PARENT SESSION
Poster Session #42: Carbon Storage.
Thursday, August 9, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

The role of temperature in controlling CO₂ uptake dynamics in a high-elevation subalpine forest.

Huxman, Travis¹, Turnipseed, Andrew¹, Sparks, Jed¹, Harley, Peter^1,2, Scott-Denton, Laura¹, Sparks, Kim¹, Monson, Russell¹, ^{1 2}

ABSTRACT- During the 1999 growing season at the Niwot Ridge Ameriflux Site, we evaluated the role that temperature plays as a controller of forest CO₂ exchange dynamics. In this subalpine forest, maximum net ecosystem CO₂ exchange (NEE) occurred at relatively cool temperatures, between 8 and 12^oC. Chamber-based measurements of net photosynthesis rate made on twigs from the dominant tree species were consistent with the relatively cool temperature optimum for NEE. Above the temperature optimum, increases in ecosystem respiration caused reductions in NEE, though there was no change in the apparent ecosystem quantum yield or maximum assimilation rate (A_max), measured as the initial slope and saturated region (corrected for respiration) of the response of NEE to the incident photon flux density. These results suggest that, despite reductions in photosynthesis at warm temperatures in isolated twigs, the influence of temperature on CO₂ uptake at the ecosystem level is dominated by the response of soil respiration, not plant respiration. Under cool conditions, NEE did not decrease significantly until canopy temperatures were less than 3^oC. In this case, declines in NEE occurred through decreases in quantum yield and A_max. Thus, the low temperature constraint on NEE is associated with physiological processes in the trees, not soil. When taken together, the results provide a foundation for understanding temperature-driven dynamics in NEE for this high-elevation ecosystem.

KEY WORDS: subalpine forest, canopy photosynthesis, net ecosystem exchange, ecosystem respiration