Component carbon fluxes and their contribution to ecosystem carbon exchange in a pine forest: an assessment based on eddy covariance measurements and an integrated model

We used a combination of eddy flux, canopy, soil and environmental measurements with an integrated biophysical model to analyze the seasonality of component carbon (C) fluxes and their contribution to ecosystem C exchange in a 50-year-old Scots pine forest (Pinus sylvestris L.) in eastern Finland (62degrees47' N, 30degrees58' E) over three climatically contrasting years (2000-2002). Eddy flux measurements showed that the growing Scots pine forest was a sink for CO2, with annual net C uptakes of 131, 210 and 258 g C m(-2) year(-1) in 2000, 2001 and 2002, respectively. The integrated process model reproduced the annual course of daily C flux above the forest canopy as measured by the eddy covariance method once the site-specific component parameters were estimated. The model explained 72, 66 and 68% of the variation in daily net C flux in 2000, 2001 and 2002, respectively. Modeled annual C loss by respiration was 565, 629 and 640 g C m(-2) year(-1), accounting for 77, 77 and 65% of annual gross C uptake, respectively. Carbon fluxes from the forest floor were the dominant contributors to forest ecosystem respiration, with the fractions of annual respiration from the forest floor, foliage and wood being 46-62, 27-44 and 9-10%, respectively. The wide range in daily net C uptake during the growing season was largely attributable to day-to-day fluctuations in incident quantum irradiance. During just a few days in early spring and late autumn, ecosystem net C exchange varied between source and sink as a result of large daily changes in temperature. The forest showed a greater reduction in gross C uptake by photosynthesis than in C loss by respiration during the dry summer of 2000, indicating that interannual variability in ecosystem net C uptake at this site was modified mostly by summer rainfall and vapor pressure deficit

Leaf photosynthesis of Betula albosinensis seedlings as affected by elevated CO2 and planting density

Birch (Betula albosinensis Burk.) seedlings were grown under two CO, concentrations, 350 mu mol mol(-1) (ambient COD and 700 mu mol mol(-1) (elevated CO,), and in two planting densities, 32 plants/m(2) (low density, LD) and 72 plants/m(2) (high density, HD). The objectives were to characterize the responses of leaf photosynthesis to long-term elevated CO, in birch seedlings in different planting densities, and to assess whether elevated CO, regulates the photosynthetic capacity of leaves, in terms of nitrogen concentration (N), the activity of ribulose bisphosphate carboxygenase (Rubisco), the photosynthetic efficiency of Rubisco and the concentration of nonstructural carbohydrates (TNC). In both planting densities, the leaves of birch seedlings grown under elevated CO2 but measured at 350 mu mol mol(-1) CO2 had a lower leaf N concentration, net CO2 assimilation rate (A), transpiration (E) and stomatal conductance (g(s)), and a higher water use efficiency (WUE) than those of birch seedlings both grown and measured under ambient CO2 concentration. On the other hand, the values of A, E, g, and WUE were significantly affected by planting density under ambient CO2, whereas these parameters were not affected by planting density under elevated CO2. In contrast, the levels of sucrose, soluble sugars, starch and TNC in the leaves were not significantly affected by planting density under ambient CO2 whereas these parameters were significantly affected by planting density under elevated CO2. Our results demonstrated that there are different acclimations of leaf photosynthesis in birch seedlings, as affected by elevated CO2 and planting density, and they highlighted the importance of the CO, level and planting density for the physiological ecology of woody plants. (c) 2007 Elsevier B.V. All rights reserved