Temperature As A Control Over Ecosystem CO2 Fluxes In A High-Elevation, Subalpine Forest

We evaluated the hypothesis that CO₂ uptake by a subalpine, coniferous forest is limited by cool temperature during the growing season. Using the eddy covariance approach we conducted observations of net ecosystem CO₂ exchange (NEE) across two growing seasons. When pooled for the entire growing season during both years, light-saturated net ecosystem CO₂ exchange (NEE_sat) exhibited a temperature optimum within the range 7–12°C. Ecosystem respiration rate (R_e), calculated as the y-intercept of the NEE versus photosynthetic photon flux density (PPFD) relationship, increased with increasing temperature, causing a 15% reduction in net CO₂ uptake capacity for this ecosystem as temperatures increased from typical early season temperatures of 7°C to typical mid-season temperatures of 18°C. The ecosystem quantum yield and the ecosystem PPFD compensation point, which are measures of light-utilization efficiency, were highest during the cool temperatures of the early season, and decreased later in the season at higher temperatures. Branch-level measurements revealed that net photosynthesis in all three of the dominant conifer tree species exhibited a temperature optimum near 10°C early in the season and 15°C later in the season. Using path analysis, we statistically isolated temperature as a seasonal variable, and identified the dynamic role that temperature exhibits in controlling ecosystem fluxes early and late in the season. During the spring, an increase in temperature has a positive effect on NEE, because daytime temperatures progress from near freezing to near the photosynthetic temperature optimum, and R_e values remain low. During the middle of the summer an increase in temperature has a negative effect on NEE, because inhibition of net photosynthesis and increases in R_e. When taken together, the results demonstrate that in this high-elevation forest ecosystem CO₂ uptake is not limited by cool-temperature constraints on photosynthetic processes during the growing-season, as suggested by some previous ecophysiological studies at the branch and needle levels. Rather, it is warm temperatures in the mid-summer, and their effect on ecosystem respiration, that cause the greatest reduction in the potential for forest carbon sequestration.