Rock water as a key resource for ecosystems with thin soils: Digging deep trees subsidize patches of surficial grasses

Mediterranean mountainous areas of shallow soil often display a mosaic of tree clumps surrounded by grass. During dry seasons, evapotranspiration (ET) cannot be met by soil moisture. However, the combined role and dynamics of water extracted from the underlying rock, and the competition between adjacent patches of trees and grass, has not been investigated. We quantified the role rock water plays in the seasonal dynamics of evapotranspiration, and its components, over a patchy landscape in the context of current and past seasonal climate changes, and land-cover change strategies. Soil water budget, using precipitation (P), ET, and soil moisture changes (ΔS; ~17 cm soil layer), suggests deep water uptake by roots of trees (f_d; 0.8 – 0.9 mm/d), penetrating into the fractured basalt below clumps and the surrounding pasture, subsidized grass transpiration in spring through hydraulic redistribution. However, in summer trees used all the deep water absorbed (0.79 mm/d; f_d > tree transpiration). A 15-year dataset shows that, with increasing seasonal drought-severity (potential ET/P) to >1.34, the vertical water flux through the bottom of the thin soil layer transitions from drainage to uptake in support of ET. A hypothetical grass-covered landscape, with no access to deep water, would require 0.68 – 0.85 mm/d more than is available from P and ΔS, forcing shortened growing season and/or lower leaf area. In summer, ET in such a landscape would be half that of the existing mosaic, with consequences to energy balance. The vegetation mosaic may represent trending equilibrium, as long-term decreasing winter precipitation and increasing spring potential evaporation suggest drying climate. Intervention policies to increase water yield by reducing tree cover will curtail grass access to rock moisture, while attempting to increase tree-related products by increasing forest cover will limit water availability per tree leaf area. Both changes may further reduce ecosystem stability.