Long-term temporal characteristics of vegetation clumping index remote sensing products at mid/high latitudes in the Northern Hemisphere – a case study

Clumping index (CI) is a vital structural parameter characterizing non-random spatial distribution of vegetation canopy leaves． Quantitative research on CI started relatively late compared to other vegetation structural parameters． This is especially true for the study of seasonal variations in CI, resulting in significant controversies． In this work, we conducted a case study on seasonal characteristics of typical CI pixels in mid/high latitudes vegetation phenology-sensitive regions in the Northern Hemisphere, using recently developed long-term MODIS CI products developed by us． We selected 84 high-quality representative pixels from 13 land cover types designated by the International Geosphere-Biosphere Program (IGBP)． We applied an enhanced dynamic threshold method in conjunction with discrete Fourier transform to extract start of season (SOS) and end of season (EOS) for various land cover types, using normalized difference vegetation index (NDVI) as reference． We established empirical Ω values for both growing season and dormant period across various landcover types． CI exhibits distinct phenological and seasonal variations, to even detect biannual cropping signs in cropland． In comparison with the relatively stable seasonal variations of NDVI, CI exhibits somewhat large variability and uncertainty in most land cover types． SOS and EOS vary within approximately 100 and 300 days, respectively; the growing season typically spans around 200 days． The optimal threshold values for extracting phenological parameters vary dependent on extraction period and landcover type． The thresholds for SOS vary within the range from 40%-80%, but for EOS, from 80%-90%． Empirical Ω values show the strongest clumping effect in coniferous forests and the weakest in croplands．