We argue that hydrologic niche segregation can be a useful lens through which to interpret these differences in vulnerability to climatic drought and climate change.Ībstract = 'Drought extent and severity have increased and are predicted to continue to increase in many parts of the world. Despite this persistent overall influence of the subsurface environment, individual species showed markedly different response patterns. Along hillslope gradients, channel locations supported similar environments in terms of water stress across a wide climatic gradient, indicating that subsurface hydrology mediates species' experience of drought, and that areas associated with persistent access to subsurface hydrologic resources may provide important refugia at species' xeric range edges. At the site scale, access to deep subsurface water, evidenced by stemwater stable isotope composition, was related to canopy condition across all species.

Regionally for blue oak, severity of dieback was related to the bedrock lithology, with higher mortality on igneous and metamorphic substrates, and to regional reductions in groundwater. rainfall), subsurface processes explained variation in drought vulnerability within and across species at both scales. We found that in addition to climatic drought severity (i.e. With California's 2013-2016 extraordinary drought as a natural experiment, we studied four co-occurring woodland tree species, blue oak (Quercus douglasii), valley oak (Quercus lobata), grey pine (Pinus sabiniana), and California juniper (Juniperus californica), examining drought vulnerability as a function of climate, lithology and hydrology using regional aerial dieback surveys and site scale field surveys. The influence of subsurface hydrologic processes is particularly important in water-limited ecosystems, and is an under-studied aspect of tree drought vulnerability. Understanding tree vulnerability to drought at both individual and species levels is key to ongoing forest management and preparation for future transitions in community composition.

Drought extent and severity have increased and are predicted to continue to increase in many parts of the world.

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