The importance of bedrock water for tree water relations under drought: experimental evidence and challenging questions

Large portions of terrestrial habitats are dominated by shallow (< 50 cm) soils overlying compact bedrock. Experiments suggest that some rock types can store appreciable amounts of water, which can be released in a water potential interval compatible with physiological ranges of root water uptake. However, it is not clear how and how much of this water can be exploited by plants under drought, depending on rock characteristics (porosity, hydraulic conductivity), root hydraulics and rock-root interaction. Indeed, Fraxinus ornus trees facing summer drought maintained a better water status when growing on a more porous bedrock formation (breccia, with higher plant available water content, AWC) than on dolostone (less porous, lower AWC). Potted F. ornus saplings grown in soil-rock mixtures were subjected to well-watered and drought conditions and the hydraulic conductivities of the single components of the system, i.e. rocks (breccia and dolostone), soil, roots and whole plant were quantified. Data were implemented into 2D model simula@ons of the rock-root water exchange. Simulations indicated higher exploitation of water pools in the breccia than in the dolostone systems in the dry range (soil water potentials between -1.0 and -1.5 MPa). These results support the field evidence and suggest to consider bedrock available water for a better understanding of spatial variations in vegetation water status and dieback patterns under drought in rock-dominated landscapes.