Health.
Global warming.
Environmental benefits of correctly managed biofuel production.
Biodiversity.
Soil-carbon cycles.
Biomass is produced using widely varying strategies related to site specific parameters, the scope of which makes it difficult to provide more than general guidelines or principles. It is essential that strategies for sustainable biomass growth are developed for each ecological region, however, research is already highlighting benefits and some areas of concern with large-scale bioenergy plantations.
Very little detailed and reliable environmental data is presently published on the impact of large-scale plantations. One notable exception has arisen from a collaboration between a multinational company and a leading NGO, namely Shell International Petroleum Company and WWF, which has resulted in the recent publication of the "Tree Plantation Review." {Anon, 1993} However, to our knowledge there are four major conferences planned during 1993 specifically targeting this issue. The research for these conferences should supply sufficient data to provide coherent guidelines for the establishment of environmentally sympathetic and sustainable bioenergy programmes. A synopsis of present environmental knowledge is provided below.
Atmospheric pollution must be decreased for health and global environmental reasons. To be successful, this requires action at the individual house, community, regional and global levels. Woodfuel use (specifically firewood) has been associated with many respiratory diseases and specifically linked to chronic bronchitis and lung/throat cancer. These are associated with: i) the emission of particulates (large airborne particles) and ii) carcinogens released from the wood through combustion.
Products of Incomplete Combustion (PICs) can be adequately dealt with through the use of more efficient stoves and ventilation, and is the subject of numerous reports and aid projects. Such stoves deal with the problem in two ways.
Firstly, increased efficiency requires more complete combustion so reducing the concentrations of particulates and carcinogens. Secondly, the controlled air flow which is required for efficient combustion, often needs an externally vented flue thereby reducing the concentrations of PIC's within confined cooking areas. {Smith, 1991, 1990} These improvements are of particular importance as the effects of indoor pollution may be delayed (and be cumulative) so afflicting mothers and young children.
It is important to note that such health problems are more a result of inefficient cooking conditions and equipment than simply the fuel source; for example, the widespread use of low-grade coal in China has significantly increased the rate of lung cancer. {Smith K.R., 1991} Some fuels, such as kerosene, are inherently lower PIC fuels and are perceived to be advantageous to health. {Ellegård, 1991}. However, improved health and development can result from improvements in the efficiency and the quality of conversion systems e.g. improved cooking stoves and fuel sources, and not just from a change in fuel type.
Biofuels provide an opportunity for the provision of the modern fuels and services required for development, whilst at the same time avoiding fossil-fuel derived CO2 emissions. However, present intensive agricultural methods can only be used for the production of liquid fuels e.g. diesel from rapeseed, if the associated residue production is used for energy. Thus the use of C-flows and energy output/input ratios to monitor net CO2 production must be integral to the study of the Greenhouse effect and the development of strategies for energy provision. (Chapter 2)
Other by-products arising from the production and use of biomass energy are involved in the complex physics and chemistry of the greenhouse effect. These include methane emissions, mainly from rice paddy cultivation and also landfills; however, emissions are directly related to management practices and soil type, and thus have the potential to be reduced substantially. {IPCC, 1992}
Bioenergy systems emit less sulphur dioxide (since they are naturally low in S, table 16) and nitrous oxides (NOx) than equivalent fossil-fuel derived energy. SO2 emissions are implicated in acid rain and increased nutrient depletion from soils. On the other hand, sulphate aerosols, derived from SO2 emissions to the atmosphere, have been shown to play a role as cloud condensation nuclei, and are thus postulated to moderate the global warming effect. Hence, increased use of bioenergy would theoretically reduce the levels of SO2 in the atmosphere thus removing this negative feedback effect. {IPCC, 1992.}
Increases in the levels of all the major greenhouse gases result from land clearing (devegetation), and thus well managed biofuel programmes which lead to revegetation will result in the decrease of greenhouse gases. Land-use changes which result in a permanent increase in the level of the carbon inventory (vegetation) will thus play a role in ameliorating the greenhouse effect (see section 2, and below).
