Tropical forest conversion, shifting cultivation and clearing of secondary vegetation make significant contributions to global emissions of greenhouse gases today, and have the potential for large additional emissions in future decades. Globally, an estimated 3:1 10 9 t of biomass carbon of these types is exposed to burning annually, of which 1:1 10 9 t is emitted to the atmosphere through combustion and 49 10 6 t is converted to charcoal (including 26-31 10 6 t C of black carbon). The amount of biomass exposed to burning includes aboveground remains that failed to burn or decom- pose from clearing in previous years, and therefore exceeds the 1:9 10 9 t of aboveground biomass carbon cleared on average each year. Above- and belowground carbon emitted annually through decomposition processes totals 2:1 10 9 t C. A total gross emission (including decomposition of unburned aboveground biomass and of belowground biomass) of 3:41 10 9 t C year 1 results from clearing primary (nonfallow) and secondary (fallow) vegetation in the tropics. Adjustment for trace gas emissions using IPCC Second Assessment Report 100-year integration global warming potentials makes this equivalent to 3:39 10 9 to f CO 2-equivalent carbon under a low trace gas scenario and 3:8310 9 t under a high trace gas scenario. Of these totals, 1:0610 9 t (31%) is the result of biomass burning under the low trace gas scenario and 1:50 10 9 t (39%) under the high trace gas scenario. The net emissions from all clearing of natural vegetation and of secondary forests (including both biomass and soil fluxes) is 2:0 10 9 t C, equivalent to 2.0-2:4 10 9 to f CO 2-equivalent carbon. Adding emissions of 0:4 10 9 t C from land-use category changes other than deforestation brings the total for land-use change (not considering uptake of intact forest, recurrent burning of savannas or fires in intact forests) to 2 :4 10 9 t C, equivalent to 2.4-2:9 10 9 to f CO 2-equivalent carbon. The total net emission of carbon from the tropical land uses considered here (2:4 10 9 t C year 1 ) calculated for the 1981-1990 period is 50% higher than the 1:6 10 9 t C year 1 value used by the Intergovernmental Panel on Climate Change. The inferred (D 'missing') sink in the global carbon budget is larger than previously thought. However, about half of the additional source suggested here may be offset by a possible sink in uptake by Amazonian forests. Both alterations indicate that continued deforestation would produce greater impact on global carbon emissions. The total net emission of carbon calculated here indicates a major global warming impact from tropical land uses, equivalent to approximately 29% of the total anthropogenic emission from fossil fuels and land-use change.

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Global warming and tropical land-use change: greenhouse gas emissions from biomass burning, decomposition and soils in forest conversion, shifting cultivation and secondary vegetation

Termite assemblages are considered as complex systems containing species with several modes of feeding and nesting, which have a major though not necessarily dominant role in decomposition and C mineralization processes, and which influence soil properties and structure. Sampling methods for species richness, abundance and biomass and the estimation of food consumption rates are reviewed; transect methods are recommended for biodiversity surveys as they are efficient and have acceptable accuracy. Biases introduced by sampling methods which focus on mounds only and by consumption assays based on baits lead to underestimates of assemblage diversity, abundance and ecological impact. The range of abundance and biomass of termites in major ecosystems and biogeographical regions is discussed and representative data are tabulated. In savanna systems, the turnover of organic matter by termites is roughly comparable to that of mammalian herbivores and bush fires, and as much as 20% of C mineralization may be directly attributed to termites. In forests, absolute C fluxes through populations are generally larger, owing to higher termite biomass, but the relative contribution to C turnover is less. Functional group heterogeneity rather than species richness per se is considered the key link between termite biodiversity and ecosystem functions.

Termites in Ecosystems

Soil carbon changes from conversion of forest to pasture in Brazilian Amazonia

Deforestation in Brazilian Amazonia is a significant source of greenhouse gases today and, with almost 90% of the originally forested area still uncleared, is a very large potential source of future emissions. The 1990 rate of loss of forest (13.8 × 103 km2/year) and cerrado savanna (approximately 5 × 103 km2/year) was responsible for releasing approximately 261 × 106 metric tons of carbon (106 t C) in the form of CO2, or 274–285 × 106 t of CO2-equivalent C considering IPCC 1994 global warming potentials for trace gases over a 100-year horizon. These calculations consider conversion to a landscape of agriculture, productive pasture, degraded pasture, secondary forest, and regenerated forest in the proportions corresponding to the equilibrium condition implied by current land-use patterns. Emissions are expressed as ‘net committed emissions’, or the gases released over a period of years as the carbon stock in each hectare deforested approaches a new equilibrium in the landscape that replaces the original forest. For low and high trace gas scenarios, respectively, 1990 clearing produced net committed emissions (in 106 t of gas) of 957–958 for CO2, 1.10–1.42 for CH4, 28–35 for CO, 0.06–0.16 for N2O, 0.74–0.74 for NOx and 0.58–1.16 for non-methane hydrocarbons.

Adelmar Gomes Bandeira

Papers

Deforestation and methane release from termites in Amazonia