PARENT SESSION
Oral Session #76: Plant Ecology: Nutrient Uptake, Movement, Use. Presiding: J. Schade.
Friday, August 10, 2001. 8:00 AM to 11:45 AM. Hall of Ideas G.
Influences of nitrogen deposition on an annual plant: Integrating resource availability and herbivory.
Throop, Heather1, Lerdau, Manuel1, O'Rourke, Megan 1, 1
ABSTRACT- Deposition of fixed nitrogen from the atmosphere is one of the major global-scale changes affecting ecosystems today. Nitrogen deposition affects plants both directly, by altering the availability of inorganic nitrogen, and indirectly, by affecting interactions with herbivores that are driven by changes in plant nitrogen composition. In field experiments, we studied the direct and indirect impacts of nitrogen deposition on common ragweed, Ambrosia artemisiifolia (Asteraceae). We subjected A. artemisiifolia to factorial combinations of three experimental nitrogen deposition treatments (0, 5, and 20 g N m-2 yr-1 additions) and thee herbivory treatments (control; aphids, Hemiptera: Uroleucon ambrosiae; and leaf beetles, Chrysomelidae: Ophraella communa caged onto individual plants). We measured leaf chemistry, photosynthetic rates, relative growth rates, and above-ground vegetative and reproductive biomass. Nitrogen deposition alone significantly decreased foliar C:N, increased photosynthetic and growth rates, and caused increases in both vegetative and reproductive biomass. Increases in herbivory by both aphids and beetles in the low nitrogen treatment counteracted the positive influence of nitrogen deposition on biomass production. At high levels of simulated nitrogen deposition, however, neither type of herbivore caused a significant decrease aboveground biomass. These results suggest that altered interactions between plants and herbivores are important in understanding the full implications of atmospheric nitrogen deposition on plant ecology.
KEY WORDS: nitrogen deposition, global change, herbivory, Asteraceae