Recent C4 vegetation decline is imprinted in atmospheric carbon isotopes

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.