Continued response of Oregon oak to release treatments 20 years after initiation in western Washington, United States

Abstract

Fire suppression has increased competitive tree encroachment of Oregon white oak (Quercus garryanna Douglas ex Hook) ecosystems, threatening maintenance of this important species. Restoration of oak ecosystems is ideal to address this threat but not always possible, giving rise to a need for novel treatments that will allow oak to persist on an altered landscape. We tested the effect of three release treatments centered around individual Oregon oak trees within four forested sites that had developed following encroachment and overtopping by Douglas‐fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii). The release treatments entailed removal of competitive tree species (>10 cm dimaeter at breast heigh [DBH]) within zero (as control), half, or full distance of the associated Oregon oak trees' height. There was no effect of release treatment on height, but diameter was significantly increased in the full release treatment relative to both the control and half release treatments. The effect on diameter increased up to year 10 and then decreased to year 20. Growth response to canopy damage from an ice storm in year 12 of the study varied significantly with treatment; growth was negatively related to the percent crown damage in both release treatments but not the control treatment. Full release treatments around individual Oregon oak trees can have long‐term positive effects on growth, providing managers with an option to maintain oak within altered ecosystems.