These benefits may include: soil and watershed protection, raising or maintaining biodiversity (see below), CO2-neutral fuel source (or C-sink), low S content, lower NOx's, maintenance of the hydrological cycle and promotion of rural development (including the provision of permanent skilled and semi-skilled employment). It is a decentralised and modular approach, allowing the flexibility to avoid environmentally degrading facets as they are encountered.
Initially, it is envisaged that the use of intensively farmed modern row crops for energy e.g. rape to Rape seed Methyl Ester (RME), will continue to increase. The ease with which biodiesel (such as RME) fits into the present farming and energy transportation infrastructure i.e. it is derived from an annual crop (rape) and can be run in unconverted diesel engines, will result in a rapid increase in its use.
The environmental and productivity advantages of perennial woody biomass crops should result in annual bioenergy crops being regarded only as a transition to sustainable agroenergy production based on perennial crops. Continued R&D and monitoring should result in the confidence to transfer woody perennial crops from the trial plot to the farmers field once a demand for bioenergy has been established. The transition to biomass-derived energy sources will also be facilitated by the rationalisation of the tariff and subsidy structures allowing renewable energy to become economically competitive with fossil fuels, (chapter 6)
Land degradation may occur where an incorrect vegetation type is chosen for a particular bioclimatic (agroecological) zone leading to erosion, leaching, or the mining of water resources. Land-use conflicts, e.g. the replacement of tropical forest with intensive monoculture fanning, or even for use as grazing land as has happened on a large scale in South America, may result in continuing land degradation. Well managed biomass plantations would provide a sustainable alternative land use, and may be able to upgrade the productivity and quality of degraded lands.
In the US, the instigation of the Conservation Reserve Programme (CRP, established by the 1985 Food Security Act), has been credited with reducing soil losses on fragile farm land under this scheme from 8.5 t/ha/yr. to 0.6 t/ha/yr. {CAST 1990} Soil stability models suggest that biomass systems can provide very stable land management systems, with the possible exception of annual row-crop energy production. With soil formation rates of between 2 and 5 t/ha/yr. perennial biomass agriculture would be sustainable in terms of soil retention. {Ranney, 1993}
Careful siting of perennial energy plantations may result in nitrogen filtering benefits, due to the high nitrogen retention ability of these plantations compared to annual crops. The N-filtering capacity could theoretically be used to protect low lying water courses from excessive nitrogen leaching from intensively managed annual crop fields. The nutrient run-off may in some circumstances provide sufficient nitrogen for the biomass crop, whilst buffering the water-course from the detrimental effects of excessive nitrification. {Ranney, 1993}
The production of stillage from sugarcane provides a good example of a noxious effluent resulting from biofuel production. The dumping of raw stillage into convenient watercourses has caused considerable environmental damage, as the stillage is both highly acid and nutrient rich. Its uncontrolled dumping has caused algal blooms increased chemical oxygen demand (COD) and biological oxygen demand (BOD). Novel methods of stillage disposal have allowed its use as an organic fertiliser, allowing reductions in the levels of artificial fertiliser application. In fact, the Triangle project in Zimbabwe has used stillage to substitute for an estimated US$ 1.1 million per annum of K-fertilisers. There have also been un-quantified benefits in reductions of P and N fertilisers which are also highly concentrated in stillage. {Scurlock et al., 1991}
Instead of dumping waste stillage into nearby watercourses, stillage is now diluted and reapplied to the fields in carefully controlled quantities in both Brazil and Zimbabwe., The soils in the fields are monitored to match the soil type and nutrient requirements with the quantities of stillage to be applied. In addition, sugarcane-stillage is providing a supplementary energy source from the production of biogas from secondary digestion in Brazil, Puerto Rico and India.
