Multi-Scale Topographic Influences on Tree Growth: A Case Study of Quercus lobata in the Tehachapi Mountains, California

Abstract

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 remains an open question. Dendrochronology, the study of precisely dated tree rings, has been a valuable tool for assessing tree growth response to climate variability across topographic gradients. In this study, six new tree-ring chronologies were developed from valley oak (Quercus lobata), a long-lived, endemic species in California. Trees were sampled growing in upland and riparian hillslope 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 and drought sensitivity. Valley oaks are thought to be highly dependent on groundwater, so I hypothesized that higher groundwater availability at the riparian sites would mediate growth sensitivity to drought. Results showed that tree growth patterns and drought sensitivity varied substantially in association with hillslope position and elevation. Valley oak radial growth showed a consistently weaker response to precipitation at riparian sites, supporting my hypothesis. The influence of hillslope position on drought sensitivity varied with elevation, such that the riparian buffering effect was strongest at the low elevation sites and weaker at higher elevations. At upland sites, precipitation and tree growth covaried on annual to decadal time scales, depending on elevation. The strong topographic influence on valley oak growth and drought sensitivity observed here has implications for climate refugia planning and paleohydrology using this species.