Effects of natural and anthropogenic environmental influences on tree community composition and structure in forests along an urban-wildland gradient in southwestern Ohio

Abstract

Unprecedented population growth and urban expansion are rapidly transforming natural and agricultural settings into highly modified urban and suburban landscapes in developed areas worldwide. A myriad of anthropogenic environmental disturbances invariably accompany such dramatic shifts in land use, and disentangling the effects of these multiple stresses on ecosystem variation pose major challenges as the world’s urban population continues to increase. Here, we quantitatively characterized environmentally-mediated variation in tree-community composition along an Urban-Wildland gradient in southwestern Ohio, using tree censuses conducted in sample plots at six study sites categorized as Urban, Exurban, or Wildland based on analyses of a broad suite of natural and anthropogenic environmental variables. Several direct measures of the tree-census data, including species richness, abundance, diversity metrics, and evenness, exhibited significant increases from Urban to Wildland sites. Non-Metric Multidimensional Scaling (NMDS) showed that tree composition within study plots also varied systematically along the gradient, with significant differences among sites captured primarily on NMDS Axis 1. The dominant tree species at Urban and Exurban sites was Acer saccharum (sugar maple). While this species was also abundant at Wildland sites, two other species, Asimina triloba (paw paw) and Juniperus virginiana (red cedar) tended to be more abundant. Regression-tree analyses demonstrated that tree-community composition was controlled by a complex combination of natural and anthropogenic factors, with primary roles for population density and other indicators of urbanization at broader and intermediate scales; natural factors, such as aspect, soil drainage, elevation, slope, and soil pH were important in differentiating among Urban, Exurban, and Wildland sites already primarily categorized by anthropogenic agents.