PARENT SESSION
Poster Session 2: Forest Ecology
Monday, August 8, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

The Effects of elevated CO2 and O3 on litterfall in aspen stands and mixed stands of aspen and birch.

Giardina, Christian^*,1, Parsons, William², King, John³, Loya, Wendy ³, Pregitzer, Kurt ³, Lindroth, Richard⁴, ¹ USDA Forest Service, Houghton, MI² Laval University, Montreal, Quebec³ Michigan Technological University, Houghton, MI⁴ University of Wisconsin, Madison, WI

ABSTRACT- Quantifying the effects of rising CO2 and O3 concentrations in the atmosphere on the timing and amounts of litterfall is critical to modeling terrestrial primary productivity and C cycling. We measured litterfall for aspen and mixed aspen plus birch stands at the FACTS-II-FACE experiment in Rhinelander, Wisconsin, which exposes forest vegetation to atmospheric CO2 and O3 concentrations projected for the year 2050. For mixed aspen-birch stands, we also quantified the fraction of litterfall that was composed of birch or aspen leaves. We found that after 4 years of fumigation, litterfall in the elevated CO2 rings was 30 to 50% higher than litterfall in control rings. This effect was consistent across community type, and there was no decline in these effects over the three year study (2001 to 2003). In addition to increasing litterfall mass, the elevated CO2 treatment delayed leaf fall by about 2 weeks. Elevated O3 had the opposite effect, reducing litterfall mass and accelerating leaf fall across community type and year. At the species level, litter in the birch-aspen stands was more abundant than the aspen stands across trace gas treatments. In the mixed species stand, both treatments generally increasing birch litter relative to aspen litter. There was moderate inter-annual variability in litterfall mass and for the mixed stands, litterfall composition. However, O3 consistently reduced aspen litter relative to birch litter when compared to the control. Results indicate that elevated CO2 may increase litter production in young north temperate forests, independent of other climate variables, and that O3 reduces it. Further, increased concentrations of CO2 and O3 each exert a direct influence on the relative proportions of litter by species in multiple species stands. The interactions of elevated CO2 and tropospheric O3 reported here illustrate the importance of accounting for effects of both pollutants for large regions of the forested landscape where concentrations of both are expected to increase in the future.

Key words: Global Change, FACE, Litterfall, Forest canopies