Stand age and species richness dampen interannual variation of ecosystem-level photosynthetic capacity.

The total uptake of carbon dioxide by ecosystems via photosynthesis (gross primary productivity, GPP) is the largest flux in the global carbon cycle. A key ecosystem functional property determining GPP is the photosynthetic capacity at light saturation (GPP _sat ), and its interannual variability (IAV) is propagated to the net land-atmosphere exchange of CO ₂ . Given the importance of understanding the IAV in CO ₂ fluxes for improving the predictability of the global carbon cycle, we have tested a range of alternative hypotheses to identify potential drivers of the magnitude of IAV in GPP _sat in forest ecosystems. Our results show that while the IAV in GPP _sat within sites is closely related to air temperature and soil water availability fluctuations, the magnitude of IAV in GPP _sat is related to stand age and biodiversity (R ² = 0.55, P < 0.0001). We find that the IAV of GPP _sat is greatly reduced in older and more diverse forests, and is higher in younger forests with few dominant species. Older and more diverse forests seem to dampen the effect of climate variability on the carbon cycle irrespective of forest type. Preserving old forests and their diversity would therefore be beneficial in reducing the effect of climate variability on Earth's forest ecosystems.