Ecosystems &. Environment. Effects of slash-and-burn agriculture and deforestation on climate change. P. Bernard Tinker a,*, John S.I. Ingram a, Sten Struwe b.
Agriculture Ecosystems & Environment ELSEVIER
Agriculture, Ecosystems and Environment 58 (1996) 13-22
Effects of slash-and-burn agriculture and deforestation on climate change P. Bernard Tinker a,*, John S.I. Ingram a, Sten Struwe b a GCTE Focus 3 Office, Department of Plant Sciences, University of Oxford, Oxford, UK b Department of General Microbiology, University of Copenhagen, Copenhagen, Denmark
Abstract Tropical forest felling can be for the purpose of traditional shifting cultivation, after which forest is re-established, or for permanent land-use change, which is defined as deforestation. Recent decades have seen a dramatic increase in tropical deforestation caused by slash-and-bum clearing for the establishment of more permanent agriculture, plantations and pastures, which often result in degraded grasslands or degraded fallows. The net CO 2 balance in shifting cultivation is near zero if the forest returns to its original biomass and soil organic carbon status, although there is a small net release of other greenhouse gases during the cropping cycle. Deforestation by contrast normally causes large losses of CO 2 from the soil and vegetation. Greenhouse gas (GHG) emissions are still difficult to quantify. Deforestation may lead to changes in evapotranspiration, runoff and local climate but there are few data. If it occurs in large continuous areas, the rainfall may be decreased, according to modelling studies. There is now no doubt that "human activities are substantially increasing the atmospheric concentrations of the greenhouse gases", and that "these increases will enhance the greenhouse effect" (IPCC, 1990. Climate Change: The IPCC Scientific Assessment). The questions we address in this paper are to what extent slash-and-bum of forest is responsible, and how land conversion of this type will affect the climate system, including its impact on local and regional hydrology. Keywords: Slash-and-bum; Deforestation; Carbon balance; Climate change; Land use
1. The driving forces of climate change Climate change is one of three main driving variables of global change; the others are changes in atmospheric composition and land use (IGBP, 1990). Change in land use, particularly deforestation, is a direct cause of change; it alters the land cover of the globe and causes erosion and other problems. However, it also has important effects on climate change through the production of greenhouse gases (GHGs, notably CO 2, C H 4 and N 2 0 ) and o f aerosols (e.g, * Corresponding author. 0167-8809/96/$15.00 Published by Elsevier Science B.V. SSDI 0 1 6 7 - 8 8 0 9 ( 9 5 ) 0 0 6 5 1 - 6
smoke and dust particles), which affect climate by reflecting and absorbing radiation. Equally, changes in the climate and atmosphere (only in part due to land-use change) feed back onto land-use effects, through vegetation response to changed growth conditions. These changes, therefore, form a very complex web of interacting processes; here we try to elucidate the role of types of agriculture that require slash-and-burn of forest. The terminology tends to cause confusion here (Houghton et al., 1985). Slash-and-burn agriculture was usually taken to mean shifting cultivation, in which the land reverts to forest regrowth after a short
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time. However, the term now seems to be used more loosely to refer to any felling and burning of forest. In this paper we use slash-and-burn agriculture to mean traditional shifting cultivation. If this becomes so intense that high forest can not re-establish itself, or if other non-forest land uses are established, we prefer to call this process deforestation, although obviously there is a gradation between the two processes: it is only the latter that has major global change implications. Any land clearing has important consequences: the felling and burning of forest, leading to GHG and aerosol emissions; the subsequent tillage causing further releases of GHGs as soil carbon is oxidized, and nitrogen is mineralized and denitrified; changes in water infiltration rates and runoff rates of the soil. Permanent deforestation may additionally affect climate and hydrology.
