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).