Asymmetric patterns and temporal changes in phenology-based seasonal gross carbon uptake of global terrestrial ecosystems

Aim To study global patterns and temporal changes in the seasonal dynamics (quantity and seasonal distribution) of terrestrial gross carbon uptake in response to global environmental change. Location Global. Time period 2000-2016. Major taxa studied Terrestrial ecosystems. Methods Following a phenology-based definition of photosynthetic seasonality, we decompose gross primary production (GPP) into three periods, green-up, maturity and senescence, and derive their corresponding GPP (GPPgp, GPPmp and GPPsp, respectively) from a newly developed time series of satellite-based global GPP to study spatio-temporal dynamics of seasonal GPP. Results We find that the global fraction of GPPsp (19.8%) is larger than GPPgp (14.3%), indicating a globally asymmetric seasonal distribution of gross carbon uptake by terrestrial ecosystems. Globally, GPPmp plays a dominant role in shaping spatial patterns and increasing/decreasing trends in GPP, while GPPgp/GPPsp contributes to increasing GPP at the regional scale. Higher fractions of GPPgp/GPPmp (lower of GPPsp), as well as the co-occurrence of increasing GPP and non-tree vegetation cover in major croplands, are likely to be caused by agricultural intensification. Global changes in GPPgp and GPPsp are closely related to changes in their seasonal distributions (R = .86/.8, respectively), whereas this relationship is weaker for GPPmp (R = .53). Finally, high correlations are observed between changes in GPPgp and GPPsp and changes in their durations (R = .78/.78, respectively), while GPPmp shows a relatively lower correlation with its duration (R = .67). Main conclusions The asymmetric spatio-temporal patterns in the seasonal dynamics of global terrestrial gross carbon uptake found here have been substantially reshaped by anthropogenic land-use/cover changes and changes in photosynthetic phenology. Compared to calendar-based meteorological seasons more suitable for temperate/subpolar ecosystems, our phenology-based approach is expected to provide an alternative starting point for a better understanding of global spatio-temporal changes in the seasonal dynamics of terrestrial ecosystem processes and functioning under accelerating global change.