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

Experiments show that elevated atmospheric CO (eCO) often enhances plant photosynthesis and productivity, yet this effect varies substantially and may be climate sensitive. Understanding if, where and how water supply regulates CO enhancement is critical for projecting terrestrial responses to increasing atmospheric CO and climate change. Here, using data from 14 long-term ecosystem-scale CO experiments, we show that the eCO 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, CO 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 eCO than can grassland ecosystems. Overall, and across different precipitation regimes, woody systems had markedly stronger CO 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 eCO impacts on the productivity of terrestrial ecosystems.