Effects of elevated CO2 and O3 on leaf area index in mixed aspen and birch stands.
Giardina, Christian*,1, Kubiske, Mark2, McDonald, Evan 2, Karnosky, Dave4, Pregitzer, Kurt4, Loya, Wendy4, 1 North Central Research Station, Houghton, MI, USA2 North Central Research Station, Rhinelander, WI, USA4
ABSTRACT- Leaf area index has been well characterized for many ecosystems and across wide gradients in temperature and moisture, but our understanding of how forest canopies will respond to global change remains rudimentary. Concentrations of CO2 and O3 in the atmosphere have increased dramatically in the past century, and quantifying the effects of these changes on canopy size and duration is critical to modeling terrestrial primary productivity and C cycling. We used a litterfall-based method to estimate LAI in 2002 for mixed aspen and birch stands at the FACE experiment in Rhinelander, Wisconsin, which includes both elevated CO2 and O3 treatments. This method permitted us to estimate total LAI, seasonal LAI patterns, and the fraction of LAI that was composed of either birch or aspen leaves. We found that after 5 years of fumigation, the elevated CO2 treatment increased total LAI by 30% relative to control rings, and increased growing season length by about 1 week. O3 had the opposite effect, reducing both total LAI and growing season length. At the species level, birch fared better than aspen under both trace gas treatments. In the high CO2 rings, birch LAI nearly doubled while aspen LAI was reduced by 20%. The elevated O3 treatment had little effect on birch LAI, but aspen LAI was just 35% of what it was in control rings. These results indicate that elevated CO2 alone may increase growing season length in north temperate forests, independent of other climate variables, and that O3 alone may shorten growing season length. Further, increased concentrations of CO2 and O3 can each exert a direct influence on the species composition of northern hardwood forests.
Key words: Elevated CO2 and O3 , Northern hardwood forest, Leaf Area Index, FACE
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