Chapter 2: Environmental links between forestry and food security

Contents - Previous - Next

2.1 Trees and the microclimate
2.2 Windbreaks, soil erosion and food crop yields
2.3 Tree's role in preventing water erosion
2.4 Protection afforded by forests in critical or hazardous areas
2.5 Forests and water supply
2.6 Forests, sediment and water quality
2.7 Forests and the global climate
2.8 Forests, rainmakers?
2.9 Forests and genetic resources

Sustainable food production depends on a favourable and stable environment. At a local, as well as a regional and global level, trees and forests may have a profound influence on the environment. By protecting the soil from erosion, and stabilising hillsides, exposed coastlines and other fragile areas, they can help preserve the integrity of agricultural land. They may also affect climate and water regimes, both of which are crucial to agriculture.

In some cases the environmental benefits that trees provide are clearly visible. The damage caused by erosion is unmistakable, for example, when steep slopes are cleared of forest. Other environmental influences are much harder to measure. Particularly at the regional and global level it is often difficult to isolate the effects of trees from other factors. A number of controversies remain, and not all of the popular beliefs about the benefits of trees can be backed up by scientific evidence. Care is therefore needed when considering the environmental links between forestry and food security. It is important to distinguish effects that can be clearly demonstrated and relied upon from those that are still speculative, and may depend heavily on local conditions.

Environmental links between forestry and food security


2.1 Trees and the microclimate

2.1.1 Temperature and humidity
2.1.2 Shade
2.1.3 Moisture availability

Interactions between trees and food production are most apparent at the micro-level. Trees, for example, when planted within agricultural areas, have been shown to have a variety of effects on the local microclimate influencing temperature and humidity, moisture availability, and light conditions.

2.1.1 Temperature and humidity

Tree cover can have a considerable influence in moderating air and soil temperatures, and increasing relative humidity (Lal and Cummings, 1979). Both these effects are generally beneficial to crop growth, a fact that is made use of in many agroforestry systems (Weber and Hoskins, 1983; Vergara and Briones, 1987).

The extent to which these benefits are realised in practice depends on the number of trees involved. An isolated tree planted on farmland can only be expected to have a minor and localised effect. The more the system resembles a closed forest in its canopy structure and tree spacing, the greater the beneficial effect on humidity and temperature.

2.1.2 Shade

The shade cast from trees can have both negative and positive effects. Shade on crops or pasture reduces their photosynthetic activity and beyond a certain point will affect growth rates. Under prolonged and complete shade most annuals and shade-intolerant perennials will die. However, because trees alter temperature and humidity as well, these factors may more than compensate for the reduction in light.


In some cases therefore, varying amounts of shade may benefit different crops. Some types of coffee, for example, are deliberately grown under partial shade. Grevillia robusta has traditionally been used for this purpose in parts of Latin America. A Spanish name for Gliricidia septum, "madre de cacao" (mother of cacao), indicates its widespread use as shade for cacao plantations. In Sri Lanka, different tree species are used together; some of the best-managed tea estates have a tall shading of either Albizia lebbek or Grevillia robusta, and an intermediate canopy of Gliricidia septum or Erythrina sp.

Shade may also be very desirable in animal husbandry, particularly in hot climates (Daly, 1984). Though shade will tend to reduce forage production under trees, this is compensated by the protection a tree affords to animals and humans against the hot midday sun. Even single trees are prized in desert or semi-arid situations, such as in the Sahelian and Sudanian zones in Africa, where "every tree is an oasis" (Gorse, 1985).

In this connection, the African species Acacia albida has the unusual feature of being leafless in the rainy season so that there is no shading on crops cultivated beneath it, but full-canopied during the hot, dry season, providing important shade for livestock (Weber and Hoskins, 1983). Manure accumulates where the animals rest, and results in increased fertility not only for the tree, but for crops planted near it (Bonkoungou, 1985).

The overall benefits of shade are not always clear cut. In large intensively-managed monoculture systems shade may prove to be a disadvantage, whereas for less intensive systems, on smallholder properties and on less productive soils shade may provide many advantages (Beer, 1987). Site specific factors are crucial. The benefits of shade depend on the local climate and soils, as well as the particular species involved. For an individual farmer, management requirements of the trees themselves and the marketability of tree products are also important factors.

