PS 62-45
“Over the river and through the woods”: An analysis of understory and canopy plant diversity in urban riparian forests
Urban development has been shown to drastically alter native habitats and to severely degrade biodiversity. However, urban landscapes also hold the potential to harbor and support valuable biodiversity especially if managed correctly. We studied the understory herbaceous plant and overstory tree canopy diversity of seven forested ravines in the small cities of Rock Island and Moline IL surrounded by a range of urban development intensity, and one rural ravine site in a nearby forest preserve. The goal of the project was to determine the relationship between urbanization and habitat fragmentation and the composition of the plant communities. The understory herbaceous community was sampled with the line point method, while the overstory tree community was sampled with the point-centered quarter method. The plant community was quantified with species richness, Shannon’s diversity index (SDI), and Floristic Quality Index (FQI). Urbanization was quantified as the mean imperviousness and percent developed land cover, and habitat fragmentation as percent forest cover within a 250m radius of the site.
Results/Conclusions
Combined understory and overstory species richness was found to be negatively correlated by regression with mean imperviousness (r² = 0.7689) and positively correlated with percent forested land cover (r² = 0.639). FQI was also found to be negatively correlated with mean imperviousness (r² = 0.429). Cluster analysis split the herbaceous understory communities into three groups (MRPP using species data: A = 0.21, P < 0.015), and showed that all of the urban sites were more similar to one another than they were to the rural site. Mean imperviousness and percent forest cover also differed significantly among groups (MRPP using environmental predictor data: A = 0.07, P < 0.011). A cluster analysis of the overstory communities yielded four different groups (MRPP using species data: A = 0.18, P < 0.005) and no significant predictor variables. Overall, the FQI of the sites ranged from 14.03 to 24.34, indicating that the ravines are of minimal to moderate biodiversity significance, typical of degraded urban habitats, yet some sites show promise for improvement. Given that we found urbanization intensity to have a significant negative effect on native biodiversity, we suggest that the ravines be properly managed in order to improve the ecological and social well being of Rock Island and Moline, including removal of impervious surfaces, use of native species landscaping, and social awareness of the value of local forests.