Douglas-fir encroachment reduces drought resistance in Oregon white oak of northern California

Abstract

Oregon white oak (Quercus garryana Douglas ex Hook.) is experiencing increasing competition from Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) across much of its range at the same time as climate variability is increasing, including more frequent and extreme droughts. These combined factors suggest that Douglas-fir encroachment is not only leading to range reduction from competition, but also likely exacerbating drought stress for a keystone oak species in the Pacific West of North America. Our research examines this by evaluating radial tree growth and drought response in Oregon white oak (n = 104 trees) and Douglas-fir (n = 104 trees) along a gradient of encroachment in an oak woodland in the North Coast region of California. Linear mixed-effects models were used to evaluate the effects of tree diameter, Douglas-fir competition, oak competition, and climate on drought resistance (2013–2015) and recent tree growth (2002–2016). Oregon white oak growth was more resistant to prolonged drought than Douglas-fir. However, Oregon white oak drought resistance declined with increasing Douglas-fir competition and increased with increasing oak competition. Oregon white oak and Douglas-fir growth related to different seasonal climate factors, but both species were more strongly limited by Douglas-fir competition than climate. Oregon white oak may be better suited to future climate conditions than Douglas-fir, although Douglas-fir encroachment will likely continue to reduce Oregon white oak resistance to future drought. These results present a strong case for the need to release Oregon white oak by controlling Douglas-fir in areas that were historically oak woodlands with diverse understory species, wildlife, and important traditional ecological values and services.