Globally Consistent Patterns of Asynchrony in Vegetation Phenology Derived From Optical, Microwave, and Fluorescence Satellite Data

Abstract

Climate change is impacting vegetation phenology with important implications and feedbacks to global carbon, water, and nutrient cycling. Satellite remote sensing can monitor multiple aspects of land surface phenology and its sensitivity to climate. Normalized difference vegetation index (NDVI) tracks vegetation chlorophyll content. Vegetation optical depth (VOD) tracks the total water content of aboveground biomass. Solar-induced chlorophyll fluorescence (SIF) more directly approximates vegetation gross photosynthesis. Yet it remains unclear how these different indices, representing independent vegetation development processes, covary over the course of a growing season and across the global domain. To address this gap, we synthesize information from all three indices and enable a deeper understanding of seasonal phenology that goes beyond seasonal photosynthesis. We derive and evaluate 9-year average timing of start of growing season (SOS), peak of growing season (POS), and end of growing season (EOS) for each of these satellite indices globally. We found SOS occurs relatively synchronously, but they become increasingly asynchronous as growing season progresses, such that POS and EOS from SIF occurred first, followed by NDVI and finally by VOD. In contrast to the relatively quick and synchronous start-of-season transition, senescence appears to be a relatively prolonged transition, beginning with reduced photosynthetic activity, then greenness/chlorophyll, and finally plant water content. Comparisons with gross primary productivity from eddy covariance flux tower observations confirm that SIF most closely tracks seasonal photosynthesis. NDVI, VOD, and SIF provide independent, complementary information on seasonal vegetation transitions and reveal new insights into the complex underlying functional and structural processes that control vegetation growth and senescence.