The trade-offs a farmer has to make in choosing the optimum shade cover are shown clearly in a study in North-east Thailand, where trees are a common feature in most rice paddy fields. The shade they provide was found to be the primary reason for retaining trees on farmland. During the hot, dry season, livestock spend much of their time resting in their shade and grazing in or near shade. Farmers were well aware of the negative effects of too much shade on rice (faster and taller growth - which makes it susceptible to lodging combined with fewer tillers, less grain and less filled grain). But they felt that the benefits outweighed these costs and controlled shade in many cases by lopping the trees. The species Phyllanthus polythyllus was particularly valued because its sparse foliage does not create too much shade. Its roots help stabilise the crumbly bunds and its branches can be used for bean poles, fences, fuelwood and charcoal (Grandstaff et al 1986).

2.1.3 Moisture availability

Trees influence the availability of soil moisture in their immediate vicinity. The interception of precipitation by tree foliage influences the amount of moisture reaching the soil. Under a densely crowned tree little or no precipitation may reach the ground in a short, light shower. Only when the tree canopy is fully wetted will most of the precipitation reach the ground. In addition, a tree influences the distribution of moisture reaching the ground. It may come as throughfall (raindrops falling between the leaves), leaf drip or stemflow. The exact pattern depends on the shape of the tree. Understorey plants may find places where stemflow or crowndrip concentrate moisture creating an especially hospitable micro-environment.

Some water is lost through evaporation from the tree canopy. In humid areas, evaporation losses from tree canopies can account for 10 to 30 percent of gross annual rainfall (Vis, 1986). Although a certain amount of evaporation will occur from any surface where water is temporarily held, losses from tree foliage are usually greater than from soil litter or close-to-ground plants, primarily a result of canopy roughness and height (Hamilton and Pearce, 1986).

Uptake of water by tree roots can also have a significant effect on local moisture availability. The impact on crop yields, however, will depend on the extent to which water stress limits crop growth. The drier the environment, the more this is likely to be a problem. The impact will also vary between different species; trees with horizontal surface roots will compete with crops to a much greater extent than deep-rooted species.


2.2 Windbreaks, soil erosion and food crop yields

2.2.1 Trees: barriers against erosion
2.2.2 Other benefits of windbreaks
2.2.3 Effects of windbreaks on crop yields

One of the most widely-recognised benefits of trees on their immediate environment is their ability to reduce wind speeds. Farmers in many parts of the world use windbreaks - or more elaborate multi-species shelterbelts - to protect crops, water sources, soils and settlements. In addition, windbreaks are essential first steps for sand dune stabilization.

There are numerous examples that can be quoted. Tall rows of Casuarina line thousands of canals and irrigated fields in Egypt. In Chad and Niger multi-species shelterbelts protect wide expanses of crop land from desertification. In China, there has been a massive programme in recent years to establish 'forest nets' throughout the exposed central plains region. These consist of a grid of windbreaks, each 'net' enclosing between 4 and 26 hectares of farmland, depending on the severity of the wind problem. Paulownia sp. has been the main species used, because of its deep roots and relatively light shading.

2.2.1 Trees: barriers against erosion

Reducing windspeeds helps substantially in preventing wind erosion, and the damage it causes (Chepil, 1945). This includes both damage due to the loss of nutrient-rich topsoil, and damage, as a result of physical injury to crops and livestock, or the partial burial of fields. Soils are most susceptible to wind erosion when they are dry and tare. Thus, overgrazing or any cropping activity that removes plant cover makes soils more vulnerable to wind erosion. The hazard increases with the length of time the soil surface is bare and with the degree of soil dryness.

A well-developed windbreak or shelterbelt can have a considerable influence in reducing wind velocity at the soil surface. Where the barrier is at right angles to the wind direction, this effect has been found to extend up to 5-10 times the height of the barrier to the windward side, and 30 to 35 times the height to the leeward, or down-wind side. Small reductions in wind speed can have a significant impact on soil erosion, in part because the drying rate of the soils is reduced after rain storms.

A windbreak using a mix of species provides an efficient semi-permeable barrier to wind over its full height. This produces a diverse shape to the windbreak as well as ensuring a long life of the windbreak (by mixing species with varied growth rates). In addition, a mixture of species provides risk protection against unexpected attack from diseases or insects that could destroy single species stands. Trees scattered throughout the fields such as the Acacia albida parkland savanna of West Africa can have the effect of breaking up wind patterns resulting in impacts similar to more formal windbreaks and shelterbelts.

