A critical question for the critical zone: how do plants use rock water?

Background The paper by Korboulewsky and co-authors in this issue ofPlant and Soiladdress some of the central questions of critical zone ecohydrology: how do plants interact with rocks that exclude roots but hold plant-available water? Scope I compare plant water uptake from stony soils and fractured bedrock in the critical zone, suggesting that the two cases may represent endpoints of a continuum along which the proportion of available space for root growth changes. Conclusions Rhizosphere models could be improved and generalized by structuring the layers of the critical zone into volume fractions that can be rooted and fractions from which roots are excluded. I hypothesize that plant-available water capacity of the rooted fraction governs productivity, while plant-available water in the unrooted fraction governs drought resilience.