Above- and Below-ground Biomass, Net Ecosystem Carbon Exchange, and Soil Respiration in a Poplar Populus deltoides Bartr.) stand : Changes after 3 years of Growth under Elevated CO2

Stands of cottonwood (Populus deltoides Bartr.) trees were grown as a coppiced system under ambient (40 Pa), twice ambient (80 Pa), and three times ambient (120 Pa) partial pressure CO₂ for the past three years in the Intensively-managed Forest Mesocosm (IFM) of the Biosphere 2 Center. Over three years Net Ecosystem CO₂ exchange (NECE) was measured continuously and in the third year, nine whole trees were harvested from each CO₂ treatment over the growing season. Both above- and below-ground parameters were measured. Three years of growth under elevated CO₂ showed the expected stimulation in foliar biomass (8.7, 11.9, and 13.1 kg for the 40, 80, and 120 Pa treatments, respectively). Rates of NECE also followed an expected increase with elevated atmospheric CO₂ concentrations, with maximum CO₂ uptake rates reaching 10.5, 15.6, and 19.6 μ moles m^-2 s^-1 in the 40, 80, and 120 Pa treatments, respectively. However, above ground woody biomass and root biomass were not much stimulated beyond 80 Pa CO₂. Wood/foliage and above/below ground biomass ratios reflect this decline. Under conditions of non-limiting nutrients and water, we found consistent increases in the above/below ground biomass ratio and wood to foliage biomass ratios in the 80 compared to the 40 Pa pCO₂. Woody biomass production and the above/below ground biomass ratio were lower under the 120 Pa than any other treatment. Although biomass production did not change appreciably between 80 and 120 Pa CO₂ treatments, both substrate induced and in-situ soil respiration values are also significantly higher in the 120Pa treatment, though no differences were present prior to CO₂ treatments (Murthy et al. 2003). The unique closed-system operation of the IFM allowed for measures of soil CO₂ efflux to be measured at both the soil collar and stand scales using a box model that takes into account all inputs and outputs from the stand. In-situ soil respiration rates increased significantly with increased atmospheric CO₂ concentrations (8.4, 11.8, 14.3 for the 40, 80, and 120 Pa treatments, respectively). Stands of cottonwoods grown at elevated CO₂ concentrations showed expected CO₂ "fertilization" in terms of foliage, wood, and root biomass responses at 80 Pa, but little further response at 120 Pa. Instead, respiration of the plantation soil was greatly stimulated at 120 Pa, suggesting increased carbon secretion to the soil, or respiration associated with more rapid fine root turnover. From a system perspective, more rapid carbon acquisition in response to elevated CO₂ concentrations appears to saturate, possibly due to more rapid carbon cycling.