Release of Oregon White Oak from Overtopping Douglas-fir- Effects on Soil Water and Microclimate

Many former Oregon white oak (Quercus garryana) woodland and savanna stands in the coastal Pacific Northwest have been invaded by Douglas-fir (Pseudotsuga menziesii) during the past century as a result of fire suppression. Douglas-fir overtop and suppress the shade-intolerant oak, causing its eventual mortality. Removal of Douglas-fir is necessary for restoration of these oak ecosystems, but such action will influence belowground and near-ground conditions, affecting residual trees and understory communities. In a three-year study on a glacial outwash soil near Olympia, Washington, we compared soil and microclimate conditions near overtopped and released oak trees to determine how soil water content (SWC), throughfall, soil and air temperature, and vapor pressure deficit (VPD) are affected when oak is released from overtopping Douglas-fir. In each year, volumetric SWC near all trees declined from ~0.25 m3 m-3 to ~0.10 m3 m-3 during the growing season, but this decline was delayed approximately one month in the released condition. Additionally, minimum SWC during late summer was 0.02 to 0.03 m3 m-3 greater near released trees than near overtopped trees. The understory in the released condition consumed more soil water than that in the overtopped condition, but only in the first year after release. During light rain events from May through July, throughfall was 170% greater in the released condition than in the overtopped condition. Release from Douglas-fir increased soil temperature, maximum air temperature, and maximum VPD. Release of oak trees from overtopping Douglas-fir reduced early- to mid-summer competition for soil water, which will likely benefit the formerly suppressed oak trees.