Urbanization and second-growth forests in the Pacific Northwest: Land-use change, fragmentation, and policy implications

Background/Question/Methods

To help limit land conversion and its attendant environmental impacts, growth management policies in the Puget Sound Region of Western Washington prioritize increased housing density within urban growth areas, while limiting densities outside urban areas through use of low-density zoning.  However, studies show that policies of low-density development have resulted in major changes in landscape pattern, including significant loss and fragmentation of forestlands in urban fringe areas.  The purpose of this study was to examine the effects of fragmentation on forests in urbanizing areas by comparing the composition of understory vegetation, complexity of forest structure, and effects of fragment size and time since fragment isolation in 52 fragments of upland second-growth forest in urban, suburban, and exurban landscapes in the Seattle metropolitan area.  Data was analyzed using ordination by nonmetric multidimensional scaling, indicator species analysis, analysis of variance, non-parametric multivariate analysis of variance, and correlation analysis. 

Results/Conclusions

This study confirmed the presence of an urban to exurban gradient of effects: the composition of understory vegetation and forest structure changed gradually from urban to exurban areas.  However, forest structure and understory composition differed significantly among landscape types. This study also confirmed the impacts that urbanization are expected to have on natural systems.  The effects of fragmentation were most pronounced in urban fragments. Urban fragments had significantly more exotic species, exotic cover, and exotic tree seedlings. In contrast, there was little difference in richness or cover of native species.  However, exotic species were observed in every fragment studied, regardless of density of residential development. Urban forest fragments were structurally simpler than exurban fragments; indicators of forest complexity (density of trees, large conifers, snags and logs, and canopy cover) increased significantly as the landscape became less urban.  There were few significant effects of fragment size or time since fragment isolation.  These findings have major implications for management of forests in urbanizing areas.