Twenty years of change in the urban tree community in Forest Park, Portland, Oregon
It is important to understand how expanding human development affects ecological processes in surrounding natural areas. Although researchers have been studying the impact of urbanization on forest structure, few have looked at long-term changes in tree community structure. This study examined changes in the tree community including mortality and recruitment by young trees along an urban-rural land use gradient in an urban, forested park. In 1993, 24 permanent sites were randomly located in Forest Park in Portland, Oregon with one additional site in the Ancient Forest Preserve, a mature forest 4km northwest of Forest Park (called old-growth). The sites were distributed along the perceived urban-rural land use gradient. Three, 250 meter square quadrats were located at each site. All trees within each quadrat were identified to species and the dbh (diameter at breast height) of each tree was measured. Saplings were members of tree species less than 10 cm in diameter but taller than 2 meters, and saplings were members of tree species less than 2 meters in height; basal diameter was measured for saplings. Data were initially collected in 1993. In 2003 and 2013, transects were relocated and measurements taken using the same methods as in 1993.
We found significantly fewer live trees and live saplings in each decade with the highest density in 1993. Although the rate of tree mortality was relatively high in 2003, it appeared to be slowing in 2013. When examined by section of the study area (city, middle, far, old-growth), the far section had significantly more live trees and saplings than did the city section, but there were significantly fewer live trees in all sections of Forest Park in 2013 as compared to 1993. The high rate of tree mortality may be a part of successional processes, but the loss of trees with the concomitant lack of recruitment may lead to a dramatic change in forest structure in the future. Our findings may have important implications for management of Forest Park.