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

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Summary Widespread drought-induced forest mortality highlights the ecological consequences of climate change, yet our ability to explain, let alone predict, the spatial patterns of forest mortality 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 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 water stress (midday water potentials) and climate or soils. Instead, 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. Plain Language Summary Below-ground structure complicates our understanding of plant water stress. We show that below-ground complexity and highly seasonal environments can both disconnect trees from their above-ground climate and disconnect drought mortality risk from growth and trait development.

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