Sustainable food production requires a favorable and stable environment. At the local level, as well as at the regional and global levels, trees and the forest can have a profound influence on the environment. By protecting soils from erosion, stabilizing slopes, exposed shores and other fragile areas, they can help preserve the integrity of agricultural land. They can also exert an influence on the climatic and hydrological regimes, both of which are fundamental for agriculture.
In some cases, the ecological benefits of trees are immediately apparent. The ravages of erosion are characteristic, for example, when a steep slope has been deforested. But other environmental factors are much more difficult to measure. In particular, at the regional and global levels, it is often difficult to distinguish tree-related effects from other factors. A number of controversies continue to be fed, and not all fashionable convictions about the benefits of trees are supported by scientific evidence. We must therefore be careful when examining the environmental relationships between forestry and food security. It is important to distinguish the undoubted effects with which we must reckon from those that remain in the field of speculation, and may depend closely on local conditions.
The interactions between trees and food production are particularly visible at the micro-environment scale. It has been shown that by exerting an influence on the temperature, the humidity, the availability of water in the soil and the illumination, the trees planted in agricultural areas have various effects on the microclimate of the place.
Tree cover can have a considerable influence by moderating air and soil temperatures, and by increasing relative humidity (Lal and Cummings, 1979). These two effects are generally beneficial to crop growth, and are taken advantage of in many agroforestry systems (Weber and Hoskins, 1983; Vergara and Briones, 1987).
The materialization of these theoretical advantages depends on the density of the canopy. An isolated tree that stands on agricultural land can only have a minor and localized effect. The more the system is similar, due to the structure of the vault and the spacing of the trees, to the dense forest, the more sensitive the effects on humidity and temperature are.
The shadow that the trees cast has positive effects and negative effects. The shadow cast on crops or grasslands reduces the activity of photosynthesis, and beyond a certain density, slows down growth. Subjected to prolonged or permanent shading, most shade-intolerant annuals and perennials die. But the presence of trees also changes the temperature and humidity, and these two factors can largely compensate for the loss of illumination.
So in some cases, the amount of shade will favor some crops over others. Some types of coffee trees, for example, are deliberately grown in a semi-shaded situation. Grevillia robusta is used in particular as a shading essence in certain areas of Latin America. One of the names of Gliricidia sepium in Spanish is “madre de cacao” (mother of the cocoa tree), which denotes its widespread use to shade plantations. In Sri Lanka, different essences are used in combination; some tea plantations, often the best managed, are shaded by Albizia lebbek or Grevillia robusta, and by an intermediate foliage of Gliricidia sepium or Erythrina sp.
Shade can also be very favorable for livestock, especially in hot climates (Daly, 1984). Even if the grasslands are less abundant under the trees, the loss is compensated by the protection that each tree offers to animals and people against the heat of the sun in the middle of the day. Even isolated trees are of great value in arid or semi-arid regions, such as in the Sahelian and Sudanian zones in Africa, where “every tree is an oasis” (Gorse, 1985).
By the way, the African species Acacia albida has the uncommon characteristic of not bearing leaves during the rainy season, and therefore of not providing shade on the crops grown under its cover, while in the hot and dry season its dense foliage offers a very useful shading to livestock (Weber and Hoskins, 1983). Manure accumulates where the animals remain at rest, which benefits both the tree and the crops grown nearby (Bonkoungou, 1985).
The net balance of the benefits of shading is not always easy to establish. In large intensive monoculture systems, shade can ultimately be disadvantageous, while in less intensive systems, on small farms and on modestly productive soils, it will have many advantages (Beer, 1987). Site-specific factors are of primary importance. The advantages of shade are linked to the climate and the soil of the place envisaged, as well as to the plant species considered. For the grower, individually, the specific requirements of the trees and the possibility of marketing their product are also important factors.
The comparative advantages of trees, which the farmer must evaluate when determining the optimum shade density on his crops, are well highlighted in a study carried out in the north-east of Thailand, where trees are commonly present in most rice fields. The shading they provide is the main reason for their conservation on agricultural land. In the hot and dry season, the cattle rest for a long time in the shade, and graze nearby. Farmers are well aware of the disadvantages of too dense shading for rice (faster growth at a higher height, which has more room for pouring, less tillering, fewer grains and less full grains), but they judge that the advantages outweigh the disadvantages, and they master shading by periodically pruning the trees. Phyllantus potythyllus is particularly appreciated, because its relatively sparse foliage gives a light shade. Its roots help to stabilize fragile diguettes, and its branches serve as a guardian for beans, are used to build fences, give firewood and are used for charring (Grandstaff et al., 1986).
The trees act on the soil moisture in their immediate vicinity. The interception of precipitation by the foliage has an influence on the amount of moisture that reaches the soil. Under a tree with dense foliage, the amount of water that reaches the ground during a light and short downpour can be zero or very low. Only when all the leaves are saturated with water, most of the rain reaches the ground. In addition, any tree changes the distribution of moisture reaching the soil. Water can fall through the canopy (between the leaves), drain from the leaves, or trickle down the branches and stem. The flow then depends on the shape of the tree. The plants of the lower floor can find a particularly hospitable micro-environment in places where the runoff coming from the trunk or foliage is most abundant.
