Overstory #78 - Reforestation of Degraded Lands
This edition of The Overstory joins combines topics in preparation, species selection, planting materials, and tree maintenance towards reforesting degraded lands.
Degradation of tropical land is a physical, chemical, and biological process set in motion by activities that reduce the land's inherent productivity. This process includes accelerated erosion, leaching, soil compaction, decreased soil fertility, diminished natural plant regeneration, disrupted hydrological cycle, and possible salinization, waterlogging, flooding, or increased drought risk, as well as the establishment of undesirable weedy plants. There is a strong relationship between inappropriate land-use practices and land degradation. In some places, degradation is manifest (e.g., desertification), where in others it is inferred (e.g., declining crop yields).
Deforestation in mountainous regions is one of the most acute and serious ecological problems today. Disturbance of vegetative cover on montane areas with thin soil and steep slopes results in land instability (e.g., landslides) and soil erosion. Excessive erosion not only impairs site productivity but may also adversely affect other sites or water bodies farther down the watershed.
Conversion of tropical moist forest into farm or grazing land commonly results in rapid depletion of the soil's plant nutrient supply and accelerated soil erosion. In some places the degradation process leads to takeover by persistent, aggressive weed species of low nutritive value. Often the combined problems of low soil fertility and weed infestation become so great that the land is abandoned. Such lands are subject to frequent uncontrolled fires and are often covered by coarse grasses. Whenever the vegetation is burned, erosion may increase and productivity may be reduced further.
In many arid and semiarid open woodlands, overgrazing and repeated fires have converted the vegetation to a degraded fire climax stage. Consequently, soils become dry and little woody plant regeneration occurs. Fire-tolerant vegetation—commonly unpalatable to animals—persists leading to a desert-like state. Over 20.5 million ha of tropical arid lands become desertified every year (Wood et al 1982).
Each year, over 500,000 ha of excessively irrigated lands become saline or alkaline as a result of inadequate drainage or use of salty irrigation water. Capillary action draws moisture to the soil surface where it evaporates, leaving salts in or on the topsoil. In some cases, salts can be leached from upland soils and bedrock, raising the salinity of runoff from deforested slopes. The increased runoff harms agricultural soil in lowland areas by causing temporary or lasting waterlogging and salinization.
The best solution to such problems is to prevent inappropriate land-use practices on forested lands. Where it is too late for this approach, reforestation is an alternative. Trees planted on degraded lands will not give such high yields as trees planted on rich, fertile lands. However, it may be the only way to raise the productivity of the most degraded lands. Furthermore, in many countries, fertile sites are reserved for agricultural activities. Given the dwindling reserves of good land and the increasing amount of degraded tropical lands, reforestation is a technology with potential to rehabilitate soils and to provide many goods and services. For example, fuelwood plantations can alleviate the worsening shortage of firewood in some areas and prevent shortages from occurring in others.
This section summarizes the mechanics of reforestation and focuses on pertinent issues and problems that may prevent reforestation success.
Many degraded sites need some type of preplanting preparation, such as clearing stumps and competing weedy vegetation. Under some circumstances, site cultivation controls weeds and improves soil aeration, soil biochemical activity, percolation of water, pH regulation, nutrient application, and surface evenness. The degree and type of land preparation depends on several factors: site and soil conditions, vegetative cover, species to be planted, and available capital and labor.
Land preparation can be done by hand or by machine. Manual methods are less constrained by the rainy season, they require few skills, and the capital cost is relatively low. They also provide temporary employment to laborers and cause minimal damage to soil. A disadvantage of manual clearing, however is the need to recruit, manage, and provide logistical support in remote areas for large numbers of laborers. Mechanical clearing, on the other hand, requires high capital inputs for equipment maintenance supplies of fuel and spare parts, and operator training and supervision. And heavy machines degrade the site through topsoil disruption. Yet, in general, mechanical clearing is cheaper than manual clearing (Evans 1982). The choice between manual and mechanical land preparation must be made on a case-by-case basis, determined by all these considerations.
Sometimes, artificial barriers of brushwood or other materials constructed in a grid or contour pattern, or grasses (such as vetiver) and trees planted in a similar pattern, can be used to immobilize drifting sand. Plowing the soil surface to increase water infiltration, ripping across the slope to retain water, plus construction of bench terraces on steeper slopes, and funneling moisture onto a smaller area are all conservation measures used to maximize planting success. Minicatchments built to concentrate water into the rooting zones of individual trees are a particularly important technique in arid zones.
