Global convergence of phenological and physiological controlling on annual terrestrial gross carbon dioxide uptake

Background/Question/Methods

Our ability to predict future terrestrial CO2 cycling is seriously limited by the poor understanding of spatiotemporal variability of gross primary production (GPP), which is the largest CO2 flux and driver of the contemporary carbon cycle in terrestrial ecosystem.  Basically, annual GPP of a terrestrial ecosystem is controlled by the phenological time span during which it keeps CO2 uptake (i.e., CO2 uptake period; CUP) and its physiologically maximal capacity to fix CO2 from the atmosphere (i.e., GPPmax) during the CUP.  Although the contributions of plant phenological properties or physiological processes to terrestrial GPP variability have been extensively studied, it is unknown how the phenological and physiological aspects synergistically control terrestrial annual GPP.  In this study, we analyzed observed GPP from a flux-net data set with 763 globally distributed site-year and the MODIS GPP product during 2000-2100 in North America, and attempt to examine the controlling of CUP length and GPPmax on annual GPP in terrestrial ecosystem. The CUP length and GPPmax were calculated from a fitted curve to the observed GPP in each year for all site-year (flux site) and grid cell (MODIS data).

Results/Conclusions

We found the ratio of terrestrial annual GPP against the product of the length of CUP and GPPmax (ε) converges to a narrow range (with the most frequent value about 0.62). Such a convergence of ε mainly result from the positive linear dependence of the duration of the stable phase of GPPmax on the CUP length. In North America, the ratio (ε) converges to 0.6‒0.7 in most regions ranging from the Arctic down to the middle of the U.S., and gradually approaches to 1.0 in the tropical regions in the west coast of Mexico and the Caribbean region.  The results suggest the interannual and spatial variability of terrestrial annual GPP is jointly controlled by that of phenological and physiological aspects. This finding provides insights into the controlling mechanism of terrestrial GPP variability under the ongoing multiple environmental changes. We suggest that more studies are needed to explore the intrinsic mechanisms of this global convergence of phenological and physiological controlling on annual GPP in terrestrial ecosystem.