Recent forest development on Yellow Island, San Juan County, WA

Abstract

Yellow Island, a small member of the San Juan Island group in Puget Sound, was nearly treeless at the beginning of the 20th century. A significant increase in woody species has since occurred. In the central forested area of the island, Douglas-fir (Pseudotsuga menziesii) is the dominant tree species, with four regeneration peaks (250, 150, 75, and 45 years) apparently related to forest fires. These fires were patchy and of moderate to low intensity, with a fire rotation of about 80 years. In the nonforested zones on the island, significant tree invasion has occurred in the last 20 years. Shrub invasion of these areas is believed to have begun sometime in the last 20–50 years. Recent tree invasion of the grasslands is independent of fire history and is associated with above-average summer precipitation. The invading trees have initial height growth similar to trees established in the 1930s in the forest zone but higher diameter growth. Recent grassland invaders (after 1970) have almost twice the annual diameter increment of the early invaders. This is hypothesized to be the result of amelioration of desiccating summer winds by the early invaders, extending the growing season for recently established trees. If significant grassland zones are to be maintained on Yellow Island, tree and shrub removal will be required.