Increasing CO2 levels fertilize C4 grass production

Résumé fourni par la source

Abstract Rising atmospheric CO 2 concentrations are impacting the global terrestrial biosphere through indirect climate effects and direct effects on plant performance 1–3 . In tropical forests, long-term monitoring indicates a substantial CO 2 -driven carbon sink 4 . C 4 -grass-dominated tropical and subtropical savannas contribute approximately 30% of terrestrial net primary production 5 , and yet equivalent long-term analyses of CO 2 responses are lacking. Here we show a clear and consistent result across a meta-analysis of 70 CO 2 -addition experiments and 32 years of in situ field observations from southern Africa: CO 2 fertilization of wild C 4 grasses is widespread in dry conditions. In experiments, grasses reduced stomatal conductance under higher levels of CO 2 , limiting water loss while increasing carbon gain. In the field, improved water use efficiency translated into increased C 4 grass biomass production across three decades of observations. Finally, simulations via the Community Land Model 6 suggest that CO 2 fertilization of C 4 grass aboveground productivity may continue to increase under future conditions. Together, these results challenge the view that C 4 grasses are unresponsive to increasing levels of CO 2 , demonstrating instead that annual aboveground production of grasses in the field in southern Africa has increased by 28% over three decades (a CO 2 -driven increase of 75.1 g m −2 (95% confidence interval of 74.5–75.8 g m −2 ) or 0.37 tons C ha −1 of annual production). Although the fate of this carbon is uncertain (depending on feedbacks with fire, herbivory and woody vegetation), effects on the global carbon cycle may be profound.

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