Increased nutrient recycling may prove essential in sustainably managed plantations. Thus, the ash produced from the combusted biomass should be applied to the plantation to ensure that inorganic nutrients are recycled, further reducing the need for fertiliser inputs. Studies on the recycling of nutrients via the returning of ash to the harvested plantation areas are already under way in Sweden at the Vattenfall (60 MW) biomass-powered Integrated Gasifier Combined Cycle demonstration project for which construction is scheduled to start in 1993 and operation by 1996. {Lindman et al, 1992; Williams and Larson, 1992)} Reduced combustion temperatures may help to retain most of the inorganic nutrients in the ash, and modern conversion technologies may be able to operate at such low temperatures.
Where degraded land is to be rehabilitated or significant growths rates are required, the use of exotic species can be justified. For example, Eucalyptus was planted in the highlands of Ethiopia, on degraded soils which would no longer support any indigenous vegetation. As a result, some areas now have soil of sufficient stability to support indigenous vegetation once more. However, it is our belief that indigenous species should be used in preference to exotics wherever possible. The use of exotics will probably occur in combination with fast-growing exotics since indigenous species may already incorporate significant resistance to local stress conditions, pests and diseases. They may also respond well to management practices normally used with fast-growing exotics. Thus, indigenous species can be used as a risk abatement strategy, protecting the plantation from complete destruction by disease or drought. The use of a wide range of species and clones is now viewed as an essential risk abatement strategy for the same reasons as the use of indigenous varieties.
Bioenergy plantations show improved biodiversity over previous vegetation type if the plantations are grown on degraded or arable land. Replacing indigenous forest or natural habitats with SRWC would undoubtedly lead to a decrease in biodiversity. Unless the carbon sequestration benefits outweigh the loss in biodiversity the use of natural forest systems may be environmentally detrimental. This would almost certainly be the case if slow growing tropical rainforests with high standing stocks are replaced with plantations, (section 2.)
Rehabilitation of degraded land with multi-purpose species (inc. agroforestry) is a long term process but can lead to undoubted improvements in both the quality of the environment and benefit the local people (see Kenyan Baringo project, chapter 2). One interesting aspect revealed through a study of 19 Agroforestry projects in Africa, showed that local farmers were far more interested in fruit trees, or trees which could be used as construction poles rather than trees specifically for fuelwood. {Kerkhof, 1990} Thus, it can be seen that projects which aim to encourage local populations to promote revegetation schemes need to utilise multi-purpose species which provide for the diverse requirements of the local people. The establishment of multi-product systems will also result in increased diversity.
Increases in the variety of species planted would habitat diversity especially for insect and bird populations. Agroforestry can offer a wide range of products on relatively small areas of land. For example, a That farmer who used to rely on intensive agriculture, but became disillusioned with the diminishing returns from hybrid crops, switched to agroforestry techniques. In doing so he reduced the area of land he "cultivated" from 200 rais (about 30 ha) mostly planted with rice to 9 rais (1.35 ha). He now has 413 species of food and medicinal plants which require minimum tillage. This switch resulted in large increases in both biodiversity and security for the farmer and his family in terms of the supply a highly varied and nutritious diet even in drought years. {BUN, 1992} Agroforestry systems offer the potential for highly productive and diverse land uses, even on marginal land. However, these systems are by no means pre-destined to success. The promotion of agroforestry will require considerable governmental backing, including: the provision of extension services, seedlings, site specific research and inter-departmental cooperation. {Erskine, 1991} However, the sustainable production of food and energy may be highly efficient using agroforestry systems, and thus deserves more attention.
