Date: Tue Jul 29 16:29:39 2008
Posted By: Susan Letcher, Grad student, Ecology and Evolutionary Biology, University of Connecticut
Area of science: Botany
the ability to fix nitrogen is a fairly rare trait in the plant kingdom. To be precise, the plants themselves don’t have the ability to fix nitrogen; instead, they have symbiotic relationships with bacteria or cyanobacteria that do. The major plant family with nitrogen-fixing symbionts is the Fabaceae (the bean family), but other groups, like the alders you mentioned, can also fix nitrogen.
In the Northeastern deciduous forest, the native alder species are much smaller than the red alders of the Pacific Northwest, and they are generally confined to riparian zones. There are few native legume trees in the Northeast, although some exotic species such as locust (Robinia pseudoacacia; native to the Southeastern US) have recently begun to invade Northeastern forests.
Interestingly, it appears that much of the nitrogen fixation in Northeastern forests comes from lichens on decaying wood (Roskoski 1980). Non-woody legumes may also play a role (Crews 1999). Currently, though, a lot of the nitrogen inputs into Northeastern forests come from anthropogenic (i.e., human-derived) sources (Aber et al. 2003). A huge influx of nitrogen into Northeastern forests comes from coal-fired power plants and industries located downwind. The changing nutrient balance in these forests is already changing many aspects of the ecosystem (Foster and Aber 2004), and potentially affecting the competitive balance of species. A friend of mine is studying the decline of Desmodium cuspidatum in the Northeast. Desmodium is a nitrogen-fixing plant, and one of the factors implicated in its decline is that in high nitrogen levels, it cannot compete effectively with non-nitrogen-fixing species; the N-fixing symbiosis is energetically expensive for the plants (Mahon 1977), and in high nitrogen levels an N-fixer loses its competitive advantage. Harvard Forest has an ongoing research program to study the effects of nitrogen deposition in Northeastern forests.
For more information about nitrogen fixation in forests, here is a recent response I wrote to another MadSci question on the topic.
Aber, J.D., C.L. Goodale, S.V. Ollinger, M.L. Smith, A.H. Magill, M.E. Martin, R.A. Hallett, and J.L. Stoddard. 2003. Is nitrogen deposition altering the nitrogen status of Northeastern forests? BioScience 53: 375-389.