2. The scale factor It is important to identify the scale and type of operation. In global change terminology, some processes are truly global in that their effects are spread relatively evenly around the globe; examples are change in atmospheric composition or rise in sea level. Other processes operate at a regional scale, like changes in climate. A third set operate at the local scale, like soil erosion or soil salinization; however, if these local processes are widespread, their collective effect can be said to be global. Slash-and-burn agriculture was traditionally practiced on a small or local scale by many farming societies, with long fallow periods, and hence a large relative area of undisturbed forest. It was then a fully sustainable system, with no net input of CO 2 or significant input of other GHGs to the atmosphere. The dramatic change in recent decades has been the intensity and extent to which slash-and-burn of forest is practiced worldwide. Increasing pressure on the land gradually reduces the fallow period until only scrub woodland regenerates, yields are reduced, and the average biomass on the land is greatly reduced. In the limit, however, there is deforestation in favor of some quite different land use. Everything then depends upon the type and extent of this use in relation to the soil and climate.
If the forest is replaced by tree crops such as oil palms or plantations, the micrometeorological conditions may ultimately be little changed and the local climate may be little affected. However, the amount of carbon in the vegetation will almost certainly be less than under forest, and soil may suffer from erosion and loss of organic matter. If lower growing vegetation such as grasses is the replacement, the water-energy exchange will be altered much more. Finally, if the land is used for annual cropping, the hydrology, micrometeorology and soil conditions will be radically changed. If these changes are on a large scale, the regional climate may be affected: these changes are discussed below.
3. The contribution of forest felling to global greenhouse gas emissions In the natural state of a forest, or even when traditional slash-and-burn agriculture is practiced, the carbon and nitrogen fluxes effectively balance. Carbon fixation through photosynthesis equals the sum of carbon loss through respiration; above- and below-ground biomass and the soil organic carbon level remain constant. Nitrogen inputs through either biological nitrogen fixation or rainfall balance leaching losses and denitrification. This balance is seriously disturbed when good secondary forest can no longer regenerate because the fallow period is too short, when there is then a permanent loss of carbon from vegetation and soils. There has been growing concern about the increasing areas undergoing landuse change of this type; this, with direct permanent change in land use, is defined as deforestation (Table 1). There is a recent suggestion (Fisher et al., 1994) Table 1 Summary of global annual deforestation estimates(104 km2) (IPCC, 1992) Reference Year of Closed canopy Closedand reforestation forestonly open canopy forests Myers, 1980 1979 7.3 FAO/UNEP, 1976-80 7.3 11.3 1981 Myers, 1989 1989 13.9 FAO, 1991 1981-90 14.0 17.0 WRI, 1990 Late 1980s 16.5 20.4
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that pasture establishment may lead to carbon sequestration below ground. Over the period 1850-1985, Houghton et al., 1991, estimate that the total net release of carbon from a range of land uses in Latin America was about 30000 Mt, about 5% of which was contributed by slash-and-burn agriculture in the sense used here; by 1985, this contribution had grown to 20%. Skole and Tucker (1993), using Landsat/Thematic Mapper (TM) and Geographic Information System (GIS) to stratify the region on the basis of cover types, report an average annual rate of deforestation of 15000 km 2 year-1 for the period 1978-1988 for Amazonia including areas identifiable as regrowth forests, i.e. areas recently under shifting cultivation: this figure therefore encompasses more than our definition of deforestation. This figure is considerably lower than other estimates, which ranged from 21 000 to 80000 km 2 year-1 (Skole and Tucker, 1993). In the early 1980s, the total global area of tropical forest cleared was estimated to be 16 000 km 2 year-~ for agriculture, but the differences of terminology and definition, which are discussed in the paper, make comparisons with other data difficult (Houghton et al., 1985). Increasing concentrations of GHGs will lead to elevated mean global temperatures and changes in precipitation. While the magnitude of the overall change is still being debated, all global circulation
Table 2 The annual global CO 2 balance sheet in 1990 a (Jarvis and Dewar, 1993); and for 1980-1989 b (IPCC, 1995) Sources
Gt y e a r - ~
Sinks
Gt y e a r - i
Fossil fuels
5.7 a 5.55:0.5 b 2.1 a
Atmosphere
3.2 a 3.2±0.2 b 1.0 a
Tropical deforestation (gross) CO z from tropical land use (net) CO and CH 4 from burning vegetation and soil changes
Total sources
0.7 0.7 a
8.5 ~
Oceans
Temperate and boreal forests Tropical forests and grasslands
Other terrestrial sinks Total sinks
2.0±0.8 b 1.8 a 2.5
1.4±1.5 b 8.5 ~
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models (GCM) agree that there will be significant global and regional change. Tropical deforestation for a variety of purposes (pasture, logging, permanent agriculture and shifting agriculture) accounts for about 25% of carbon released as CO~ at present (Jarvis and Dewar, 1993; Table 2). However, the table also includes large sink terms to tropical forests and grasslands, so the net contribution from deforestation is still uncertain. IPCC, 1995, estimates the net contribution of tropical deforestation to be 1.6 Gt year- 1. Table 2 shows that the burning of fossil fuels currently contributes by far the greatest amount of the gross input (about 67%) and is still rising; it is much more difficult to obtain accurate data on the size and trend of the contribution from deforestation, and the estimate of emissions from changing land use in the tropics may possibly be modified in the near future (IPCC, 1995).
