Community trait differentiation and seedling diversity in urban remnant forests and rural forests

Background/Question/Methods

We rely on the ecosystem services that urban forests provide such as carbon storage, recreation, and pollution mitigation. Urban foresters use functional traits such as specific leaf area (SLA) to predict tree species’ stress tolerance and suitability for the urban habitat as well as estimating the ecosystem services of urban trees. However, few studies have measured the forest structure and functional traits of regenerating urban forests in multiple size classes across an urban-rural gradient. Such research can provide valuable insight into the impacts of urbanization on the trajectory of forest fragments near urban areas. We asked whether the community structure and function of mature canopy trees, saplings, and understory trees differed across an urban to rural gradient in Cleveland, Ohio. Further, we asked whether urban remnant forests differed in seedling diversity compared to rural areas. To test for community differences, we recorded tree species composition and basal area in 60 plots and estimated community weighted specific leaf area using TRY database values. We collected leaf samples from five randomly chosen understory trees at each site to estimate the SLA in situ. Finally, we counted tree seedlings in five subplots at each site to evaluate seedling biodiversity.

Results/Conclusions

NMDS visualization and PERMANOVA tests showed that the tree community composition of urban forests significantly differ from those of rural areas. Community weighted means of SLA of canopy trees showed a significant negative relationship with increasing impervious surface area (i.e. more urban locations). However, preliminary analyses of SLA in the understory showed the opposite trend. We also found a trend toward lower seedling diversity in urban locations. The difference in trends in SLA values among size classes is intriguing and the exact cause requires further investigation. Nevertheless, our results show divergent community composition and leaf traits in urban and rural areas and varying trajectories towards future forest structure and function.