Ecosystem processes along an urban-to-rural gradient MARK J. McDONNELL* University of of Connecticut, Bartlett Arboretum, 151 Brookdale Road, Stamford CT 06903, USA STEWARD T. A. PICKETT, PETER GROFFMAN and PATRICK BOHLEN Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545, USA RICHARD V. POUYAT and WAYNE C. ZIPPERER U.S.D.A. Forest-Service-NEFES, SUNY-CESF, 5 Moon Library, Syracuse, NY 13210, USA ROBERT W. PARMELEE Ohio State University, Department of Entomology, 1735 Neil Ave., Columbus, OH 43210, USA MARGARET M. CARREIRO Fordham University, The Louis Calder Center, 53 Whippoorwill Road, Drawer K, Armonk NY 10504, USA KIMBERLY MEDLEY Miami University, Department of Geography, 217 Shideler Hall, Oxford, OH 45056, USA In order to understand the effect of urban development on the functioning of forest ecosystems, during the past decade we have been studying red oak stands located on similar soil along an urban-rural gradient running from New York City ro rural Litchfield County, Connecticut. This paper summarizes the results of this work. Field measure- ments, controlled laboratory experiments, and reciprocal transplants documented soil pollution, soil hydrophobicity, litter decomposition rates, total soil carbon, potential nitrogen mineralization, nitrification, fungal biomass, and earthworm populations in forests along the 140 × 20 km study transect. The results revealed a complex urban-rural environmental gradient. The urban forests exhibit unique ecosystem structure and function in relation to the suburban and rural forest stands; these are likely linked to stresses of the urban environment such as air pollution, which has also resulted in elevated levels of heavy metals in the soil, the positive effects of the heat island phenomenon, and the presence of earthworms. The data suggest a working model to guide mechanistic work on the ecology of forests along urban-to-rural gradients, and for comparison of different metropolitan areas. Keywords: urban; rural, forests; gradients; ecosystems Introduction The conversion of natural or agricultural landscapes throughout the world to highly modified urban landscapes is expected to continue and many urbanized areas are expected to become even more highly modified (Alig and Healy, 1987; Richards, 1990; Douglas, 1994). The need for comprehensive studies of the ecological impacts of urbanization is great (Brown and Roughgarden, 1989; Rogers, 1994; Penner, *To whom correspondence should be addressed. Urban Ecosystems, 1997, 1, 21–36 1083-5155 © 1997 Chapman & Hall