2.2.2 Other benefits of windbreaks

As well as reducing wind erosion, windbreaks and shelterbelts can benefit agriculture in a variety of other ways:

* windbreaks and shelterbelts help prevent mechanical damage caused by high winds (Guyot, 1986). Winds in excess of 8 metres per second, for example, can break off twigs and small limbs in orchard crops. Such losses of photosynthetic surface reduce production, and can adversely affect flowering and fruiting the following year. Flowers on crops are particularly susceptible to high winds, and fruits may also be damaged or dislodged. With cereal crops, stem breakage or flattening (lodging) is an increasing hazard as the crop matures;

* shelter provided by windbreaks helps reduce the rate of water loss from crops through evapotranspiration; this can extend to as much as 30 times the height of the tree barrier (Konstantinov and Struzer, 1965);

* reductions in wind velocity can prevent adverse physiological changes in crops - such as the reductions in leaf area and photosynthetic rate that are characteristic of some crops when exposed to high winds (Whitehead, 1965);

* trees and shelterbelts protect livestock, particularly young animals, against the damaging effects of both cold and hot winds;

* windbreaks provide an essential element for dune stabilization;

* trees planted along coastlines can protect crops from salt spray and thus allow farming to be extended closer to the shore. The trees selected for such "salt-breaks" must have some degree of salt tolerance, for they will concentrate salt under their crowns. Species that have been used successfully include Casuarina equisetifolia, Casuarina glauca, Pinus pinaster, Pinus radiata and Cupressus macrocarpa;

* windbreaks can reduce evaporative losses from ponds, irrigation canals and other water bodies, thus making more water available to food production;

* by reducing wind velocities windbreaks can help improve insect pollination of crops. This is particularly important in fruit orchards (Caborn 1965). Beekeepers also find wind protection for their hives to be desirable in areas with strong, cold or hot winds;

* windbreaks may benefit crop yields by reducing the incidence and severity of pest damage. Studies of the Colorado beetle, for instance, showed large reductions in populations of eggs and larvae near to windbreaks, and higher predator densities closer to the trees (Karg, 1976). The effects are not uniform, however, since windbreaks can harbour harmful pest species as well as pest predators (Janzen, 1976). Trees have been traditionally thought to encourage tse-tse flies, though this view is not universally accepted. Kenyan and Tanzanian experience suggests that windbreaks need not shelter tse-tse flies if the under-storey is relatively open, the over-storey is high, and the ground surface is kept weeded;

* windbreaks can help prevent the spread of plant diseases by inhibiting the aerial dispersal of disease spores. This effect may be offset, however, by the more rapid development of disease spores near to windbreaks resulting from higher relative humidity (Guyot, 1986).

Colorado beetle

As well as reducing windspeeds, windbreaks and shelterbelts provide a range of direct benefits from the fodder, fruit, wood and other products they supply. Even in the harsh desert environment of Yemen, a two row windbreak of Conocarpus lancifolius yielded 350 m3 of wood per kilometre every 20 years, which was more than enough to offset the establishment costs, without considering the additional agricultural benefits (Costen, 1976). In the Majjia Valley in Niger, pollarding of windbreaks every four years is estimated to bring local residents US$ 800 worth of construction poles and wood per kilometre of windbreak (USAID, 1987). Several books and manuals on the subject of windbreak design are available (see Guyot, 1986; Bhimaya, 1976; Weber, 1986).

2.2.3 Effects of windbreaks on crop yields

The effects of windbreaks on crop yields is illustrated in Figure 2.1. Close to the windbreak, yields are reduced because of shading, root competition, and the physical space taken up by the trees. Moving further away the benefits become more apparent, until at a certain distance they start to drop off again as the influence of the trees diminishes.

Some of the most dramatic yield increases have been reported in China, where the hot, dry summer winds represent a major limitation on agricultural production. In Hetian Prefecture, where 110,000 hectares of land were planted with Paulownia sp. windbreaks in the early 1980s using the 'forest net' system, increases in grain yield of 60% have been recorded, along with a 70% increase in natural silk production, and a 300% increase in cotton output (Wang Shiji, 1988).

