Maximum water stress is decoupled from climate, traits, and growth in a xeric oak

Our ability to explain and predict the spatial patterns of forest mortality remains limited. We tested the predictability of water stress across populations and explored how water availability and plant allocation mediate spatial variation in tree growth in a widespread oak species (Quercus douglasii Hook & Arn.). We conducted a range-wide survey of drought stress, xylem water isotopes, growth, and allocation traits across 15 sites spanning almost 1000 m difference in annual precipitation. We found little relationship between end-of-season water availability (predawn leaf water potentials) or maximum stress (midday water potentials) and climate or soils. Instead, stem water stable isotopes indicated that water availability was predicted by access to deep water resources derived from large winter storms. We also found a three-way decoupling of water stress, prior growth, and allocation to leaf tissue, such that simulated hydraulic risk from a mechanistic model was uncorrelated with observed performance. Our results reveal that deeply rooted trees can be hydrologically decoupled from aboveground climate, and that the seasonality of growth, trait development, and hydraulic risk are phenologically disconnected. The complicated relationship between carbon gain and hydraulic risk in seasonal environments and limited data on critical zone hydrology are key challenges to predicting drought vulnerability.