4. C O 2 s o u r c e s
4.1. The burn
Prompt release of CO 2 comes from burning the above-ground biomass (Crutzen and Andreae, 1990), although not all is necessarily burnt on the first attempt but has to be reignited. If the fire burns hotly and rapidly, there is a greater ratio of CO2:CO in the combustion gases than in a cooler, smoldering burn. A mixture of biomass types will burn much more slowly, with a greater smouldering phase. Setzer and Pereira (1991) report that burnings associated with recent deforestation have a flaming period of about l h, after which the smoldering may persist for many hours, or even over 1 day where embers are immersed in ashes or soil. A net release of CO 2 is associated with a decrease in the standing stock of biomass, as is the case during the cropping phase of slash-and-bum agriculture. If, during the fallow phase, biomass and soil organic carbon (SOC) accumulate to the same level as before clearing, the long-term net release is zero. This is discussed below. In either case, however, Crutzen and Andreae (1990) point out that there is a net transfer of particulates and trace gases other than CO 2 to the atmosphere, which potentially plays a significant role in altering climate.
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It is hard to derive clear estimates of the regional net release rate of C O 2 because of several uncertain factors: (1) the actual areas of land being cleared annually; (2) the different estimates of biomass cleared per hectare and (3) the different amounts of slash burnt, unburnt (but susceptible to slower microbial decay), or left as charcoal, and (4) the regrowth rate of vegetation in the cleared areas. However, using a refined comparison of Advanced Very High Resolution Radiometer (AVHRR) and Landsat/TM methods for Brazil's Amazon Basin, Setzer and Pereira (1991) estimated 1700 Mt CO 2 and 10 Mt C H 4 w e r e released gross during 1987, although it is difficult to tell forest sources from fires in other types of vegetation.
4.2. Cultivation: loss of soil organic carbon (SOC) It has long been known that SOC levels decline during the cultivation phase following burning of forest (Nye and Greenland, 1960), although the effects can be highly variable. The method of clearing influences SOC decline, with bulldozer clearing being the most disruptive, because it scrapes topsoil into windrows and much of the plant biomass and nutrients are removed (Sanchez et al., 1985). After the site is cleared for cultivation, three factors contribute to SOC decline: (1) higher topsoil temperatures leading to higher decomposition rates; (2) lower litter inputs and (3) increased SOC oxidation caused by tillage (which increases aeration and breaks down aggregates), While less SOC may be oxidized during the slash-and-burn agriculture cycle than during continuous cropping (Lugo and Brown, 1992), Houghton et ai. (1991) summarize a range of studies indicating 20-30% of C in the top 1 m may be lost during a slash-and-burn agriculture cycle. For a soil with 1% SOC in the top 25 cm before cultivation, losses would amount to ca. 6 - 9 t C ha- l In addition to in situ oxidation of SOC, accelerated erosion increases SOC loss from the site by selectively removing the carbon-rich surface horizons. However, SOC lost in this way will not necessarily contribute to CO 2 emissions, if it is adequately protected when deposited. The quantification of SOC losses and gains resulting from land-use change is a prerequisite to under-
standing GHG emissions. Davidson et al. (1993) point out that this quantification is made difficult because of a dearth of comparative information on below-ground carbon budgets of tropical forests and the ecosystems replacing them. This is clearly an area that needs much more attention.
5. CH 4 and N 2 0 sources
Because of the logistic difficulty of making measurements in relevant regions, and the complexity of the ecosystems and combustion dynamics involved, our understanding of the GHG emissions from slashand-burn systems is poor. It is nevertheless known that while the main GHG produced during the burn is CO2, significant amounts of CH 4 and N20 may also be released, depending on the fire conditions (e.g. fuel load, rapidity of burning, stem size). Biomass burning in African savannas and forests has, however, been investigated recently in largescale campaigns (e.g. Angelletti et al., 1992). It is estimated that 2.5 Gt vegetation of African savannas is burned annually, and on the basis of a value of 29 ppmv of CH4 in the smoke plume in savanna fires, Bonsang et al. (1992) estimated the very high total figure for C H 4 emission at 204 Mt C year-1. Both these estimates seem very high by comparison with global figures (IPCC, 1995). The emission of N20 has been estimated in similar ways based on the ratio between N20 and CO 2 in plumes from big fires. Delmas et al. (1992) provide a figure of 0.37 Mt N20 lost from the whole of Africa in fires. Clearly, fires from slash-and-bum will contribute significantly to the global emission of GHGs, but the quantification is still very poor, as is true for almost all the sources of trace GHGs. There is only limited evidence from measurements in regions with slash-and-burn agriculture of the long-term effects of this type of cultivation on the emission of CH 4 and N20 from soils. It is known, however, that CH 4 and N20 emissions during the cultivation phase will be highly influenced by the cropping systems employed, and the soil carbon and nitrogen at clearing. During the period following clearing, if the soils are not so dry as to limit microbial activity, roots and other plant material are decomposed by soil fungi and bacteria. When the
P.B. Tinker et al./Agriculture, Ecosystem and Environment 58 (1996) 13-22
soil solution nitrate concentration is high, and both the pH and the availability of oxidizable material are low, the C H 4 production is low, but the production of N 2 0 may be high (Arah and Smith, 1990). More intensive reducing conditions will lead to higher CH 4 production, but will also favor N 2 production over N20. The effects of soil fauna such as termites are also important.
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y e a r - I in above-ground biomass can be accumulated. Given a sufficiently long fallow period, the site clearly recovers to near its original carbon status. The rate of carbon sequestration clearly varies greatly depending on the local soil fertility and other factors, and figures well above those quoted may well be found.
6. Regain of carbon in slash-and-burn agriculture
7. Impact of slash-and-burn agriculture on hydrology
There is no doubt that large amounts of GHGs are released during the clearing/burning/cultivating period of slash-and-burn agriculture. There is a growing realization, however, that these concerns are perhaps less critical in the long term, as this statement does not take account of the development of subsequent land cover, such as regeneration following slash-and-bum agriculture or planting with perennial cropping systems. The debate in recent years has therefore moved to how long the net loss stage lasts, and whether replacement land management systems can subsequently act as net sinks for carbon (Lugo and Brown, 1992, Lugo and Brown, 1993). Data presented in Table 3 show that during the period of forest succession following abandonment, up to 2 t SOC h a - I y e a r - l and 2-3.5 t C h a -
The removal of forest can in principle affect the hydrology o f an area in two ways. Firstly, the ene r g y / w a t e r balance of the area will be changed. This will certainly change variables such as local air and soil temperatures, but there is a possibility that, over large areas, the rainfall may be decreased (Salati and Vose, 1984). The total areas that have been deforested are extensive (Table 1), but little is known about how the total area is spatially distributed. Secondly, the change in the water balance will almost certainly alter the drainage, runoff and water yield. Mechanism and consequences of alteration of water/energy exchange. The albedo of high rainforest is around 0.11-0.13. Almost all other land uses will give a higher albedo - up to 0.20 with grassland. Reflected radiation is therefore considerably larger,
Table 3 Processes that create carbon sinks and their potential magnitude in the tropical closed forest landscape (from Lugo and Brown, 1992) Process Magnitude Source (t C ha- i year- l) Biomass accumulation in forests greater than 60-80 years old and logged forests a Biomass accumulation in secondary forest fallows, 0-20 years old a Biomass accumulation in plantations b Accumulation of coarse woody debris in a Forests greater than 60-80 years old Forests 0-20 years old Accumulation of soil organic carbon Forest succession Conversion of annual crops to pasture/grassland
1-2
Original authors' data
2-3.5
Brown and Lngo, 1990a
1.4-4.8
Brown et al., 1986
0.20-0.40 0.17-0.30
Original authors' data Brown and Lugo, 1990a
0.5-2.0 0.30-0.42
Brown and Lugo, 1990b Lugo et al., 1986
a Convened to carbon units by multiplying organic matter by 0.5.b Weighted average rates across all species and age classes,c Two studies given in Brown and Lugo (1990a) report an average amount of coarse woody debris at age about 20 years of 8.5% of the above-ground biomass; we assumed this percentage of the biomass accumulation rate goes into coarse woody debris during the 20-year period, i.e. 8.5% of 2-3.5 t C h a -R yearS' is 0.17-0.30 t C ha -1 year-z.
