Understanding a system’s historical conditions is a key first step in mapping out restoration goals and strategies. We examined the age structure and stem density of a stand dominated by Garry oak (Quercus garryana) and Douglas-fir (Pseudotsuga menziesii), and related this structure to its environmental history. Our reconstruction was based on 139 increment cores and 197 stem cross-sections collected from trees in 21 plots. Individuals of both species were more than 200 years old, indicating that the stand had a mixed composition for centuries. The historical tree density was ca. one-tenth that present prior to restoration (oak release) activities. This open oak/Douglas-fir savanna was maintained for centuries by fires set by Native Americans. It began to infill with oak, and later Douglas-fir, in the 1800s, particularly following Euro-American settlement in the 1860s. Douglas-fir encroachment continued throughout the 1900s, with a very large cohort becoming established in the early 1970s. This recent wave of recruitment has occurred in many sites in the region, and may reflect interactions among climatic and environmental conditions, together with changes in land use, including the cessation of livestock grazing and logging. Oak release actions were undertaken to re-open the forest structure, and involved the removal of 55% of the trees, primarily small-diameter Douglas-fir. Comparisons with historical information suggest that the post-release stand is still much denser and biased towards Douglas-fir than the stand was historically.
For more articles from the Spring 2011 issue of Northwest Science please refer to the link below: