Aboveground Carbon Accumulation Research Articles

A METHOD FOR SCALING VEGETATION DYNAMICS: THE ECOSYSTEM DEMOGRAPHY MODEL (ED)

The problem of scale has been a critical impediment to incorporating im- portant fine-scale processes into global ecosystem models. Our knowledge of fine-scale physiological and ecological processes comes from a variety of measurements, ranging from forest plot inventories to remote sensing, made at spatial resolutions considerably smaller than the large scale at which global ecosystem models are defined. In this paper, we describe a new individual-based, terrestrial biosphere model, which we label the eco- system demography model (ED). We then introduce a general method for scaling stochastic individual-based models of ecosystem dynamics (gap models) such as ED to large scales. The method accounts for the fine-scale spatial heterogeneity within an ecosystem caused by stochastic disturbance events, operating at scales down to individual canopy-tree-sized gaps. By conditioning appropriately on the occurrence of these events, we derive a size- and age-structured (SAS) approximation for the first moment of the stochastic ecosystem model. With this approximation, it is possible to make predictions about the large scales of interest from a description of the fine-scale physiological and population-dynamic pro- cesses without simulating the fate of every plant individually. We use the SAS approxi- mation to implement our individual-based biosphere model over South America from 15 8 Nt o1 5 8S, showing that the SAS equations are accurate across a range of environmental conditions and resulting ecosystem types. We then compare the predictions of the biosphere model to regional data and to intensive data at specific sites. Analysis of the model at these sites illustrates the importance of fine-scale heterogeneity in governing large-scale eco- system function, showing how population and community-level processes influence eco- system composition and structure, patterns of aboveground carbon accumulation, and net ecosystem production.

The Potential for Carbon Sequestration Through Reforestation of Abandoned Tropical Agricultural and Pasture Lands

AbstractApproximately half of the tropical biome is in some stage of recovery from past human disturbance, most of which is in secondary forests growing on abandoned agricultural lands and pastures. Reforestation of these abandoned lands, both natural and managed, has been proposed as a means to help offset increasing carbon emissions to the atmosphere. In this paper we discuss the potential of these forests to serve as sinks for atmospheric carbon dioxide in aboveground biomass and soils. A review of literature data shows that aboveground biomass increases at a rate of 6.2 Mg ha−1 yr−1 during the first 20 years of succession, and at a rate of 2.9 Mg ha−1 yr−1 over the first 80 years of regrowth. During the first 20 years of regrowth, forests in wet life zones have the fastest rate of aboveground carbon accumulation with reforestation, followed by dry and moist forests. Soil carbon accumulated at a rate of 0.41 Mg ha−1yr−1 over a 100‐year period, and at faster rates during the first 20 years (1.30 Mg carbon ha−1 yr−1). Past land use affects the rate of both above‐ and belowground carbon sequestration. Forests growing on abandoned agricultural land accumulate biomass faster than other past land uses, while soil carbon accumulates faster on sites that were cleared but not developed, and on pasture sites. Our results indicate that tropical reforestation has the potential to serve as a carbon offset mechanism both above‐ and belowground for at least 40 to 80 years, and possibly much longer. More research is needed to determine the potential for longer‐term carbon sequestration for mitigation of atmospheric CO2 emissions.

Aboveground peat and carbon accumulation potentials along a bog-fen-marsh wetland gradient in southern boreal Alberta, Canada

Production-to-decomposition quotients and asymptotic limits of peat accumulation were determined to estimate peat and carbon accumulation potentials along a bog-fen-marsh wetland gradient in southern boreal Alberta. The wetlands were a bog, a poor fen (PF), a wooded moderate-rich fen (WRF), a lacustrine sedge fen (LSF), a riverine sedge fen (RSF), a riverine marsh (RM), and a lacustrine marsh (LM). First year mass losses increased along this gradient (bog 14%, fens 25–61%, marshes 57–62%), with second year total mass losses increasing from 18 to 38% from the bog to the moderate-rich fens. Ratios of aboveground net primary production to decomposition and asymptotic limits of peat accumulation showed decreasing trends from the bog to the fens to the marshes as decay rates increased along the same gradient. TheSphagnum-dominated sites (bog, PF) showed greater peat accumulation potentials than the brown moss-dominated sites (WRF, LSF) and those sites with an insignificant-to-no moss stratum (RSF, RM, LM), which is paralleled by their decreasing peat thicknesses. Rates of litter accumulation in the first year averaged 170 g m−2 yr−1 inSphagnum-dominated sites, 130 g m−2 yr−1 in brown moss-dominated sites, and 103 g m−2 yr−1 in sites with an insignificant-to-no moss stratum. All three wetland types showed similar carbon accumulation potentials (83, 67, and 50 g m−2 yr−1, respectively) after the first year of decomposition. Peat depth, asymptotic limits of peat accumulation, and production-to-decomposition ratios correlated negatively with water levels, pH, and Ca2+, and they correlated positively with moss and woody plant production (shrubs, trees). Peatlands with strong moss and shrub/tree strata (bog, PF, WRF) accumulate more peat than those wetlands dominated by graminoids (LSF, RSF, RM, LM). In the bog, high peat accumulation potentials may be related to low rates of decomposition. The peat accumulation potentials of some fens (PF, WRF) are similar to the bog and may be maintained by higher decomposition rates, which are offset by higher litter inputs. In the graminoid-dominated fens and marshes, peat accumulation potentials are lowest and may be related to higher litter quality, resulting in higher decomposition rates.

The First Five Years in the Reorganization of Aboveground Biomass and Nutrient Use Following Hurricane Hugo in the Bisley Experimental Watersheds, Luquillo Experimental Forest, Puerto Rico

Five years after Hurricane Hugo reduced the aboveground biomass by 50 percent in two forested watersheds in the Luquillo Experimental Forest of Puerto Rico, regeneration and growth of survivors had increased the aboveground biomass to 86 percent of the pre-hurricane value. Over the 5 yr, the net aboveground productivity averaged 21.6 Mg.ha-1 yr-' and was faster than most plantations and secondary forests in the area. Woodfall and associated nutrient fluxes never attained pre-storm values but by the fifth yr, mean daily total litterfall, and N, P, K, Ca, and Mg fluxes in litterfall were 83, 74, 62, 98, 75, and 81 percent of their pre-disturbance values, respectively. Aboveground nutrient pools of these nutrients ranged from 102 to 161 percent of their pre-disturbance values and were larger after 5 yr because of higher nutrient concentrations in the regeneration compared to the older wood that it replaced. The following sequence of ecosystem reorganization during this first 5 yr period is suggested. An initial period of foliage production and crown development occurred as hurricane survivors re-leafed and herbaceous vegetation and woody regeneration became established. During this period, 75 to 92 percent of the nutrient uptake was retained in the aboveground vegetation and there was a relatively low rate of aboveground carbon accumulation per mole of nutrient cycled. This initial period of canopy development was followed by a peak in aboveground productivity that occurred as early successional species entered the sapling and pole stages. This period was followed by the establishment of the litterfall nutrient cycle and an increase in the net productivity per mole of nutrient cycled. During this 5 yr period, the Bisley forest had some of the lowest withinstand nutrient-use-efficiencies and some of the highest levels of aboveground productivity ever observed in the LEE The study demonstrates that high levels of productivity and rapid rates of aboveground reorganization can be achieved with rapid within-system cycling and inefficient within-stand nutrient use.