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hence evapotranspiration is less and humidity will be lower than with forest. There is also a large effect in the interception of rainfall by forest and its rapid subsequent evaporation, without ever reaching the soil. Surface roughness will be changed, altering turbulent transfer of energy and water (Pitman et al., 1993). Finally, it is likely that the effective rooting depth of the replacement vegetation is less than that of the original forest. If so, extraction of stored soil water in periods of drought will be less. With such a variety of processes involved, the net result will be highly dependent upon local conditions. If local rainfall is largely the return of locally evaporated water, then lower evapotranspiration must have an effect on rainfall. Salati et al. (1986) noted from the differences in oxygen isotope signatures that roughly half of the rain falling in the Amazon was locally derived. This result agrees with the observation of Shuttleworth (1988) that the average flux of water vapor into the air near Manaus was 3.6 mm day -1, whereas the rainfall was 7.2 mm day - t . This effect would of course be expected to be largest in extensive areas of land, and much less near coasts or on islands where water vapor is brought in from the ocean. This agrees with the modelling results of Henderson-Sellers (1993), who found that removal of forest would have much more widespread effect on rainfall in the Amazon than in Southeast Asia. Similarly, Polcher and Laval (1993) modelled the energy budget of the surface with and without forest for the three main areas of rainforest in the world and predicted that the percentage difference in evapotranspiration following deforestation would be 10% in the Amazon, 8% in Central Africa and only 3% in Southeast Asia. As yet there are no precise measured data from the field, because of the extreme difficulty of getting reliable before-and-after data for large areas. This has, therefore, encouraged the use of modelling, using various global circulation models. The complexities of this type of modelling and the uncertainties in the results are well outlined in HendersonSellers et al. (1993), who compared their modelling results with those of earlier authors. They found that precipitation would decrease by about 30% in the wet season and 10% in the dry season in the Amazon but that temperature differences were small. However, the decrease in runoff was unexpectedly large.