Significant increases have also been reported in a number countries with Mediterranean type climates. According to one survey covering Argentina, Bulgaria, California, Egypt, Israel, Italy, Saudi Arabia and Tunisia, well-designed windbreaks have given a net increase in crop yields of between 80 and 200 percent (Jensen, 1984). Similar increases have been reported in studies in the Antilles on vegetable yields (Guyot, 1986).

In the Sahel, while statistically valid results are not yet available, initial trials with millet and sorghum suggest that in fields protected by windbreaks yields can be as much as 23 percent higher than in unprotected fields (Bognetteau-Verlinden, 1980). In a year with poor rainfall, even relatively small differences in crop yields can be of major significance to local people.

However, the overall effects of windbreaks on crop yields varies considerably. In some cases yields are increased significantly; in others the competition for water and light and the loss of planting area have been found to be detrimental to crop yields. As a general rule, where land is exposed to high winds for most of the year, or where soil erosion is a particular problem, the case for windbreaks will usually be strong. Where these conditions do not prevail, the advantages of windbreaks may be less clear. As well as the direct costs for labour and planting material, windbreaks will take some land out of crop production, and will compete for water, light and nutrients. Therefore windbreak products such as fodder, fuel, and foods, increased crops yields, and soil improvements must be sufficient to cover these costs. In many cases, for the farmer the negative effect on crop yields may be more than made up for by the wood and other products provided by the windbreak itself and having this dual production system may reduce risks should one of the systems fail.

Figure 2.1 Effect on Output of a Field Receiving Protection by a Windbreak


2.3 Tree's role in preventing water erosion

Soil erosion caused by water is a serious problem for agricultural production in many regions of the tropics and subtropics. It strips the most fertile top layers of soil, and can destroy crops themselves by flooding. Forests (and trees) can provide protection against some types of water-induced soil erosion. Surface erosion caused by water in undisturbed forests is generally less than that under other types of land use (Hamilton, 1983). Removing forest, and leaving the soil exposed, can thus have a radical effect in increasing the rate of soil erosion.

Contrary to what is often assumed, it is not the tall tree canopy that gives the most protection to the soil, but the ground cover and litter layer beneath it (Hamilton, 1986). If the ground below is bare, large water droplets falling from a tall tree canopy may actually cause splash erosion and initiate more sheetwash than rain falling on bare soil in the open (Lembaga Ekologi, 1980). Often, therefore, it is not the cutting of trees that leads to surface erosion, but the disturbance to the understorey and leaf litter, and baring of the soil, associated with tree cutting.

TABLE 2.1 Erosion under various tropical moist forest and tree crop systems (tons/ha/year)

  Minimal Median Maximal
Multistoried tree gardens 0.01 0.06 0.14
Natural forests 0.03 0.30 6.16
Shifting cultivation, during fallow period 0.05 0.15 7.40
Forest plantations, undisturbed 0.02 0.58 6.20
Tree crops with cover crop/mulch 0.10 0.75 5.60
Shifting cultivation, during cropping period 0.40 2.78 70.05
Taungya cultivation 0.63 5.23 17.37
Tree crops, clean-weeded 1.20 47.60 182.90
Forest plantations, burned and litter removed 5.92 53.40 104.80

Source: Wiersum 1984

The importance of ground cover in protecting against surface erosion has been demonstrated in studies of different forest and tree crop systems, the results of which are summarised in Table 2.1. Forest and tree crop plantations in which the ground cover had been removed were found to be far more susceptible to erosion than those in which it was retained. Similarly, taungya systems (systems where food crops are grown between young plantation trees) were more prone to erosion when the ground between trees was weeded than when a cover crop or mulch layer was maintained.

As slopes increase, in both steepness and length, the dangers of erosion become greater. A variety of soil conservation techniques can be used to reduce erosion. When combined with physical measures, such as terracing, planting of trees and shrubs can help considerably in binding the soil and preventing water erosion.

Such techniques are used in many traditional agroforestry systems (Vergara and Briones, 1987, Nair, 1984a). For example, in Amazonian Ecuador contour strips of Inga edulis (a leguminous fuelwood tree) are used in a cassava swidden system (Bishop, 1983). After the cassava has been harvested, a perennial legume ground cover of Desmodium is planted and grazed by sheep. This represents a combined tree/ground cover/livestock system that, if properly managed, maintains good soil stability and rapidly improves the soil during the fallow period.

