Canopy clumping index (CI) indicates the degree to which foliage deviates from the random distribution and is a critical canopy structural variable used in many ecosystem process models. CI has frequently been used for leaf area index (LAI) mapping, and to calculate the proportions of sunlit and shaded leaves in ecosystem process models.

CI affects the interception of radiation and its distribution within a canopy, and influences canopy photosynthesis and evapotranspiration (ET). Previous studies have shown that both the gross primary productivity (GPP) and ET will be substantially underestimated if CI is not properly considered.

Many field measurements have shown that CI exists a strong seasonal and inter-annual variations. However, a global CI product showing proper seasonal and inter-annual variations is lacking. This greatly limits the CI study and GPP and ET estimations.

Dr. WEI Shanshan  and Prof. FANG Hongliang, Institute of Geographic Sciences and Natural Resources Research (IGSNRR), and other international scholars, proposed a new look-up table (LUT) method, 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.

Global validation studies show that the new CI product has a higher accuracy than those in existing studies. The deciduous broadleaf forest and mixed forests show strong seasonal CI variation and this character could to be used to detect the vegetation phenology change.

The CI shows a negative correlation to the leaf area index (LAI) for all vegetation types, especially forests. The global CI demonstrates an inter-annual variation that correlates with the global precipitation change, and from 2001 to 2017, the global CI shows a decreasing trend, consistent with the increasing LAI.

These findings show that with the increasing amount of global leaves, their spatial distribution has become more aggregated. The study reveals, for the first time, the unique seasonal and long term variation patterns for the leaf spatial distribution and provides a new prospective to examine the global vegetation change.

The study was mainly supported by the National Key Research and Development Program of China and National Natural Science Foundation of China. The Chinese Academy of Sciences CI product (CAS-CI) is available from the National Earth System Science Data Center (www.geodata.cn) or by contacting H.F. (fanghl@lreis.ac.cn).

Reference:

Wei, S., Fang, H., Schaaf, C. B., He, L., and J. M. Chen, 2019. Global 500 m clumping index product derived from MODIS BRDF data (2001-2017). Remote Sensing of Environment. 232, 111296. https://doi.org/10.1016/j.rse.2019.111296.