Global 500 m clumping index product derived from MODIS BRDF data (2001-2017)

The vegetation clumping index (CI) quantifies the degree to which foliage deviates from a random distribution, and therefore, is an important canopy structural parameter that governs the photosynthesis and evapotranspiration processes in terrestrial ecosystems. While the temporal variation of CI has been investigated in many field studies, there has not been a global CI product that captured its seasonal and inter-annual variations. In this study, a look-up table (LUT) method was developed based on an improved Normalized Difference between Hotspot and Darkspot (NDHD) method to estimate the daily CI from the MODIS V006 MCD43 BRDF (bidirectional reflectance distribution function) product. This new CI product is consistent with field measurements at 33 sites distributed globally (R-2 = 0.38, RMSE = 0.04, and bias = -0.01). CI shows stronger seasonal variations for deciduous broadleaf forest (DBF) and mixed forests than other vegetation types. The large seasonal differences between evergreen and deciduous vegetation, especially for DBF and deciduous shrubs, allow the CI to be used to detect seasonal structural changes and classify vegetation. The global CI shows a negative correlation to the leaf area index (LAI) for all land cover types, especially forests. The global CI demonstrates an inter annual variation that correlated with global precipitation (R-2 = 0.34). In addition, the global CI shows a global decreasing trend from 2001 to 2017 (-0.007/decade) that was consistent with the increasing LAI (0.049/decade). The time series of CI provides a new perspective for understanding the structural characteristics of vegetation in global ecosystem studies.