PARENT SESSION

Symposium S1D Photosynthesis and global change
Monday August 30th, 2004 10:20 AM-12:20 PM Room 510B
Chair: Dave Layzell and Neil Baker
Co-Chair: Don Ort

Extracting photosynthetic insight from net ecosystem CO₂ exchange. Russell Monson^*,1, Chuixiang Yi¹, David Bowling³, Yiqi Luo⁴, William Sacks⁵, David Schimel⁵, ¹ Department of Ecology and Evolutionary Biology, Boulder, CO, USA³ Department of Biology, Salt Lake City, UT, USA⁴ Department of Botany, Norman, OK, USA⁵ National Center for Atmospheric Research, Boulder, CO, USA

ABSTRACT- Net ecosystem CO2 exchange (NEE) is routinely measured at over 200 eddy flux sites worldwide. NEE represents the difference between net ecosystem photosynthesis and heterotrophic ecosystem respiration. We have explored several approaches, both empirical and theoretical, to extract insight about ecosystem photosynthesis rate from long-term NEE data sets. Using stable isotopes of CO2 (13CO2/12CO2 ratio) in the air above the canopy of a temperate deciduous forest in Tennessee, we were able to partition NEE into its component photosynthetic and respiratory fluxes in a manner that was consistent with modeling approaches of the same data set. The greatest uncertainty in the isotope partitioning involved the evaluation of canopy stomatal conductance. In separate studies, we used a new nonparametric statistical model, to analyze NEE data from four forest ecosystems. We observed that differences among forests in their ability to assimilate CO2 as a function of temperature were attributable to consistent differences in the partitioned temperature dependence of photosynthesis and respiration. The statistical model was used to evaluate the partitioning of NEE according to the light-dependence of photosynthesis, while allowing the ecosystem respiration rate to vary with temperature; a nonparametric regression approach was used to obtain optimized estimates of ecosystem respiration rate, light-saturated net ecosystem CO2 uptake rate, and the quantum yield for canopy CO2 uptake. Most recently, we have used a Bayesian parameter optimization approach to evaluate an ecosystem photosynthesis model using five years of NEE data from a high-elevation, subalpine forest as the top-down constrain on model function. This approach provides our most highly constrained effort to date to derive photosynthetic insight from net ecosystem CO2 exchange data.

KEY WORDS: Stable isotopes, Ecosystem photosynthesis, Modeling, Net ecosystem CO2 exchange