As Polcher and Laval (1993) say: "the influence of deforestation on climate is model dependent and can vary from one region to another". Runoff increases in models that assume the deforested area does not quickly regain a complete plant canopy. This is seldom true in slash-and-burn agriculture, where a canopy of crops, weeds and tree regrowth is usually well established within 1 month. Furthermore, the mass of unburned logs and branches also retards erosion. It has been suggested that the climate following deforestation could be so much changed that it would no longer support rainforest, i.e. the change would be irreversible (Nobre et al., 1991). At present this worst case scenario seems unproven, but there does appear to be a rough consensus from modelling studies that deforestation of large areas such as the Amazon will diminish rainfall and evapotranspiration (Henderson-Sellers et al., 1993). It would clearly be desirable if this could be confirmed by direct observation. However, the slow development of deforestation in earlier days and the uncertainty about the succeeding vegetation cover make this very difficult. The question has been discussed frequently because of its global importance (Bruijnzeel, 1990; Deutscher Bundestag, 1990), but reports are too isolated and variable to allow any finn conclusion. Much more work is needed on this topic. The general question of the effects of smaller clearings on rainfall is even less clear. It is known from direct measurements that a number of variables, including soil temperature, will fluctuate more on a diurnal basis after the removal of forest in fairly small areas (Bastable et al., 1993) and that the total evapotranspiration will decrease. These authors make the point that all modelling is done on daily or even monthly mean values, but they found that the means differed little, whereas the variation differed greatly. Bastable et al. (1993) concluded that small clearings would be very unlikely to affect the local rainfall distribution and the regional weather patterns, but that the local near-surface climate will differ from that under forest. As clearings multiply, their effects should eventually integrate to affect the atmospheric circulation and give effects like those predicted by the GCMs. However, the scale at which the effects appear is not known, because these simulations are all at GCM grid scale or larger; in the work of
P.B. Tinker et al./Agriculture, Eco,system and Environment 58 (1996) 13-22
Henderson-Sellers et al. (1993) the grid was 4.5 ° ;4 7.5 °. This use of large grid cells means that there is little information on the effects of smaller clearings and in particular on the effects of fragmenting a large area of forest. The figure of Skole and Tucker (1993) for average deforestation (including some regrowth areas) in the Amazon over 1978-88 of 15000 km 2 year -~ is lower than almost all other estimates, yet it is probably one of the most accurate. Recent estimates of deforestation of around 11 000-13000 km z year -j (Fearnside et al., 1993) suggest even lower values. Some 4 × 106 km 2 of the 'legal Amazon' (Skole and Tucker, 1993) is forested, so the present rate of deforestation would need to proceed for 300 years to remove all forest. However, Skole and Tucker (1993) included regrowth areas as deforested, and no estimate of the real extent of deforestation is possible until it is known how much felled forest in fact goes back to forest again. If a grid cell in the GCMs used for modelling studies is, e.g. 5.0°× 2.5 ° , it represents about 125000 km 2, equivalent to some 10 years of deforestation. There is consequently a long way to go before the models deal with smaller, localized clearings. It is clear that the question of the effect of deforestation on climate will not be settled for some time. The general results from the modelling work mostly agree that evapotranspiration and precipitation will both be reduced. This is also in accordance with general physical principles (Sud et al., 1993) and based upon known differences in conditions in a forested and a grassland area. However, the difference in degree and in detail between simulations, even using basically the same model, indicates that there is some way to go before reliable results are obtained. These models of course apply only to extreme conditions of complete replacement of forest by grassland over very large areas. Information on the effects of replacement by mixed vegetation, or in broken patterns, or in smaller discrete areas is still awaited.
7.1. Changes in hydrology There is a great deal of work on the effects of afforestation or deforestation on water yield from catchments, including much in temperate climates.
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Shuttleworth (1989) has argued that temperate and tropical forest micrometeorology should not differ markedly, so that the temperate zone work can be used as an example. From this, the normally expected consequence of deforestation is an increase in river flow. This is caused by the processes listed earlier that reduce evapotranspiration. If the precipitation remains the same, the total runoff and runthrough will therefore increase and swell the rivers. If, however, the deforestation also results in a decrease in precipitation, this effect will be counteracted. The modelling studies referred to above tend to suggest rather similar percentage changes in evapotranspiration and rainfall. As the latter is roughly twice as large in the Amazon, the final effect here could be decreased runoff, as indeed the modelling predicts. For this reason, Shuttleworth (1989) suggests that the increase in river flow rates following deforestation will be greatest near coasts, where the climate is stable because of the nearness of the ocean. Thus Durbidge and Henderson-Sellers (1993) predicted that runoff would be markedly increased by deforestation in southeastern Asia, when precipitation was little affected. Similarly, it would be expected that the effects of deforestation would be smaller in climates without dry seasons, because the difference in rooting depth between the forest and other covers is then less important. The most direct way to obtain data on this question is by the use of paired catchments, assuming that they are generally similar, and that they do not leak. Bosch and Hewlett (1982) analyzed the results of many such paired catchments and concluded that where vegetation was reduced, water yield increased or remained constant, and where it was increased, water yield decreased or remained constant. Bruijnzeel (1990) summarizes a series of conclusions regarding the effect of more detailed treatments such as plantation establishment, burning, coppicing, etc. He concluded that light and careful harvesting of trees would have little effect. There have been indications that replanting with tree production species such as eucalyptus may reduce water yield to less than that with the original forest, but according to Newson and Calder (1989), this is by no means always true. The other main differences relate to the rate of infiltration and the moisture-holding capacity of the
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soil. If the infiltration rate is decreased by soil damage, there will be increased surface runoff and greater flooding. This is not, however, a necessary result of deforestation, because other vegetation covers may allow an equally good infiltration rate. As stated before, a great deal depends upon the type and level of management after the forest is felled. Bruijnzeel (1990) summed this up excellently as follows: "the adverse environmental conditions following deforestation in the humid tropics are not so much the result of deforestation per se, but rather of poor land use practices after clearing of the forest." There is a great deal of information on the pattern of runoff, flooding frequency and sediment transport, but these are issues that do not really affect climate change.
