series and the LAI time series was not significant from 2001 to 2017,but their spatial distributions were negatively correlated(k=-0.0407).Geomorphic type(0.499),elevation(0.474),and slope(0.418)were the dominant driving factors of the spatial differentiation of CI.Furthermore,the secondary driving forces of CI were vegetation type,land use type,and soil type.The climatic factors(average annual temperature,annual precipitation)and the aspect had significant influences on the spatial distribution of CI when interacting with other driving factors.There were optimal ranges or types of different driving factors for clumping effects of foliage,and the interactions between driving factors enhanced their explanatory power in terms of the spatial distribution of foliage CI.The dominant and secondary driving factors had significant differences.The analyses of spatial and temporal variation characteristics of regional CI and its driving factors will enrich the study materials in this field to a certain extent,which are of great significance to the study of ecology,climate,and biogeochemistry.Figure [62] Table [8] Reference [128]... |
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