Dealing with drought: Does plasticity or local adaptation underlie geographic variation in drought resistance in a California oak?

Leander Anderegg, Ecology, Evolution & Marine Biology, UC Santa Barbara, Santa Barbara, CA, Robert Skelton, South African Ecological Observatory Network, Cape Town, CA, South Africa, Jessica V. Diaz, Integrative Biology, University of California, Berkeley, Berkeley, CA, Prahlada D. Papper, Department of Integrative Biology, University of California, Berkeley, David Ackerly, Integrative Biology & Environmental Science, Policy, and Management, UC Berkeley, Berkeley, CA and Todd Dawson, Integrative Biology, University of California, Berkeley, CA

Background/Question/Methods
Two decades of widespread drought-induced forest mortality events on every forested continent have raised the specter of future unpredictable, rapid ecosystem changes in 21st century forests. Yet our ability to predict drought stress, much less drought-induced mortality across the landscape remains limited. This uncertainty stems in part from both incomplete understanding of spatial variation in drought tolerance within widespread species. We built on previous research showing limited within-species variation in thresholds to hydraulic damage in blue oak (Quercus douglasii) to quantify genetic and plastic variation in multiple plant hydraulic traits in both a common garden and across the species’ geographic range in California. We also quantified spatial variation in plant traits or spatial variation in end-of-growing season water stress (Pmin).
Results/Conclusions
We found limited among-population genetic variation in most traits and plastic variation in only a few hydraulic traits. We also found that site climate was a weak predictor of plant traits, and an extremely poor predictor of end-of-season water stress. Instead, it appears that many sites had become largely decoupled from the local climate by the end of the season due to access to subsurface water sources. Moreover, hydraulic traits were not well correlated with end-of-season water stress, resulting in highly variable and difficult to predict hydraulic safety margins. Collectively, our results suggestion that spatial variability in vulnerability to drought resistance is primarily driven by plastic adjustments in allocation rather than ecotypic variation, and that drought tolerance traits were remarkably consistent across populations while drought avoidance traits were more variable. This implies a fairly consistent threshold to drought-induced damage across space, but a plastically determined variable exposure to drought stress that is largely mediate by site edaphic characteristics across blue oak’s geographic range.