Tree Composition and Size Structure Based on Nativity and Deer Resistance at an Urban Forest in Southeast Pennsylvania

Abstract

Urban forests of northeastern North America are often seen as degraded ecosystems with little ecological diversity or value. This is especially true given that native tree species declines in these forests have resulted from low recruitment caused by abundant exotic species, mortality from pathogens and pests, and severe deer browse. This study assessed tree composition and size structure of species found at the Robert B. Gordon Natural Area for Environmental Studies, an urban 51 ha forest fragment located 36 km west of Philadelphia, Pennsylvania. The study quantified and compared tree species from 15 plots located in mid- to late-successional forest stands. Species were categorized as native or exotic and resistant or nonresistant to deer browse. We quantified tree densities per species and measured diameter at breast height of all individuals > 2.5 cm and used this information to quantify relative importance values and assign trees to size classes. Natives Liriodendron tulipifera and Fagus grandifolia, and exotic Acer platanoides, dominated composition (i.e., 45.3% of total relative importance), with L. tulipifera most dominant. Natives had significantly greater densities than exotics in all size classes that included both categories, except for a 10–20 cm class, regardless of whether F. grandifolia, a prolific root sprouter, was included or excluded from comparisons. When F. grandifolia was excluded, nonresistant species had significantly greater densities than resistant ones in classes ranging from the smallest to larger classes. The prominence of nonresistant species in the smallest classes was unexpected given intense deer browsing.