Conifer encroachment and removal in a Northern California oak woodland: influences on ecosystem physiology and biodiversity

Oregon white oak (Quercus garryana) woodlands across their range are becoming increasingly threatened by encroaching Douglas-fir encroachment (Pseudotsuga menziesii) as a result of fire exclusion. Using water potential (Ψ), stomatal conductance (gs), xylem water stable isotopes (dD), and three metrics of biodiversity, this study investigates the effects of conifer encroachment and removal at the ecosystem-scale. The study was set in an Oregon white oak woodland in northern California and compared three levels of encroachment before and after conifer removal. Findings indicate that heavily encroached stands have the least amount of water stress and gas exchange. A moderate level of conifer encroachment appears to buffer water stress and support high productivity throughout the growing season. Trends in Y and gs suggest that coniferous shade improves oak water status via reduced evapotranspiration but limits productivity. Further, xylem water dD confirms that oaks and Douglas-firs are likely not directly competing for water, as oaks appear to use a relatively deeper water source. Thus, physiological results indicate that oak mortality to encroachment is likely light due to light, not water, limitation. Following conifer removal, moderately encroached stands did not respond dramatically during the first and second post-treatment years. However, heavily encroached stands increased gas exchange in both post-treatment years compared to unthinned counterparts. For ecosystem biodiversity, plant and bird diversity did not meaningfully differ among encroachment levels or between treatments, but mammal diversity was greatest in encroached stands. Collectively, findings from this work demonstrate that conifer removal is physiologically beneficial for light-limited oaks and that heavier thinning treatments are likely needed to yield long-term responses and influence biodiversity