Are plant responses to drying soil and drying air coupled across species and ecosystems?

It is well-established that soil drought hinders plant water uptake whereas atmospheric drought (e.g. high vapour pressure deficit, or VPD) controls plant water loss. However, we know very little about the extent to which plant sensitivity to drying soil and drying air is coordinated across species and ecosystems. Here, using both species-level ecophysiological observations and ecosystems-scale flux tower data, we show that the sensitivity of plant carbon and water fluxes to drying soil is diverse within and across sites. In contrast, the sensitivity of these fluxes to rising VPD is largely generalizable across species and stands, and greatest when soil moisture limitations are small. Or, in other words, species and stands that are insensitive to soil drought may be especially vulnerable to limitations imposed by atmospheric drought. Moreover, we show that in most systems, canopy conductance to water is more sensitive to drought stress than carbon exchange, resulting in predictable variations in intrinsic water use efficiency during drought periods. Our results have important implications for the sensitivity of land-atmosphere carbon and water exchanges to ongoing climate change. VPD will increase globally with rising temperatures whereas shifts in soil moisture availability will be more heterogeneous and site-dependent. Thus, the species and ecosystems that are most sensitive to drought now may not be the most vulnerable in the future.