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

Jill J. Beckmann, School of Forestry, Northern Arizona University, Flagstaff, AZ; Forestry & Wildland Resources, Humboldt State University, Arcata, CA, Rosemary L. Sherriff, Geography Department and Forest, Watershed, Wildland Science Graduate Program, Humboldt State University, Arcata, CA, Lucy P. Kerhoulas, Forestry and Wildland Resources, Humboldt State University, Arcata, CA and Jeffrey M. Kane, Department of Forestry & Wildland Resources, Humboldt State University, Arcata, CA

Background/Question/Methods
Oregon white oak (Quercus garryana) is experiencing increasing competition from Douglas-fir (Pseudotsuga menziesii) 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. While anatomical and physiological differences suggest oaks may be more resistant to prolonged drought than Douglas-fir, Douglas-fir competition may interfere with the drought resistance of Oregon white oak, especially given the shade-intolerance of Oregon white oak and well-documented decline of this species with Douglas-fir encroachment. Species vary in their response and vulnerability to drought and few studies have directly examined the effects of competition on tree growth under prolonged and severe drought conditions. Our research examines this by evaluating radial tree growth, competition, and drought response in Oregon white oak (104) and Douglas-fir (104) trees along a gradient of encroachment in an oak woodland in coastal Northern California. Linear mixed-effects models evaluated the effects of tree diameter, Douglas-fir and oak competition, and climate on drought resistance (2013-2015) and recent tree growth (2002-2016).
Results/Conclusions
Both Oregon white oak and Douglas-fir responded negatively to drought in 2013-2015, but Oregon white oak growth was more resistant to prolonged drought than Douglas-fir. We also found increased resistance of Oregon white oak and decreased resistance of Douglas-fir after the first year of drought. In both species, drought resistance declined with increasing Douglas-fir competition and growth was more strongly limited by Douglas-fir competition than climate. Intraspecific competition decreased Oregon white oak growth but increased drought resistance. At our site, Douglas-fir competition interacted with climate variables to affect oak growth; specifically, late spring temperature, summer precipitation, and previous year climatic water deficit were meaningful. There were also notable differences in the season of climate factors related to growth for each species. 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. Addressing Douglas-fir encroachment in Oregon white oak woodlands will foster the benefits of increasing drought resistance and preserving biodiversity under climate change.