The Effects of Urbanization and Human Disturbance Upon Plant Community Structure and Bird Species Richness, Diversity, and Abundance in a Natural Forested Area (Forest Park) in Portland, Oregon

Abstract

Forested Area (Forest Park) in Portland, Oregon.The effects of urbanization and continual human disturbance on the plant and avian communities of Forest Park and forested lands surrounding Portland, Oregon, were studied. I examined characteristics of plant and avian communities at 25 sites, 24 which were in Forest Park and surrounding areas and one which was in the Ancient Forest Preserve (old-growth stand) northwest of Forest Park. Data were analyzed using multiple regression, ANOV A, and Bonferonni/Dunn. Seven variables were selected representing different urbanization gradients. An additional covariable coded for the old-growth stand, allowing it to be used as a control.Many tree variables, especially those related to shade-tolerant species, were positively correlated with both the distance from downtown Portland and the number of houses in the surrounding area, and negatively correlated with the distance from the nearest forest edge; however, many shrub and herbaceous variables were negatively correlated with the distance from downtown Portland. Species diversity for herbaceous and shrub species was greater at more urban sites, but diversity of trees was lower at more urban sites. There were significantly more non-native species of plants in the city section. I found significantly fewer saplings and small trees, especially shade-tolerant species, in the section of Forest Park closest to downtown Portland, although tree mortality was positively correlated with distance from Portland.Summer bird data revealed significant increases in the abundances of urban and edge species at more urban sites, with concomitant reductions in forest species. There were significantly more ground gleaning birds and short distance migratory species.I also found a significantly greater abundance of birds in the old-growth stand during the winter. This increase was positively correlated with the depth of snow in the nearby Cascade Mountains.My results indicate that Forest Park is apparently progressing in a normal successional pattern with the exception of the city section. The reduction in shade tolerant saplings and small trees in the city section suggest that rate of succession has been slower at more urban sites. Avian data suggest that urbanization affects bird species abundance and guild composition in the more urban areas. ACKNOWLEDGMENTS

The influence of urbanization on forest stand dynamics in Northwestern Oregon

“…Sites nearer the city are surrounded by significantly more houses and roads and less forest than are sites further from the city. In addition, human usage in the park is greater at sites nearer downtown Portland (Broshot 1999). This is in keeping with the management plan for Forest Park.…”

“…Sites in the city section are significantly closer to roads and surrounded by greater numbers of buildings than sites in the rest of the park. The city section of the park is closer to the population center of Portland and has significantly greater human use (Broshot 1999). Increased use by people and their pets may lead to increased trampling of seedlings and saplings during offtrail excursions.…”

The Invasion Ecology of Ivy (Hedera spp.) in Portland's Forest Park

“…Ivy's strong spatial autocorrelation across scales, and the importance of that spatial autocorrelation among environmental predictors, could be interpreted in multiple, non-exclusive ways. One possibility at the landscape scale is that the stressed conditions under which it thrives, and/or the niche vacancies enabling it to dominate understory communities, are determined by gradients of urban influence and localized disturbance (Broshot, 1999;Cameron et al, 2015); another is that its limited dispersal ability (Metcalfe, 2005) means it is still expanding into suitable habitat from a few source populations. Perhaps not surprisingly, each of the apparent ivy epicenters is along the eastern, more urbanized edge of the park, particularly clustered around roads and residential areas.…”