Exotic tree species and nitrogen cycling in an urban ecosystem (presentation)

Benjamin F. Hardt
Brown University
Center for Environmental Studies
Providence, RI 02912

Peter M. Groffman*
Institute of Ecosystem Studies
Box AB, Millbrook, NY 12545
Phone:(845) 677-5343
FAX: (845) 677-5976
E-mail: groffmanp@ecostudies.org

Nitrogen (N) is the major factor limiting productivity in temperate forests. However, excess N can cause eutrophication and saturation in terrestrial and especially in marine ecosystems, e.g. the Chesapeake Bay. Exotic tree species can affect N cycling and loss in forests via changes in water and nutrient use and the nature of the detritus that they produce. In this study, we evaluated N cycling in soils beneath two exotic tree species, Ailanthus (Ailanthus altissima) and Norway Maple (Acer platanoides). We measured a suite of N cycle variables in forest stands dominated by these exotic species and used the BES long-term biogeochemical study plots, which have no exotic tree species, as reference sites.

Sites dominated by exotic species had higher levels of soil nitrate (Figure 1) and nitrification rates (Figure 2) than native sites. These results suggest that exotic species accelerate N availability and loss. Sites dominated by exotics also had higher levels of soil moisture, suggesting either that exotics colonize sites that are inherently wet or that another effect of exotics is to increase soil moisture. The interactions between exotic species, soil moisture and N cycling are being explored in current BES research.

*Corresponding author

Figure 1. Soil nitrate in forest stands dominated by exotic (Ailanthus, Norway Maple) or native vegetation. There were two sites of each vegetation type, with two replicate plots per site. Values are mean with standard error. There were significant (p < 0.05) between Ailanthus and the other two vegetation types.

Figure 2. Potential net nitrification (measured with a 10 day laboratory incubation) in forest stands dominated by exotic (Ailanthus, Norway Maple) or native vegetation. There were two sites of each vegetation type, with two replicate plots per site. Values are mean with standard error. There were significant (p < 0.05) between Ailanthus and the other two vegetation types.