Biological control of pests and diseases. A few biological control agents readily fit into modern pest and disease control practices. For example, Bacillus thuriengensis (BT) can be applied by spraying crops and plantations with the bacterial application, or the spraying with a solution of the purified crystalline toxin which has powerful anti-lepidopteran properties. Such spraying has been widely practised over the pine forests of Scotland and elsewhere. Systemic insecticides which selectively kill only those pests which consume the host and not the pests' predators should be developed and made more widely available as part of an integrated pest control strategy. The use insect herbivores to control outbreaks of weeds has also been tried extensively, however, success has generally been limited to the control of accidentally introduced exotic weeds and pests. {Crawley, 1989}
Other forms of this sort of strategy aim to provide agrarian reserves where indigenous predators can exist at naturally low levels, increasing with pest outbreaks. Once the pest outbreak has been contained the levels of predator will naturally die back, but a reserve will be maintained in the reserve area to respond to future outbreaks. Foresters now recognise the efficacy of such approaches, and it is has become common practice for them to leave decaying branches etc within the plantation. Decaying material harbours insects and can sustain a high and diverse level of bird life useful for similar reasons to the agrarian reserves discussed above. {Beyea et al., 1992}
The benefits of longer rotation lengths can accrue not only in terms of carbon storage, but also in raising the levels of biodiversity. Habitat recovery studies on temperate short-rotation plantations show that between 3 and 5 years after planting woodland species of birds and soil microfauna start to occupy the plantation, and that diversity is increased if the plots are connected to native woodlands, due to edge effects. {Ranney, 1993}
Research to date, cited in Ranney {1993}, indicates that bioenergy systems offer a significant improvement over other land uses, especially in terms of reducing soil erosion and increasing biodiversity. However, the interaction between bioenergy systems and natural ecosystems is dynamic and complex. Novel strategies are required which can only be derived from increased research, development, monitoring and the utilisation of local knowledge. Nevertheless, preliminary results suggest that woody plantations can provide a diverse habitat for woodland bird species if management strategies incorporate reserves of natural woodland. Furthermore, such reserves may provide economic returns through decreased need for pesticides and fertilisers. More intensive bioenergy production may provide fewer environmental benefits. Certainly the commercial viability of IPM appears to be demonstrated in the commercial plantations of Brazil.
Ethanol Brazil and Zimbabwe both adopted a sugarcane-alcohol programme in order to produce ethanol for use as a petrol substitute. An immediate benefit stems from the higher octane level of ethanol compared to petrol, thus negating the need to add lead as an anti-knocking agent. High lead levels have been implicated in the lowering of children's IQ levels in inner cities of industrialised countries. However, the use of ethanol as a vehicle fuel has been associated with increased Formaldehyde emissions a suspected carcinogen.
Whilst very little work has been done to enable the provision of accurate estimates over large regions, studies on Loblolly pine and spruce-fir plantations (35 and 65 year rotations) may allow broad generalisations for all plantations.
For both the Loblolly and spruce plantations soil carbon diminished over the first 10 to 20 years after establishment. However, differences occur in the rate of decomposition under different climatic regimes. For temperate regions, vegetation decays more slowly than tropical regions due to lower ambient temperatures. Typically, loblolly pine plantations in the warmer southeast of the US have soil carbon levels between 5 and 10 tC/ha whereas, in the spruce-fir plantations of the colder northeast US soil carbon levels range between 15 and 20 tC/ha. Thus small changes in the rates of decomposition and overall levels of soil carbon over wide areas could be a significant sink or source of atmospheric CO2. {Birdsey, 1992} Levels of soil organic carbon may also be important in aiding plant productivity, due to faster rihzospheric recycling of nutrients, but very little quantitative data exists to confirm this belief. {Ranney, 1992b}
Bioenergy plantations which use coppicing species over short rotations would probably represent a hybrid between clear-cut plantations (with rotation lengths of 30 to 100 years) and annual crops. Short rotation energy coppices are harvested every 5-12 years however, their roots systems are left alone to allow fast regeneration in the next growth season. Decreasing growth rates require new root stock about every 30 years. Thus, the only physical disturbance to the below ground carbon store is through the compaction of the soil by the heavy machinery used during the harvest. Should this prove important, strategies to minimise soil disturbance can be adopted, such as harvesting when the soil is frozen or dry.
