The annual peak growth of vegetation is critical in characterizing capacity of terrestrial ecosystem productivity and shaping the seasonality of the atmospheric CO2 concentrations. The recent greening of global lands suggests an increasing trend of terrestrial vegetation growth, but whether the peak growth has been globally enhanced still remains unclear. Here we used two global datasets of gross primary productivity (GPP) and a satellite-derived Normalized Difference Vegetation Index (NDVI) to characterize recent changes in annual peak vegetation growth (e.g., GPPmax and NDVImax).
Results/Conclusions
We demonstrated that the peak in growth of global vegetation had been linearly increasing during the past three decades. About 65% of the NDVImax variation was evenly explained by the expanding croplands (21%), rising CO2 (22%), and intensifying nitrogen deposition (22%). The contribution of expanding croplands to the peak growth trend was substantiated by measurements from eddy-flux towers, sun-induced chlorophyll fluorescence, and a global database of plant traits, all of which demonstrate that croplands have a higher photosynthetic capacity than other vegetation types. The large contributions of CO2 was also supported by a meta-analysis of 466 manipulative experiments and 15 terrestrial biosphere models. Furthermore, we showed that the contribution of GPPmax to the change in annual GPP was less in tropics than in other regions. These multiple lines of evidence reveal an increasing trend in peak growth of global vegetation. The findings highlight the important roles of agricultural intensification and atmospheric changes in reshaping the seasonality of global vegetation growth.