Oak woodland restoration: understory response to removal of encroaching conifers.

Abstract

Oregon white oak (or Garry oak, Quercus garryana) woodlands and savannas of the coastal Pacific Northwest are legacies of an anthropogenic fire regime that ended with European settlement in the mid-1800s. Historically, these oak stands had a sparse overstory and an understory dominated by fire-tolerant grasses and forbs. Post-settlement fire suppression resulted in widespread invasion and subsequent overstory dominance by conifers, causing mortality of shade-intolerant oak trees and shifting understory plant communities to shade-tolerant species. In a study on four southwestern Washington sites, our objective was to determine the effects of overstory conifer removal, primarily Douglas-fir (Pseudotsuga menziesii), on microclimate, native and nonnative understory cover, and sapling growth. Overstory conifer removal created a warmer, drier understory microclimate during summer months. Conifer removal had little effect on native understory cover during 5 years posttreatment; however, cover of nonnative plants, primarily grasses and woody understory species, increased significantly during the same period. Height growth of Oregon white oak and Douglas-fir saplings exhibited a delayed, but positive, response to overstory conifer removal, although the treatment response of Douglas-fir was 133 percent greater than that of oak. Increases in nonnative understory cover and the rapid growth of young Douglas-fir indicate the importance of pre- and posttreatment understory management to control undesirable plants and promote native species such as Oregon white oak.

Citation

Devine, Warren D.; Harrington, Constance A.; Peter, David H. 2007. Oak woodland restoration: understory response to removal of encroaching conifers. Ecological Restoration. 25(4): 247-255.