It is important to recognise, however, that planting trees does not guarantee effective soil erosion control. The design and management of such systems are crucial. Simply putting trees into a cropping or grazing system, and even complete reforestation, will not eliminate surface erosion.

Forestry activities such as plantations can also increase the possibilities of water-induced soil erosion. For example, serious erosion problems have been reported under teak plantations in Trinidad. This was due to the lack of understorey vegetation and surface litter (Bell, 1973). For the same reason, introducing trees as part of agroforestry systems will not cure the erosion problem if the soil between the trees is bare for most of the year (Hamilton, 1986).

Nevertheless, it is widely recognised that combining trees with other soil conservation techniques can greatly extend the possibilities for sustainable crop cultivation on sloping land. At some stage, however, even the best soil conservation techniques come up against economic or physical barriers which make them impractical. On these sites, there is a very strong case for maintaining, or restoring, undisturbed forest cover.


2.4 Protection afforded by forests in critical or hazardous areas

2.4.1 Unstable slopes
2.4.2 Coastal protection
2.4.3 Riparian forests
2.4.4 Areas prone to salinisation
2.4.5 Dune stabilisation

In environmentally sensitive areas, forests can play an important indirect role in enhancing food security by protecting cropland and grazing areas from natural hazards such as landslides and coastal erosion. In such hazardous areas the removal of forests can seriously jeopardise agricultural production.

2.4.1 Unstable slopes

The effect of landslides on downslope agricultural production and human settlements can be disastrous. In addition to the immediate physical destruction, the dumping of large amounts of sediment into streams and rivers also affects the quality of water and survival of fisheries downstream. The loss of food supplies combined with increased incidence of diseases (a result of poor water quality) can have a major negative impact on household food security.

Unstable slopes

Deep-seated slides need to be considered separately from shallow ones. Deep-seated slides are induced largely as a result of the nature of geological material and are little influenced by the presence or absence of trees (Megahan and King, 1985). They can occur on relatively gentle slopes as well as on steep slopes. These sites are risky even for timber production, since logging can have a triggering and destabilising effect. Areas prone to this type of erosion should be left undisturbed, or harvested manually.

Shallow slides or slips are greatly influenced by vegetation. Tree roots can increase substantially the stability of slip-prone slopes. Studies in New Zealand found that tree roots provide up to 80 percent of the soil shear strength under saturated soil conditions (O'Loughlin and Watson, 1981). In this respect, trees are much more effective than either crops or grasses. Removing them can increase the frequency of slides by as much as seven times (Swanson et al, 1981). In these types of areas forest management (as well as some agroforestry techniques) can help protect against slides. Where trees are to be harvested for wood, coppicing species are most appropriate as their root systems remain alive and maintain shear strength.

2.4.2 Coastal protection

Trees have an important function in some coastal areas in protecting coastlines against wave damage during storms. They can also help dampen the effects of extreme tidal surges, thereby protecting inland areas from inundation and physical damage. Thus trees may help sustain agricultural production in coastal regions.

Mangrove forests have a particularly significant role in this context, providing agricultural lands and human settlements with protection on exposed coastlines (Hamilton and Snedaker, 1984). While mangrove forests cannot prevent tidal waves and other natural disasters, they can help in mitigating their effects. In the case of the Sunderbans forest in Bangladesh, for example, where tidal surges have resulted in major loss of life and property damage, the effects would undoubtedly be worse if the mangroves were to be removed. In addition mangrove regions provide a protective habitat for many species of coastal fish and crustacea, thus protecting and sustaining an important food source for coastal communities.

2.4.3 Riparian forests

Forests bordering lakes and streams, known technically as riparian forests, are also valuable for maintaining environmental stability. They act as vegetative buffers that help prevent sediment reaching rivers and streams, as well as providing an important habitat for wildlife. By trapping agricultural chemicals and pesticides from overland water flow a forest buffer zone can also contribute to downstream water quality. Riparian buffer zones are also important for the maintenance of some fish species; they help maintain stable water temperatures and prevent sedimentation both of which are important for maintaining fish populations.

Trees, in addition, can help stabilise river banks and prevent erosion damage and flooding during storms. In streamside areas that are experiencing problems, establishing a forest strip can be an important supplement to structural measures.

The effects of trees along streamsides is not always positive, however. Trees can use large volumes of water, and in arid or semi-arid areas this may reduce downstream water yields, particularly during the dry season (Hough, 1986).

