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Harvard Forest REU Symposium Abstract 2010

• Title: Urban-to-rural differences in tree growth rates: A comparison of native Quercus rubra and invasive Ailanthus altissima
• Author: Leah Nagel (Middlebury College)
• Abstract:

  Urban ecosystems represent a complex mix of competing positive and negative influences on plant growth rates. In this study, we examined the growth rates of two deciduous trees species at the end points of an urban-to-rural gradient between Boston and the Harvard Forest. Red oak (Quercus rubra) is a mid-successional tree with a moderate growth rate found throughout the Northeast, while Ailanthus (Ailanthus altissima) is a very fast-growing, highly pollution-tolerant invasive that is found throughout North America and is often targeted for eradication. We cored and measured tree rings from both species in urban and rural locations to compare the ring widths, total and annual carbon sequestration, and growth rates as a function of tree size within species at opposite ends of the gradient and between species grown in the same location. The maximum ring width for Ailanthus was substantially higher than that of oak with widths of 1.31 and 0.93cm, respectively. However, the mean ring widths for Ailanthus were smaller than oak (0.16±0.06 as opposed to 0.19±0.01 cm), likely due to the differences in the mean size of the individuals sampled. Normalized by the mass of the individual tree, Ailanthus showed a higher mean growth rates than oak, 14.90±3.25 and 5.79±2.33% yr-1, respectively, and rural Ailanthus sequestered more carbon annually than oaks growing in a rural setting, (9.98±16.31 and 8.41±26.07 KgC). Sequestration rates as a percentage of overall size declined with tree size and age across the gradient, with Ailanthus individuals attaining large sizes much faster than oaks. Ailanthus is an invasive species, but it can account for ~25% of urban biomass (Nowak 2002); if extrapolated to the Boston area, this would represent 7 Mg C ha-1. Efforts to remove all Ailanthus individuals due to its invasive status would result in the removal of a significant amount of sequestered carbon from the city that would not be replaced for decades.

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