Annual Carbon Accumulation Research Articles

Photosynthesis and carbon balance of a Sahelian fallow savanna

AbstractEddy‐covariance measurements of CO2 exchange above a Sahelian savanna consisting of small shrubs over a near‐continuous herb layer were made during the HAPEX‐Sahel experiment in Niger, West Africa. The measurements were made near‐continuously during an 8‐week period, covering the main part of the rainy season and three weeks at the beginning of the dry season. The measurements were corrected for in‐canopy storage of CO2 and the night‐time measurements used to derive respiration functions for the soil, roots and above‐ground plant material. Photosynthetic CO2 uptake was estimated and compared to simulations using a biochemical photosynthesis model in a simple, ‘big‐leaf’, implementation. The model satisfactorily reproduced the measurements (coefficient of determination 0.80) using parameters defined from the literature and based on soil nutrient concentrations. When the quantum yield (α) and rubisco capacity (Vmr) were fitted to the data with allowance for physiological changes through the season, an excellent agreement between model and measurements was obtained (coefficient of determination 0.93, RMS error 1.46 μmol m–2 s–1).The fitted photosynthesis and respiration model was used to estimate the carbon balance of the savanna site during the growing season of 1992 and for the complete calendar year. Harvest estimates of net plant biomass accumulation during the growing season and annual wood accumulation agreed well with modelled net photosynthesis and annual net carbon accumulation, respectively. Peak instantaneous ecosystem CO2 uptake was comparable to peak values observed in other biomes, but annual photosynthesis and carbon sequestration were considerably lower than observed elsewhere.

Dissolved organic and inorganic carbon mass balances in central Ontario lakes

Mass balances of dissolved organic carbon (DOC) and dissolvedinorganic carbon (DIC) based on stream and precipitation inputs andoutflows were measured for seven unproductive lakes in central Ontariobetween 1981 and 1989. Net annual CO2 evasion occurred in sixof the seven study lakes with minor net invasion in the seventh. Atmosphericinvasion might have been significant at certain times of the year, particularlyduring the growing season. Net evasion rates were greater than DIC loadingrates, indicating partial mineralization of the terrestrially-derived DOC in thelakes. A steady state mass balance model adequately described the variationin DOC retention between lakes. Net annual carbon accumulation of forestcommunities based on estimates of net ecosystem production may beoverestimated because of significant export of carbon to lakes via streamsand groundwater, particularly in catchments with extensive peatlands.

Simulating seasonal soil water balance in contrasting semi-arid vegetation communities

Soil water and nutrient dynamics of a forest ecosystem process model have been modified to simulate productivity and seasonal water use patterns in Artemisia, Agropyron, and Bromus communities in south-central Washington, USA. Measured soil water profiles indicate that each of the three communities exerts substantially different influence on the hydrologic balance of the intermountain sagebrush-steppe, and each requires unique model parameterization to accurately simulate the natural balance between soil water availability and annual carbon accumulation. Yearly simulations predicted soil moisture content for each community for 56 dates in 1992 with R 2 values ranging from 0.93 to 0.98. Model relationships between soil water availability, transpiring leaf area, water use efficiency, and respiration costs of substantial below-ground productivity illustrate key ecophysiological considerations for arid land process modeling.