The evaporation that occurs on the treetops leads to a loss of water for the soil. In humid regions, this evaporation eliminates from 10 to 30 percent of the gross annual precipitation (Vis, 1986). Even if some evaporation occurs from any surface where the water temporarily stays, the losses caused by the foliage of the trees are generally greater than those that occur in a leaf litter, or on a low vegetation cover, especially due to the irregularity and the height of the crowns of the trees (Hamilton and Pierce, 1986).
The removal of water by the roots of trees can also have a significant effect on the availability of water. The effect on crop yield, however, will depend on the extent to which a possible water deficit will limit the development of plants. The drier the environment, the more likely the sampling will be a problem. This effect is also variable according to the species; trees whose root system develops horizontally near the surface of the soil compete with crops much more than those whose roots go deep.
One of the best demonstrated effects of trees on their immediate environment is that they reduce wind speed. In many parts of the world, farmers plant windbreaks, or complex shelter curtains composed of several species, to protect crops, waterways and canals, soils and human settlements. In addition, shelter curtains are the first weapon to be used to stabilize the sand dunes.
There are many examples that could be cited. High rows of Casuarina trees border thousands of canals and irrigated fields in Egypt. In Chad and Niger, composite shelter curtains protect large agricultural areas from desertification. In China, a massive program has been underway for a few years to establish a “forest net” over the entire area of the threatened region that constitutes the central plains. This net is a mesh network of curtain shelters, which each surround from 4 to 26 hectares of agricultural land, depending on the severity of the effects of the wind. The main species planted is Paulownia sp., due to the depth of its roots and its relatively light shading.
Reducing the wind speed makes it possible to prevent wind erosion and the damage it causes to a large extent (Chepil, 1945). The damage in question ranges from the loss of the surface earth, rich in nutrients, to the physical damage suffered by plants and livestock, and the burial of cultivated fields. It is when they are dry and bare that the soils are most susceptible to wind erosion. Overgrazing and any agricultural activity that removes the protective vegetation cover expose the soils to the effects of the wind. The risks increase with the length of time the soils remain bare and with the degree of dryness of the earth.
A well-designed and realized shelter curtain can have a considerable effect on the wind speed at ground level. When the barrier thus constituted is exactly perpendicular to the direction of the wind, this effect is felt over a distance reaching 5 to 10 times the height of the curtain on the windward side, and 30 to 35 times this height on the leeward side. Even small reductions in wind speed can have significant effects on erosion, partly because the soils dry less quickly after downpours.
The composite shelter curtains constitute semi-permeable barriers to the wind over their entire height. The curtain then takes on a particular shape, and its service life is extended by mixing more or less fast-growing species. Mixing the essences also makes it possible to guard against unexpected attacks of diseases or insects that could completely destroy the pure stands. Trees in sparse formations, such as Acacia albida stands in the wooded savannah of West Africa, can have the effect of disrupting the flow of air streams, which partly joins the effect of shelter curtains planted by man.
In addition to reducing wind erosion, shelter curtains promote agriculture in various ways:
· they contribute to preventive action against damage caused by strong winds (Guyot, 1986). Winds with a speed of more than 8 m/s, for example, are capable of breaking twigs and small branches of fruit trees. This loss of surface suitable for photosynthesis reduces production, and can adversely affect flowering and fruiting the following year. The plants are particularly sensitive to strong winds at the time of flowering as well as when they bear fruit, which can be damaged or torn off. In the case of cereals, the risk of breakage of the stem and fall of the aerial apparatus (pours) increases as the plant approaches maturity;
· the protection provided by shelter curtains contributes to reducing the rate of water loss of crops by evapotranspiration; this protection is sensitive over a width of up to 30 times the height of the tree curtain (Konstantinov and Struzer, 1965);
· the reduction of the wind speed makes it possible to escape the undesirable physiological modification of cultivated plants, for example the reduction of the leaf area, therefore of the rate of photosynthesis, which characterizes certain species when they are exposed to strong winds (Whitehead, 1965);
* trees and shelter curtains provide protection for livestock, especially young animals, from hot or cold winds;
* shelter curtains are an essential element of dune stabilization;
· trees planted by the sea can protect crops from salt spray, and thus make it possible to extend the cultivated area on the land closest to the shore. The trees chosen to constitute these “salt barriers” must have at least a certain tolerance to the saline environment, because they will concentrate the salt in the soil under their crown. Mention may be made, among the essences successfully used, of Casuarina equisetifolia, Casuarina glauca, Pinus pinaster, Pinus radiata, and Cupressus macrocarpa;
* shelter curtains reduce evaporation losses from ponds, irrigation canals and other bodies of water, thus leaving more water for food production;
· by reducing the wind speed, shelter curtains promote the pollination of crops by insects. This effect is particularly important in fruit orchards (Caborn, 1965). Beekeepers also consider it desirable to protect their hives in areas of strong winds, whether cold or hot;
· shelter curtains can improve crop yields by reducing the incidence and severity of pest damage. Studies on the Colorado potato beetle, for example, show a sharp reduction in egg and larval populations near tree curtains, and a higher density of predators near trees (Karg, 1976). However, this effect is not uniform, because, in addition to useful predators, trees can harbor pests (Janzen, 1976). It is usually believed that trees favor the presence of the tsetse fly, but this point of view is not universally shared. The experience of Kenya and Tanzania suggests that shelter curtains will not necessarily attract the tsetse fly if the lower floor is sufficiently ventilated, and the upper floor is high enough, the soil must be free of weeds;
· shelter curtains can help prevent the spread of plant diseases by inhibiting the aerial dispersion of vector spores. This effect can, if necessary, be cancelled out by the faster development of the spores near the curtains, due to the higher relative humidity that prevails there (Guyot, 1986).