2.4.4 Areas prone to salinisation

Soil salinisation and related phenomena are among the most serious problems threatening land productivity in arid and semiarid regions, especially in irrigated areas. In some instances trees can help mitigate against increasing soil salinisation. In addition, in some cases forest removal will result in increased salinisation. Trees often absorb more water than agricultural crops, forest clearance can thus lead to a rise in the watertable level (Hamilton, 1983). Where groundwater levels come to within one metre of the soil surface, capillary action can draw water to the surface where salts may then be concentrated by surface evaporation (Hughes, 1984). Seepage of water may also result in downslope salinisation, and if salts enter water courses this can impair fish life and make the water unfit for irrigation uses.

Identifying problem areas prior to forest clearance is essential if salinisation is to be avoided. Where salinisation has already taken place, the introduction of trees can often play a useful role in the rehabilitation of the land for agriculture.

2.4.5 Dune stabilisation

Moving sand dunes pose a major threat to agriculture in many countries. Combined with other measures, including various mechanical fixation approaches, trees play an important part in stabilising dunes and preventing the damage they cause (Weber, 1986; FAO, 1985). Maintaining as complete a vegetative cover as possible, and reducing wind velocities using windbreaks are often the best ways of preventing soil movement.

Once dune erosion has begun, however, the first step is to determine why natural vegetation is not re-colonising the area (Weber, 1986). If animals or fire are causing the problem, then tree planting or revegetation alone will not suffice. Fencing or firebreaks may be a prior requirement, or may in themselves be sufficient to allow natural regeneration.



2.5 Forests and water supply

2.5.1 The effects of forest cover on stream flow and groundwater levels
2.5.2 Forests and stormflows
2.5.3 Low flows

The impact of forests on groundwater supply and stream flow is an extremely important issue, especially with regards to food production and food security; but this subject is surrounded by a great deal of myth and misunderstanding. Hydrological systems are complex. While forests can play a variety of useful roles, the assumption that forests are always good for water supplies is a serious over-simplification. Much depends on soil depth, land-use practices, and a range of other factors.

2.5.1 The effects of forest cover on stream flow and groundwater levels

There is a common notion that more water is released into streams from forests than other kinds of land area, and that the removal of forests results in less water being available downstream. It is also widely held that forest removal lowers the watertable and thereby adversely- affects water availability from wells and springs.

While true in some instances, these assumptions are not universally valid. It is often very difficult to predict the exact impact of deforestation, or reforestation, on a particular watershed without concrete evidence from similar circumstances.

Forests use more soil water in converting sunlight to biomass than most other forms of vegetation. As a result, when forests are partially or completely removed, water consumption will drop and the total annual water yield in streams from the area can be expected to increase (Bruijnzeel, 1986; Hamilton, 1983). Increases in stream flow are greatest in the period immediately following forest removal (Bosch and Hewlett, 1982). Water levels are reduced if forest regrowth is vigorous and in some cases water "consumption" can even exceed that of the original forest (Langford, 1976).

The establishment of a forest plantation will tend to reduce streamflow. The faster the rate of growth of the trees, the more pronounced this effect will be. One study in India reported a decrease in water yield of 28 percent following establishment of Eucalypt plantations (Mathur et al, 1976). Although they have become the centre of current controversy about the undesirable effects of tree plantations on water supplies, Eucalypts are not unique in their demand for water. Any tree that is well adapted to the particular site and produces a large amount of biomass-whether it is Eucalyptus, Pinus, Leucaena, or any other species - will also consume large amounts of water.

While these are the general trends, it is important to recognise that variations and exceptions do occur. Streamflow from forests depends on the depth of the soil as this influences the water uptake by trees. On the one hand, where soils are deep, trees with deep roots can extract water that is unavailable to other plants. Uptake and transpiration therefore tend to be considerably greater than for other types of vegetation. On the other hand, on shallow soils, uptake by trees may be comparable to that of vigorous grasses and increases in total annual streamflow will therefore not be great following forest removal.

The effect of tree cover on groundwater levels is similar to that on streamflow. Where on-site groundwater levels have been measured, in the majority of cases the level has been found to rise following forest removal, and fall if an open area is planted to trees (Boughton, 1970; Holmes and Wronski, 1982).

