Invasion Ecology of Acer platanoides in an Old-Growth Urban Forest

Abstract

Acer platanoides (Norway maple) is an exotic tree species with invasive potential that has been described as a prolific seed producer, shade tolerant, and a strong competitor for limiting resources. It has invaded many forests in the northeastern United States and Canada, including the Washington Grove, a 10 ha forest in Cobbs Hill Park in Rochester, NY. To quantify the extent of the invasion at the Grove, I surveyed the forest canopy, subcanopy, seedlings, saplings, shrub cover, herbaceous cover, seed rain, and seed bank. In a primarily Quercus (oak) canopy, A. platanoides was relatively sparse at 31 individuals/ha, but was the most abundant tree species in the forest subcanopy with 215 individuals/ha. Two other key findings include the prevalence of other invasive species in the understory (e.g. Alianthus altissima [tree of heaven]), and a lack Quercus regeneration. I suspected that superior competitive ability of A. platanoides was key to its invasiveness and wanted to test this at the seedling stage. Two native species (Acer saccharum [sugar maple], and Quercus rubra [red oak]) and the invasive were used in nine different competition arrangements grown under low shade, medium shade, and high shade (85%, 91%, and 97% shading, respectively). I measured photosynthesis rate, stem height, and stem diameter in control, intraspecific, and interspecific competition arrangements. Height growth and photosynthetic rate both decreased significantly with increased shade. Q. rubra had the highest overall photosynthesis rate (mean = 1.98 ± 0.10 ?mol CO2 m-2 s-1) and A. saccharum had the greatest change in height (mean change = 23.7 ± 2.67%). In contrast to my expectations, I did not find any conclusive evidence 2 suggesting that the invasive A. platanoides was the superior competitor at the seedling stage. In conclusion, the Washington Grove is heavily populated by the invasive A. platanoides and if left unmanaged, the area will further progress to resemble a nonnative stand. However, this pattern does not appear to be due to competition at the seedling level. To limit the further spread of the established trees I recommend felling all of the established invasive trees and removing any emerging seedlings. A long term management plan of invasive removal and creating conditions to promote Quercus recruitment will help promote a native forest.