Contrasting responses of woody and grassland ecosystems to increased CO₂ as water supply varies

Experiments show that elevated atmospheric CO₂ (eCO₂) often enhances plant photosynthesis and productivity, yet this effect varies substantially and may be climate sensitive. Understanding if, where and how water supply regulates CO₂ enhancement is critical for projecting terrestrial responses to increasing atmospheric CO₂ and climate change. Here, using data from 14 long-term ecosystem-scale CO₂ experiments, we show that the eCO₂ enhancement of annual aboveground net primary productivity is sensitive to annual precipitation and that this sensitivity differs between woody and grassland ecosystems. During wetter years, CO₂ enhancement increases in woody ecosystems but declines in grass-dominated systems. Consistent with this difference, woody ecosystems can increase leaf area index in wetter years more effectively under eCO₂ than can grassland ecosystems. Overall, and across different precipitation regimes, woody systems had markedly stronger CO₂ enhancement (24%) than grasslands (13%). We developed an empirical relationship to quantify aboveground net primary productivity enhancement on the basis of changes in leaf area index, providing a new approach for evaluating eCO₂ impacts on the productivity of terrestrial ecosystems.