In order to establish the amount of carbon assimilated by the scrub oak ecosystem exposed to elevated CO2, we periodically measure net ecosystem gas exchange using the open top chamber fitted with a ventilated top to prevent the backflow of outside air. (Hymus et al. 2003). Net ecosystem gas exchange (NEE) was shown to be very close to the measurements obtained by eddy flux methods in scrub oak of the same age (Dore et al. 2003). Measurements of NEE at elevated CO2 shows growth in elevated CO2increased maximum NEE from 50% in the winter to 180% in the summer (Hymus et al., 2003). When expressed per unit leaf area the stimulation of NEE ranged from 7% to 60%, with the increase depending on soil water content: the greater the soil water, the greater was the stimulation of NEE . At night, CO2 effluxes from the ecosystem were on average 39% higher in elevated CO2and varied between 6% and 64%: increased biomass of leaves, high temperature and wetter soil produced greater ecosystem respiration during summer (Dore et al. 2003) Partitioning of NEE at night into its below- and above-ground components was done in the winter only near the time when shoot biomass had been measured. CO2 stimulation of NEE at night in December 1999 and 2000 by 35% and 27% respectively was largely due to a 26% and 28% stimulation of belowground respiration in the respective periods: soil respiration averaged 87% of NEE at night. Shoot respiration was stimulated 37% and 42% in December 1999 and December 2000 respectively. An increase in the relative amount of aboveground biomass in woody tissue, combined with a decrease in the specific rate of stem respiration of the dominant species, Quercus myrtifolia, in elevated C a , was responsible for this effect. While we could find no direct effect of elevated CO2 on respiration on leaves, either through measurement of CO2 exchange (Hymus et al., 2002; Hymus et al. 2003) or by oxygen assimilation (Davey et al.2003)), differences in allocation of carbon to tissues having lower specific respiration, such as wood, reduced ecosystem respiration in elevated CO2. Elevated CO2 had a greater effect on carbon uptake than on carbon loss resulting in greater ecosystem carbon assimilation in elevated CO2.
Inter-annual variability of plant response to elevated carbon dioxide is tightly coupled to environmental stress. In scrub oak, the relative stimulation of growth was greatest during the drought of 1998. By comparison, in the Chesapeake Bay wetland the effect of elevated CO2 on shoot density was positively correlated with salinity and sea level (Rasse et al., In Review).