Global distribution of C₃and C₄vegetation: Carbon cycle implications

Abstract

The global distribution of C 3 and C 4 plants is required for accurately simulating exchanges of CO 2 , water, and energy between the land surface and atmosphere. It is also important to know the C 3 /C 4 distribution for simulations of the carbon isotope composition of atmospheric CO 2 owing to the distinct fractionations displayed by each photosynthetic type. Large areas of the land surface are spatial and temporal mosaics of both photosynthetic types. We developed an approach for capturing this heterogeneity by combining remote sensing products, physiological modeling, a spatial distribution of global crop fractions, and national harvest area data for major crop types. Our C 3 /C 4 distribution predicts the global coverage of C 4 vegetation to be 18.8 million km 2 , while C 3 vegetation covers 87.4 million km 2 . We incorporated our distribution into the SiB2 model and simulated carbon fluxes for each photosynthetic type. The gross primary production (GPP) of C 4 plants is 35.3 Pg C yr −1 , or ∼23% of total GPP, while that of C 3 plants is 114.7 Pg C yr −1 . The assimilation‐weighted terrestrial discrimination against 13 CO 2 is −16.5‰. If the terrestrial component of the carbon sink is proportional to GPP, this implies a net uptake of 2.4 Pg C yr −1 on land and 1.4 Pg C yr −1 in the ocean using a 13 C budgeting approach and average carbon cycle parameter values for the 1990s. We also simulated the biomass of each photosynthetic type using the CASA model. The simulated biomass values of C 3 and C 4 vegetation are 389.3 and 18.6 Pg C, respectively.