When considering the link between deforestation and groundwater levels, it is important to distinguish the effect of tree cutting from what happens to the land after it has been cleared. If logging or agricultural practices are poor, the resulting compaction of the soil surface may have a significant effect in reducing water infiltration rates resulting in an overall decrease in groundwater levels - and hence lower levels in wells and less reliable springs. Although deforestation is usually blamed in such cases for the reduced water availability, this is often an oversimplification. In reality it is the way the land is treated after tree cutting that causes the problems, not tree cutting itself.

On land that is already badly compacted, tree planting may help to break up the soil structure and consequently increase infiltration rates. Although it has not been confirmed experimentally, this improved water recharge may in some cases be sufficient to compensate for the increased evapotranspiration from the trees. Again, therefore, the overall effect may be contrary to the general trend; tree planting may lead to a rise in groundwater levels.

This brief review of information on the relationship between forests and water supply highlights the complexities involved in managing forests for water supply. There can be little doubt, however, that the resulting impact - whether increasing or decreasing water supplies can have important repercussions for food production.

2.5.2 Forests and stormflows

From the point of view of agriculture, the variation of flow in rivers and streams is often as or more important as the total amount available for the entire year. Excessive stormflows, and the floods they cause, can have a disastrous impact on downstream agriculture. Fisheries may also be affected because floods usually carry large amounts of sediment, which disrupt the habitat and life cycle of aquatic species. Increased variability in stormflows, therefore, can increase the risk involved in food production.

When forest land is converted to agriculture, the effect on stormflows depends on the type of agricultural practices introduced. Any cropping or grazing methods that cause soil compaction will tend to reduce infiltration rates and result in more water entering streams than would be the case under forests, thus increasing the likelihood of floods. Some soil and land management practices, in contrast, are less likely to create problems. The centuries-old rice terraces found on steep slopes in parts of Java, Bali, Cebu, Nepal and elsewhere testify to the effectiveness of traditional water management techniques. Conversion to agriculture, therefore, does not necessarily increase the susceptibility to flooding.

It is often claimed that forests prevent floods and that the removal of forests helps cause floods. While there is some truth in this, the evidence suggests that the impact of forests is mainly localised and restricted primarily to the more frequent, short-duration storms, rather than major storm events. While studies do demonstrate that deforestation usually results in greater stormflow volumes and higher peakflows in streams in the immediate vicinity, these effects are negligible over large river basins (Reinhart et al, 1963; Douglass, 1983). There is no simple cause-effect relationship between forest cutting in the headwaters and floods in the lower basin (Hewlett, 1982 ).

The value of forests in reducing flooding is likely to be greatest on deeper soils. By breaking up the soil and improving infiltration, trees help increase water storage capacity. At a certain point, however, any soil will become saturated. Beyond that, trees cannot prevent rainfall from running off into streams.

When unusually heavy storms occur, floods are liable to happen whatever vegetation is present. Catastrophic floods on large rivers are not caused by deforestation but by too much rain falling in a given period, or by the rapid melting of snow. Even large-scale reforestation of upland areas is unlikely to have a significant impact on the frequency of these kinds of flood events. They will happen whether trees are present or not.

Forests and stormflows

2.5.3 Low flows

Water shortages caused by reduced dry season flow is also a major threat in many agricultural areas. It has been suggested that forests and forest soils may be able to play a beneficial role by acting as a 'sponge', soaking up water in the rainy season and then releasing it in the dry season. Tree clearing is supposed to eliminate this sponge effect and decrease dry season flow (Spears, 1982).

In practice, there is little scientific evidence to substantiate this. Most of the experiments that have been carried out have indicated that cutting trees increases dry season flows in streams from the treated area, and that planting of trees decreases flows (Hamilton, 1983). For example, in Northern Queensland, Australia, a stream draining the area that used to dry up periodically prior to the rainy season, remained perennial following logging (Gilmour, 1971). In Fiji, the planting of Pinus radiate in a dry grassland zone resulted in a 65 percent reduction in dry season streamflow (Kammer and Raj, 1979).

Making generalizations based on isolated examples is clearly risky, as a number of factors are at work. Although trees may help increase infiltration during the rainy season, during the dry season they draw water from lower depths than might otherwise contribute to low season river flow or a higher watertable. Which of these effects is most significant will depend on the local site conditions.

Contents - Previous - Next