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

Two decades of drought-induced forest mortality have highlighted the ecological consequences of climate change, yet our ability to predict or even explain the spatial patterns of these mortality events remains limited. We conducted a range-wide survey of drought stress, growth, and allocation traits in a widespread oak species (Quercus douglasii, blue oak) to test the predictability of drought stress across populations and explore how water availability and allocation mediate spatial variation in tree growth. Across 15 sites, we found little relationship between end-of-season water availability (predawn leaf water potentials) or maximum drought stress (midday leaf water potentials) and climate or soils. Instead, plant water availability (within and among sites) was predicted by access to deep water resources inferred from stem water stable isotopes. We also found a remarkable three-way decoupling of water stress, growth, and allocation to leaf tissue, which challenged a data-parameterized mechanistic plant model. Our results reveal that deeply rooted trees can be hydrologically decouple from above-ground 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.