Understanding the role of phenology and summer physiology in controlling net ecosystem production: a multiscale comparison of satellite, PhenoCam and eddy covariance data

Abstract

Understanding the temporal and spatial variability (SV) of net ecosystem productivity (NEP) is critical for coupling ecosystem carbon (C) cycle and climate system. Previous studies have shown responses of NEP to changes of plant phenology, but impacts of summer physiological status on annual NEP and how this may vary across different ecosystems and spatial scales were largely unknown. Combining large regional satellite derived indices (MODIS), 676 site-year local data (FLUXDATA) covering seven vegetation types, and 57 site-year regional data (PhenoCam), we found that phenological metrics and summer physiological indicators were significantly correlated with their respective gross primary production-based estimates. The interannual variability of NEP was mostly explained by summer physiology than phenology for most ecosystems, while phenology showed a better performance than summer physiology only for grassland sites. In comparison, inconsistent results were derived from three scales when explaining the SV of NEP. Summer physiology showed more potential in interpreting the SV of NEP at large regional scale, but both physiology and the length of growing season exhibited similar performances at local scale. Observations from regional scale were not able to explain NEP, given that the green chromatic coordinate signal cannot track photosynthesis in summer with a high canopy closure. The finding highlights the important role of summer physiology in controlling the C accumulation of terrestrial ecosystems and understanding the responses of summer physiology to environmental drivers is of great significance for improving the simulation accuracy of C sequestration under the global climate change.