Urban Ecosystems, 5: 71–85, 2001 c 2003 Kluwer Academic Publishers. Manufactured in The Netherlands. Factors related to tree growth across urban-rural gradients in the Midwest, USA VALASIA IAKOVOGLOU JANETTE THOMPSON jrrt@iastate.edu Department of Natural Resource Ecology and Management, 253 Bessey Hall, Iowa State University, Ames, IA 50011, USA LEE BURRAS Department of Agronomy, 1126 Agronomy Hall, Iowa State University, Ames, IA 50011, USA REBECCA KIPPER Department of Natural Resource Ecology and Management, 253 Bessey Hall, Iowa State University, Ames, IA 50011, USA Abstract. Several environmental factors influence tree growth at any site. The objective of this study was to examine the relationship between biotic and abiotic factors and tree growth rate (mean ring width averaged over the last 10 years) in settings ranging from urban to rural. Six “clusters”, each with five communities and two rural parks, were sampled in Illinois, Iowa, Minnesota, Missouri, and Wisconsin, for a total sample of 320 trees. Within each community, trees in parks, and along residential and commercial streets were sampled. Five species were sampled: silver maple (Acer saccharinum L.), honeylocust, (Gleditisia triacanthos L.), hackberry (Celtis occidentalis L.), black maple (Acer nigrum Michx F.), and basswood (Tilia americana L.). Factors were investigated for three scenarios: (i) all trees sampled in all clusters, (ii) a single species, all clusters, and (iii) all species, a single cluster. Baseline variables (cluster, place population, site, species, and age) accounted for 49–71% of observed variation in growth rate. Combined biotic factors accounted for 5 to 6% of observed variation. For all species in a single cluster, combined abiotic factors accounted for 11% of observed variation. Biotic factors related to growth rate detected using multivariate analyses included number of other trees within 9 m, presence of disease and insects, and human-induced mechanical injury. Abiotic factors that were related to tree growth included presence of pavement and core bulk density. For trees in rural parks, number of other trees within 20 m, and for trees in both rural and community parks, number of other trees within 9 m of sample trees were associated with decreased growth rate. Keywords: urban-rural gradient, urban soil, land use, increment core analysis, street trees, abiotic factors, biotic factors Introduction The high degree of heterogeneity of urban ecosystems has led to difficulty in identifying and describing meaningful gradients. Systematic analyses of specific components of urban ecosystems, such as urban trees, are uncommon in the literature. There are, however, reports that have focused on individual factors or sets of factors and their relationship to tree growth in specific settings within urban ecosystems. These factors have often been classified as either abiotic influences or biotic influences. Abiotic factors that may influence tree growth in urban ecosystems include soil physical and chemical properties (structure, texture, salt content, and pH), available soil volume,