Global convergence of optimal photosynthetic responses to atmospheric dryness

Most existing studies assume a monotonic decline in GPP with increasing VPD. Guided by an optimality framework, we predict that GPP responses to VPD are inherently non-monotonic and that an optimal VPD (i.e., the VPD at which the ecosystem shows peak GPP) exists.

Using 162 FLUXNET sites globally, we found that more than 96% exhibited an optimal VPD. The ubiquitous optimal VPD is independently observed at the leaf scale using a global gas-exchange dataset. Interestingly, optimal VPD converges to mean ambient VPD across biomes and spatial scales (from leaf to ecosystem scales), driven by dynamic optimal-ambient VPD relationships over time.