8. Climate change impacts on slash-and-burn agriculture The impact of slash-and-bum agriculture on climate change has formed the main theme of this paper. We feel it important, however, to consider briefly the other dimension of slash-and-burn agriculture and climate change: how will climate change affect slash-and-burn agriculture? Given the uncertainties in predictions of regional and local future climates, it is unwise to attempt more than a brief, scenario-based discussion here. The elevated atmospheric CO 2 levels may lead to a fertilization effect, although with respect to tropical conditions, this has only been researched at the
species level; whole ecosystems or crop experiments are required to gain a better understanding. However, these initial investigations confirm that litter quality (i.e. the C:N ratio) will change, as will the ratio of above- to below-ground carbon allocation. This will have implications for SOM dynamics and carbon sequestration. Changed climate sensu stricto in the tropics will most likely be limited to changes in the rainfall mean values and their variance; all GCMs agree mean temperature changes in tropical regions will be very small. The impact on systems currently water limited for at least part of the year (e.g. miombo woodland in southern Africa) may be dramatic with even a moderate (10%) change either up or down. The effect of changing rainfall variability and intensity on erosion will in itself be a major issue, but it will likely be overridden by the effects of changes in vegetation cover brought about by the three global change drivers working in concert. This highly complicated situation warrants detailed experimental and modelling research programs.
9. The global change soils program Many national and several international programs are vigorously addressing components of the problems described above. The overall coordination at the global level of studies in the global change debate currently falls to IGBP through its Core Projects: Global Change and Terrestrial Ecosystems
Table 4 The structure of the GCTE Focus 3 soils activity (Activity 3.3) Task 3.3.1 Global Change Impact on Soil Organic Matter
3.3.2 Soil Degradation under Global Change
3.3.3 Global Change and Soil Biology
Objectives To determine the qualitative and quantitative effects of plant physiological, vegetation, and ecosystem change on soil organic matter dynamics, in relation to changes in atmospheric composition, climate, and land use, and the consequences for natural and managed vegetation. 1. To develop the capability to predict soil degradation by water erosion caused by interactive changes in land use and climate. 2. To develop the capability to predict soil degradation by wind erosion, and subsequent deposition, caused by interactive changes in land use and climate. (Draft) to determine the impacts of global change on soil biota and biological processes, and the feedback effects of such changes on greenhouse gas emissions.
