Release of Oregon white oak from overtopping Douglas-fir: effects on soil water and microclimate.

Many former Oregon white oak woodland and savanna stands in the coastal Pacific Northwest have been invaded and subsequently overtopped by Douglas-fir during the past century. We examined soil water and microclimate conditions near overtopped oak trees and near oak trees that had been released from Douglas-fir. In each of the three study years, volumetric soil water content declined from ~25 percent to ~10 percent during the early-to-mid growing season near all trees, but this decline was delayed approximately 1 month in the released condition. Throughfall from May through July was increased by release, particularly during light rain events when Douglas-fir crowns intercepted a substantial fraction of total precipitation. Release from Douglas-fir also increased soil temperature, maximum air temperature, and maximum vapor pressure deficit.