Plant nitrogen concentration and isotopic composition in residential lawns across seven US cities.

Human drivers are often proposed to be stronger than biophysical drivers in influencing ecosystem structure and function in highly urbanized areas. In residential land cover, private yards are influenced by individual homeowner preferences and actions while also experiencing large-scale human and biophysical drivers. We studied plant nitrogen (%N) and N stable isotopic composition (δ(15)N) in residential yards and paired native ecosystems in seven cities across the US that span major ecological biomes and climatic regions: Baltimore, Boston, Los Angeles, Miami, Minneapolis-St. Paul, Phoenix, and Salt Lake City. We found that residential lawns in three cities had enriched plant δ(15)N (P < 0.03) and in six cities higher plant N (%) relative to the associated native ecosystems (P < 0.05). Plant δ(15)N was progressively depleted across a gradient of urban density classes in Baltimore and Boston (P < 0.05). Lawn fertilization was associated with depleted plant δ(15)N in Boston and Los Angeles (P < 0.05), and organic fertilizer additions were associated with enriched plant δ(15)N in Los Angeles and Salt Lake City (P < 0.04). Plant δ(15)N was significantly enriched as a function of housing age in Baltimore (r (2) = 0.27, P < 0.02), Boston (r (2) = 0.27, P < 0.01), and Los Angeles (r (2) = 0.34, P < 0.01). These patterns in plant δ(15)N and plant N (%) across these cities suggests that N sources to lawns, as well as greater rates of N cycling combined with subsequent N losses, may be important drivers of plant N dynamics in lawn ecosystems at the national scale.

Morphological characteristics of urban water bodies: mechanisms of change and implications for ecosystem function.

The size, shape, and connectivity of water bodies (lakes, ponds, and wetlands) can have important effects on ecological communities and ecosystem processes, but how these characteristics are influenced by land use and land cover change over broad spatial scales is not known. Intensive alteration of water bodies during urban development, including construction, burial, drainage, and reshaping, may select for certain morphometric characteristics and influence the types of water bodies present in cities. We used a database of over one million water bodies in 100 cities across the conterminous United States to compare the size distributions, connectivity (as intersection with surface flow lines), and shape (as measured by shoreline development factor) of water bodies in different land cover classes. Water bodies in all urban land covers were dominated by lakes and ponds, while reservoirs and wetlands comprised only a small fraction of the sample. In urban land covers, as compared to surrounding undeveloped land, water body size distributions converged on moderate sizes, shapes toward less tortuous shorelines, and the number and area of water bodies that intersected surface flow lines (i.e., streams and rivers) decreased. Potential mechanisms responsible for changing the characteristics of urban water bodies include: preferential removal, physical reshaping or addition of water bodies, and selection of locations for development. The relative contributions of each mechanism likely change as cities grow. The larger size and reduced surface connectivity of urban water bodies may affect the role of internal dynamics and sensitivity to catchment processes. More broadly, these results illustrate the complex nature of urban watersheds and highlight the need to develop a conceptual framework for urban water bodies.