Influences of conifer encroachment and removal on oak woodland ecophysiology and biodiversity—a case study from northern California, U.S.A.

Abstract

Oregon white oak (Quercus garryana) woodlands are threatened by conifer encroachment established during decades of fire exclusion. Widespread restoration efforts are underway to remove conifers from many oak woodlands in California, Oregon, and Washington. Using water potential, stomatal conductance, stable isotopes, and three metrics of biodiversity, we investigated the effects of Douglas‐fir (Pseudotsuga menziesii) encroachment and removal at the ecosystem scale across 3 years in a northern California woodland. We found that heavily encroached stands had the highest water potential and often the lowest stomatal conductance, compared to moderately encroached and open stands. Xylem water stable isotopes indicated that oaks and Douglas‐firs were likely not directly competing for water, as oaks appeared to use a relatively deeper water source. Under certain climatic conditions, heavily encroached oaks were more vulnerable to water stress than oaks in open or moderately encroached stands. Following low‐intensity conifer removal, thinned stands had slightly lower predawn water potential and higher gas exchange compared to unthinned stands, particularly under dry conditions. For ecosystem biodiversity, understory plant and bird diversity did not meaningfully vary with encroachment or thinning, but mammal diversity was greater in encroached stands compared to open stands. Findings from this work demonstrate that negative impacts of conifer encroachment on oaks are not due to increased competition for water, conifer removal is physiologically beneficial for shaded oaks, and that heavier thinning treatments are likely needed to yield long‐term responses and influence biodiversity.