Monitoring of soil water availability and uptake
In each plot, four soil psychrometer sensors (TSP-55, Wescor Inc. USA; 64 total) were inserted at 12-15 cm depth (depending on the presence of soil pockets) near tree trunks to monitor soil water potential (Ψs). To determine leaf water potential (Ψl) for the same trees, we harvested 2-3 leaf discs from healthy mature leaves and needles exposed to sunlight, allowing 25 minutes for stabilization in C-52 chambers (C-52, Wescor Inc. USA). Soil and leaf Ψ were monitored during the diurnal peaks of water stress (from 11 to 14 hrs) at biweekly intervals for 27 months (from September 2012 to December 2014). To help interpret plant responses to soil water availability, Ψl and Ψs time series were divided into three ecohydrological periods: two dry periods (depletion and recovery) followed by one wet (wet season) period. The depletion period corresponded to the time when soil humidity started declining a few weeks after the rainy season ended; it included the months with the lowest leaf and soil Ψ (October–December). The recovery period started after the leaf and soil Ψ had reached their lowest values and once leaf Ψ initiated recovering; it lasted for 3-4 months until the beginning of the rainy season (January–May). The recovery period was unrelated to precipitation input, stable isotope analysis of water used by plants during this period suggests that water was extracted exclusively from fractured rocks. The wet season corresponded to the months with monsoon precipitation, when leaf and soil Ψ fully recovered (June–September).