Topography mediates growth and drought sensitivity in California valley oak (Quercus lobata)

Complex topography can facilitate climatic and hydrologic microenvironments that buffer plants against climate change and extreme drought. However, the extent to which topographic position mediates tree growth response to climate is still poorly understood. Dendrochronology, the use of precisely dated tree rings to study environmental processes and history, has been critical for assessing tree growth response to climate variability across topographical gradients. We developed new tree-ring data from valley oak (Quercus lobata) growing in upland and riparian landscape positions along a 1,300-meter (4,265-feet) elevational transect in the Tehachapi Mountains of California to understand how topography acts as a mediating factor on tree growth, drought sensitivity, and biological reaction to environmental extremes. Because valley oak is understood to be highly dependent on groundwater, we hypothesized that higher groundwater availability at the riparian sites may mediate valley oak drought vulnerability. Preliminary results showed that valley oak growth and drought sensitivity varied substantially as a function of landscape position and elevation. Growth-climate analyses indicated consistently weaker correlations between valley oak radial growth and precipitation at riparian sites compared to upland sites, consistent with our hypothesis that increased groundwater availability at riparian sites may act to decouple valley oak productivity from precipitation variability. This study adds to a growing body of literature examining vegetation drought vulnerability in California and North America, where climate extremes and associated tree dieback are an early harbinger of future drought-induced tree mortality.