The study of forest ecosystems in urban areas is important, not only because of the spatial extent of urbanization, but because such areas provide an excellent opportunity to study how habitat fragmentation, invasive species introductions, and urban environmental changes interact to affect soil chemical and biological characteristics. As an example, older cities are more likely to have a history of heavy industrial activity than newer, less industrialized, cities and thus have a higher deposition of heavy metals than surrounding areas. Additionally, newer cities tend to be associated with more sprawling development patterns than older cities. These and other contrasts provide opportunities to compare the effect of urban environmental factors and forest fragmentation on forest soil characteristics.
In this presentation, we consider the unique abiotic and biotic aspects of forest ecosystems embedded within urban, suburban, and rural areas (i.e., urbanization gradients) to address questions relating to the effects of climate change, habitat fragmentation, and invasive species on soil physical, chemical and biological characteristics. We then present case studies to describe the use of urbanizing gradients and other “natural experiments” to conduct these investigations.
Results/Conclusions
Results thus far suggest that forest soils embedded within urban or suburban areas are altered by environmental changes occurring along urbanization gradients. For instance, forest soils within or near urban areas accumulated up to three-fold higher concentrations of lead and copper, support dense populations of invasive earthworms, exhibit three-fold higher nitrification rates and 50% smaller methane sinks, than forests more than 25 km from an urban area. Within urban areas, forest soils exhibited heterogeneous conditions that were strongly related to previous land-use legacies. To help explain patterns found in the data thus far, we build on a conceptual framework, The Urban-Rural Habitat Fragmentation Continuum, which assumes that urban and suburban forest patches are smaller in size than their rural counterparts resulting in their having higher edge-to-interior ratios. This suggests that a proportionately greater length of edge is in contact with the surrounding matrix, allowing for more frequent exchanges of species, pollutants, and human incursions between forest and matrix. Since urban and suburban matrix conditions differ markedly from rural matrix types in species composition, plant structure, and environment, the intensity and type of flows is accentuated, which corresponds with measures of soil physical, chemical, and biological characteristics observed thus far.