Recent C4 vegetation decline is imprinted in atmospheric carbon isotopes

Abstract

Abstract How changes in the abundance and distribution of C3 and C4 plants are influencing the terrestrial carbon cycle is still unclear. Here, we use a simple C3/C4 distribution model based on optimality principles to investigate the impact of climate-related changes in C3 and C4 plants on global gross primary production (GPP) and atmospheric carbon isotopic discrimination (Δ13C). We estimate that the fraction of total C4 plants including natural grasslands and crops has decreased from 17.6% to 14.4% over 1982–2016, despite an increase in the fraction of C4 crops, reflecting the declining competitive ability of C4 photosynthesis as atmospheric CO2 concentrations increased. When ignoring this decline, the model predicts a strong increase in global GPP (18.6 ± 2.1 PgC) and slight increase in Δ13C (0.003 ± 0.001‰ ppm-1). However, when considering this decline, the increase in global GPP is slightly reduced (16.5 ± 1.8 PgC) and that of Δ13C increased (0.014 ± 0.001‰ ppm-1). The rate of Δ13C increase is consistent with independent atmospheric estimates but greater than that derived from another C4 distribution model. We conclude that the magnitude of the decrease in global atmospheric δ13CO2 can be partly explained by global changes in the distribution of C3/C4 plants.