Anthropogenic old growth forests

Background/Question/Methods Anthropogenic old growth forest can have long historical records, such as detailed descriptions of forest conditions and measurements of forest species composition. In urban parks, the historical development of a particular anthropogenic old growth forest can be clarified in regard to human caused changes, such as tree removals for safety, plantings in nearby forests, and trampling seedlings, which occur in rural forests but are typically unaccounted for in long-term research. Once human caused changes are defined, old growth anthropogenic forests can be model systems for analyzing the effects of progressive long-term changes such as increasing levels of atmospheric carbon dioxide and temperature for a much longer period than their rural counterparts. The first objective of this research was to analyze the Fairmount Park forest history and long-term forest changes in three anthropogenic old growth forests of West Fairmount Park, Philadelphia, Pennsylvania, US using forest composition records from 1868, 1907, and 2007. With the information gained from the analysis, the second objective was to assess the apparent future of the forests prior to the chestnut blight and the actual changes that occurred nearly a century after the loss of chestnut.

Results/Conclusions Arboricultural plantings of native species in the estates that became Fairmount Park formed three forests dominated by native species. The forest history reveals that the frequency of fires and trampling differed in three old growth forests before and after the loss of chestnut to the blight. In 1868, chestnut was the forest dominant. In the three anthropogenic old growth forests, Beechwood, Country Club, and Robert’s Hollow, the overstory and understory dominants were respectively: chestnut and flowering dogwood; tulip-tree and choke-cherry; and rock-chestnut oak and white ash. In 1907, white oak appeared to be poised to disappear from the forests but instead the 2007 surveys revealed expansions in the middle size class in all three forests and the first reported presence of saplings in Country Club and Robert’s Hollow. Beyond the loss of chestnut by 2007, rock-chestnut oak, flowering dogwood and choke-cherry were virtually lost. In contrast, 14 species were either present for the first time or had increasing density per hectare in 2007. Considering the varying changes in tree populations over the 139 year period, there is a need to examine factors such as white-tailed deer browsing, and atmospheric temperature and carbon dioxide levels as possible causative factors in the long-term forest composition changes.