In addition to reducing the speed of the winds, tree curtains have a whole range of direct advantages, as they can give fodder, fruits, wood, and other products. Even in the harsh desert environment of Yemen, a two-row curtain of Conocarpus lancifolius yields 350 m 3 of wood per kilometer every 20 years, which largely compensates for the costs of establishing the curtain, not to mention the additional agricultural benefits (Costen, 1976). In the Majjia Valley, Niger, it is estimated that the pruning of shelter curtains every four years brings local residents the equivalent of $800 in construction poles and wood per kilometer of curtain (USAID, 1987). Several books or manuals devoted to the principle and the realization of shelter curtains are available (see Guyot, 1986; Bhimaya, 1976; Weber, 1986).
The effects of shelter curtains on crop yield are illustrated in Figure 2.1. Close to the tree curtain, yields are reduced due to shading, competition from the roots, and the physical space occupied by the trees. Further on, the advantages manifest themselves more and more clearly, until, at a certain distance, they begin to fade, the influence of the trees diminishing.
Some of the most remarkable yield increases have been reported in China where the hot and dry summer winds are one of the main factors limiting agricultural production. In the prefecture of Hetian, where 110,000 hectares have been endowed with shelter curtains ( Paulownia sp.) in the early 80s, according to the “forest net” system, yield gains of 60 percent were obtained for cereals, as well as a 70 percent increase in natural silk production and 300 percent for cotton (Wang Shiji, 1988).
Significant increases have also been reported in Mediterranean-type climates. According to a survey conducted in Saudi Arabia, Argentina, Bulgaria, California, Egypt, Israel, Italy and Tunisia, well-designed shelter curtains have made it possible to obtain a net increase in yields of between 80 and 200 percent (Jensen, 1984). Similar increases have been reported in the West Indies for the yields of vegetable crops (Guyot, 1986).
In the Sahel, although statistically valid results are not yet available, initial tests carried out with millet and sorghum suggest that, in fields protected by curtain shelters, yields can exceed those of unprotected fields by 23 percent (Bognetteau-Verlinden, 1980). In years of low rainfall, even small differences in yields can be of great importance for local populations.
But it is true that the overall effects of shelter curtains on yields vary considerably. In some cases, the yield is markedly increased; in others, competition for light and water, added to the loss of cultivable area, is exerted to the detriment of yields. As a general rule, where the earth is exposed to strong winds most of the year, or when soil erosion poses particular problems, it will be strongly recommended to establish shelter curtains. When these conditions are not dominant, the benefits are less clear. In addition to representing labor and planting material costs, shelter curtains put a certain area out of agricultural production, and compete with crops for water, lighting and nutrients. That is why the direct products of the tree curtains – fodder, fuel and food products, increased yields and soil improvement, must be sufficient to compensate for these costs. Often, from the farmer’s point of view, the loss of crop yield is largely compensated by the wood and other products that the tree curtain gives, and having a diversified production system can reduce the risks in case one of the elements fails.
Water erosion seriously harms agricultural production in many tropical and sub-tropical regions. It takes away the surface layers of the soil, the most fertile, and can destroy the crops themselves by flooding them. The forest and the trees can play a protective role against certain erosive phenomena due to water. The surface erosion caused by water in intact forests is generally less than on land exploited under any regime whatsoever (Hamilton, 1983). The clean cuts that leave the ground bare have a radical effect on the rate of erosion.
Contrary to what is often believed, it is not the vault formed by large trees that best protects the soil, but the vegetation cover on the ground, and the litter that lines it (Hamilton, 1986). If the ground is bare under the trees, the large drops of water that fall from the high foliage cause erosion by impact and a more pronounced runoff in a sheet than a rain falling on an uncovered ground (Lembaga Ekologi, 1980). Often, therefore, it is not the removal of high-growth trees that leads to soil erosion, but the fact of disturbing the lower floor and the leaf litter, and the baring of the soil which often results from logging.