CO₂ exchange in the alpine sedge Carex curvula as influenced by canopy structure, light and temperature

The temperature and light responses of CO₂ uptake (F_n) in the sedge Carex curvula were investigated in situ by IRGA technic in the Austrian Central Alps at an altitude of 2,310 m. F_n in Carex leaves reaches a maximum of 15.6 mg CO₂ dm^-2 h^-1 at a leaf temperature of 22.5°C and a quantum flux density larger than 1.0 mmol photons m^-2 s^-1 (400-700 nm). A model based on a polynomal regression analysis of the F_n responses and informations about the microclimate and the canopy structure was used to simulate F _n for individual days and for a whole season. It turned out that the major rate limiting factor is the availability of light in the canopy: The calculated photosynthetic yield for a hypothetical optimum season of clear days with fully illuminated leaves and optimum temperature as well as for a typical season with the actual light and temperature conditions in the canopy, shows that insufficient illumination of the leaves accounts for almost 40% reduction of the possible CO₂ uptake while suboptimal temperatures cause only a loss of 8%. Half of the light deficit is caused by mutual shading of the leaves. The minor importance of temperature for the annual CO₂ uptake results from the fact that temperature adaptation of F _n in this sedge allows optimal utilization of short periods with high light intensity and hence high photosynthetic yield. The weaker the quantum supply the more becomes temperature limiting. This indicates that the length of the growing season is probably less important for the success of this prominent alpine plant than the sum of hours with high radiation.