P.B. Tinker et al./Agriculture, Ecosystem and Environment 58 (1996) 13-22
(GCTE), International Global Atmospheric Chemistry (IGAC) and Biospheric Aspects of Hydrological Cycle (BAHC) (IGBP, 1990). One of these, GCTE, has a specific activity dealing with soils (Steffen et al., 1992). The outline of the GCTE soils research program is in Table 4. Task 3.3.1 concerns global change impact on soil organic matter. Change in climate, atmospheric composition and land use all affect SOM to some extent, either directly or indirectly, which in turn has an impact upon soil erosion and fertility. The predominant present manifestation of global change in much of the tropics is, however, land-use change, where SOM declines are strongly correlated with changes from natural to agricultural systems; aspects of this have been covered in this paper. Possible consequences of increased atmospheric CO 2 concentration is briefly discussed above. Existing SOM models need to be expanded to accommodate these possible changes and linked to GISs to allow their application to large areas. In the second soils task (T3.3.2), GCTE will initially concentrate on the impacts of global change on water erosion in the humid tropics and wind erosion in semi-arid regions, two phenomena that require urgent attention and are wide ranging. Both forms of erosion are caused primarily by land-use change and technologies aimed at reducing erosion often overlap with those aimed at improving soil nutritional fertility. The most common features are maintaining soil cover and improving or maintaining the soil organic matter levels. This project is thus closely linked with Task 3.3.1 at the micro scale but also needs to consider the wider issues ultimately causing erosion. Essential links have therefore been initiated with the Human Dimensions of Global Environmental Change (HDP, an international program of the International Social Sciences Council), and with International Society of Soil Science (ISSS), International Soil Reference and Information Centre (ISRIC) and FAO to refine both erosion hazard algorithms and global extent maps. The third task in the GCTE soil program (T3.3.3) concerns soil biology and covers both the direct and indirect impacts of global change on soil biota and biological processes and the critical aspect of GHG emissions. The soil processes leading to GHG emissions will be the subject of a tightly integrated
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research program with national and other international programs. From within the IGBP family, both IGAC and BAHC core projects will be involved; outside IGBP, GCTE seeks collaboration with national and international projects involved in soil biological processes such as the UK Terrestrial Initiative in Global Environmental Research (TIGER) program (essentially UK in geographic scope, but with some tropical work), and the Tropical Soil Biology and Fertility Program (TSBF). References Angelletti, G., Beilke, S. and Slanina, J. (Editors), 1992. Air Pollution Research Report 39. CEC, Brussels. Arah, J.R.M. and Smith, K.A., 1990. Factors influencing the fraction of the gaseous products of soil denitrification evolved to the atmosphere as nitrous oxide. In: A.F. Bouwman (Editor), Soils and the Greenhouse Effect. John Wiley and Sons Ltd., Chichester. Bastable, H.G., Shuttleworth, W.J., Dallarosa, R.L.G., Fisch, G. and Nobre, C.A., 1993. Observations of climate, albedo, and surface radiation over cleared and undisturbed Amazonian forest. Int. J. Climatol., 13: 783-796. Bonsang, B., Bassler, U., Boissard, C., Le Cloarec, M.F. and Menaut, J.-C., 1992. Methane, carbon monoxide and light non-methane hydrocarbons emissions from African savanna buruings. In: G. Angelletti, S. Beilke and J. Slanina (Editors), Air Pollution Research Report 39. CEC, Brussels. Bosch, J.M. and Hewlett, J.D., 1982. A review of catchment experiments to determine the effect of vegetation changes on water yield and evapotranspiration. J. Hydrol., 55: 3-23. Brown, S. and Lugo, A.E., 1990a. Tropical secondary forests. J. Trop. Ecol., 6: 1-32. Brown, S. and Lugo, A.E., 1990b. Effects of forest cleating and succession on the carbon and nitrogen content of soils in Puerto Rico and US Virgin Islands. Plant soil, 124: 53-64. Brown, S., Lugo, A.E.and Chapman, J., 1986. Biomass of tropical tree plantations and its implications for the global carbon budget. Can. J. For. Res., 16(2): 390-394. Bruijnzeel, L.A., 1990. Hydrology of moist tropical forests and effects of conversion: a state of knowledge review. UNESCO, Paris. Crutzen, P.J. and Andreae, M.O., 1990. Biomass burning in the tropics: impact on atmospheric chemistry and biogeochemical cycles. Science, 250: 1669-1678. Davidson, E.A., Trumbore, S.E. and Nepstad, D.C., 1993. Losses of carbon from pasture soils of the eastern Amazon estimated from measurements of isotopes, fluxes, and inventories. Paper presented at the International Symposium: Soil Processes and Management Systems, Greenhouse Gas Emissions and Carbon Sequestration, 5 - 9 April 1993, Columbus, OH. Delmas, R., Lacaux, J.-P., Menaut, J.-C., Abbadie, L., Leroux, X., Lorbert, J. and Helas, G., 1992. Nitrogen compound emission from biomass burning in tropical African savanna, FOS/DE-
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