Bedrock: the hidden water reservoir for trees challenged by drought

Several forest ecosystems rely on only shallow soil layers overlying more or less compact bedrock. In such habitats, the largest water reservoir can be represented by rock moisture, rather than by soil water. Here, we review evidence for the presence of water available for root water uptake in some rock types, and show examples of the physiological and ecological roles of rock moisture, especially when trees are facing drought conditions. The possible magnitude of rock–root water exchanges is discussed in the frame of current knowledge of rock, soil, and root hydraulic properties. We highlight several areas of uncertainty regarding the role of rock moisture in preventing tree hydraulic failure under drought, the exact pathway(s) available for rock–root water exchange, and the relative efficiencies of water transport in the different compartments of the rock–soil–root continuum. Overall, available experimental evidence suggests that bedrock water should be incorporated into any model describing the forest seasonal water use and tree responses to drought.