An e-publication by the World Agroforestry Centre
ALTERNATIVES TO SLASH-AND-BURN: a global initiative
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An e-publication by the World Agroforestry Centre |
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ALTERNATIVES TO SLASH-AND-BURN: a global initiative |
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West Sumatra field exercise Biophysical measurements GLOBAL RESEARCH AGENDA ON CARBON CYCLES Dr Mike Swift (TSBF) started this session with a presentation on the global carbon cycle. The cycle is controlled by the flux between conversion of organic biomass to carbon dioxide and water through organic matter decomposition in the soil and vegetation and organic carbon storage through photosynthesis and biomass accumulation. In the rainforest this process is in equilibrium. If the CO2 release is accelerated through burning, carbon dioxide accumulates in the atmosphere as a 'greenhouse gas'. The most serious effects of greenhouse gases derive from carbon dioxide emissions. Once the carbon cycle equilibrium is disturbed, it affects other processes such as nutrient cycling. In 1992-93, the combustion of different vegetation types contributed to the global carbon cycle as follows:
The following constitute some major research targets for global carbon research:
The types of measurements that need to be made occur at several levels within a land-use system are given in table 6.
Dr Sten Struwe (GCTE) gave a field demonstration on simple methods for determining gaseous emissions from soils and vegetation. The method is so robust and easy to implement that much interest was generated in using it for determining greenhouse gas emissions at remote field sites, where research on slash-and-burn is typically done.
A conventional tin can is placed over the soil in the field location where gas measurements are desired. The size of the can is flexible as long as its volume is known, but typically it might be 1-3 litres. Time is allowed for the atmosphere within the can to accumulate gaseous emissions from the underlying soil. Then a gas sample is withdrawn through a sealed rubber stopper by the use of a medical syringe. The gas sample is taken to the lab and run through a gas chromatograph to determine the concentration of CO2, N2O or other gases of interest. The gaseous flux is calculated based on the volume of the chamber and the amount of time the gas accumulated. Dr Struwe indicated that he would welcome interested researchers at the global sites to contact him for detailed information and guidance on application of the method. The group agreed that a global working group on gaseous emissions would be desirable. Through such a group the concerned researchers at the various sites could jointly develop and apply uniform methodologies to obtain authoritative estimates of greenhouse gas exchange from the various land-use systems in the forest margins and compare the effects of land-use changes from slash-and-burn to other systems.
Dr Meine Van Noordwijk (ICRAF) gave a presentation on below-ground biomass measurements and their importance. There are two methods for measuring root biomass: soil based and shoot based. Reasons for measuring it include understanding of the processes of flux in the carbon pool, root turnover, uptake efficiency and the intensity and depth of rooting. Methods used for measuring root biomass include soil-based sampling and plant-based sampling. The 'synchrony' of demand for nutrients by plants versus soil nutrient supply is especially important for shallow-rooted crops. Field demonstrations included preparing root maps and proximal root quantification.
Dr Cheryl Palm (TSBF) gave a presentation on the need for the biophysical characterization before slash-and-burn and afterwards, to measure the potential productivity of the subsequent system. She emphasized three types of measurements:
Vegetation characterization is necessary to measure nutrient stocks and distribution both above ground (vegetation, litter) and below ground (roots, soil); to measure carbon storage and losses; to record biodiversity changes and the use of forest species. The 'slash-and-burn' cycle shows periods of recovery and degradation in biomass carbon and plant-available nutrients. The amount of nutrients in the system will depend on ash production and nutrient availability processes. The fate of various nutrients differs when they are burned: carbon (carbon dioxide), nitrogen (nitrous oxide) and phosphorus, calcium, magnesium and potassium (ash). Biomass sampling. As the biomass and complexity of a system increase, measurements become more difficult, e.g. comparing grasslands (annual crops) and agroforestry (forest systems). For grasslands small quadrats (1 m x 1 m) and destructive harvests give accurate estimates of biomass. In tree/forest systems destructive harvests and allometric equations are required to estimate vegetation. Destructive harvest. Samples need to be stratified according to vegetation components (size and distribution). Small plots (1 m x 1 m) are used for materials with small, even distribution and large plots (10 m x 10 m) are used for those with large, patchy distribution. Vegetation is classified by size class: Large trees >15 cm diameter at breast height (dbh) Ash samples are taken by burying 0.25 m2 'ash trays' in three or four locations at the burn site before the burn, to collect ash samples for nutrient content after the burn. Allometric estimations. The biomass of large trees may be estimated through equations obtained by destructive harvest methods. These are usually location or land-type specific and depend on the composition of the vegetation. For example, the following equation was developed for trees in moist forests from a large sample of trees in Latin America: B = 38.4908 - 11.7883D + 1.1926D2 where biomass (B) is given in kilograms and D is diameter at breast height (dbh) measured in centimetres.
Dr Latnid (SARIF) gave a presentation on weed sampling in slash-and-burn systems. Sampling is done using a 0.5 m x 0.5 m quadrat and throwing it randomly in a field. This is replicated at least three times and the data are presented on a 1-m2 basis. There are several ways of deriving data: Visual = weed coverage by visual percentage; no differentiation by
species or group Weed control measures. There are several ways of measuring the
efficacy of weed control: |