It may be necessary to add nutrients during land preparation. Several techniques exist including mulching with organic matter, planting nitrogen-fixing trees, applying green manure (especially herbaceous legumes), and commercial fertilizers. Mulching suppresses weeds, improves soil moisture conditions, and augments soil organic matter, but it may increase problems with rodents or other pests. Nitrogen-fixing trees can improve soil with their ability to produce nitrogen fertilizer. Foliage dropped by legumes is nitrogen-rich and will augment soil fertility as it decays.
Other measures may also be used, such as the addition of small amounts of commercial fertilizers and amendments. Nutrient levels and fluxes in plantations should be monitored to determine the prospective benefits and cost effectiveness of soil amendments. One of the less obvious soil deficiencies, occurring particularly in eroded soils in the drier climates is the lack of necessary micro-organisms. An ancient and effective method to add micro-organisms is to inoculate either nursery soils or planting holes in the field with a few grams of topsoil from well-established plantations or healthy forests. The method is not practical, however, where well-established plantations and/or healthy forests do not exist.
Tree species selection is important to plantation success. If a tree is grown under unsuitable soil or site conditions, it will be stressed and thus become susceptible to attacks from insects or competition from weeds. Several factors influence species selection, including the objectives of reforestation, seed availability, and costs associated with reforestation alternatives. For many degraded sites, the species need to be those that can add nitrogen to the soil as well as provide products wanted by local communities.
The importance of matching tree species with site cannot be overstated. The problem of species selection is complicated in the tropics by intricate climatic and soil patterns, and in areas that have been deforested by the highly variable degree of site degradation. Inadequate information on planting sites is a major cause of tree planting failure (Wadsworth 1982). Since most tree species used in reforestation are found over broad geographic ranges, there are different land races (subdivisions of a species with heritable characteristics resulting from adaptation to a specific environmental condition). Tree species races are often described by referring to the geographic location where the race is found naturally. Thus, the species' suitability to a particular site varies depending on the races used. Increases in yield and resistances to disease can be achieved through selection and use of appropriate seeds. Only by planting species and races on the sites for which they are adapted can maximum yields be obtained.
Natives and Exotics, Monocultures and Polycultures
Plantations cannot substitute wholly for natural forests as reservoirs of germplasm or as components of the natural environment—they are really an agricultural crop. Plantations contribute to preservation of the natural environment because they concentrate wood, food, and forage production within a minimum area, thus relieving some demands on natural forests. However, where plantations are established on land with good potential for annual agricultural crops, the effect actually may be to increase pressure on the natural forests.
Most large-scale tropical industrial timber plantations use species that are exotic to the planted area (Gallegos et al 1982). The widespread use of exotics may be a result of the preponderance of information, experience, and research on them, especially on Pinus, Eucalyptus, and Tectona. Also important to their use is the abundance, availability, ease of storage, and germination of seeds of these exotic species. The use of exotic tree species involves risks, such as susceptibility to pests and diseases. Because of the high yields possible with exotic species, however, the risks will continue to be taken.
The potential of using native species in plantations has been largely ignored. Reasons for this vary from lack of familiarity with many tropical tree species to lack of seed supplies. Native species are adapted to the local environment and, thus, may be less susceptible to stress, serious disease, and pest damage. Local people are more familiar with their native plants and have more uses for them.
Forest plantations in the past usually served industrial purposes and grew only one product, usually sawtimber, pulpwood, or fuelwood. Now, with an increasing demand for food, fuel, and fodder, plantations are needed to serve a wider variety of objectives. Thus, the use of multipurpose trees and polycultural plantings (with many species of trees) is becoming increasingly important, especially in areas with high populations.
To reforest lands, seeds of various species must be available in great quantities. The seed supply for species most commonly used in tropical, industrial plantations is adequate. However, the seed supply for multipurpose, agroforestry species is small.
The customary way of raising planting stock in the tropics is to grow seedlings in a forest nursery either in open beds for bareroot planting or in containers. Good nursery practices are essential to produce a hardy plant with a well-balanced, straight root system. Barerooted seedlings are susceptible to desiccation. Containerized seedlings are more costly and bulky to handle in the field and are subject to root coiling if closed-bottom containers are used. The latter can be avoided if the containers have an open bottom and are suspended above the ground.
Another technique for producing planting material is vegetative propagation—reproduction of planting stock without the use of seed. Vegetative propagation is widely used for tree crops such as rubber, coconut, tea, coffee, cocoa, and oil palm. Methods include cuttings, air layering, budding, grafting, and tissue culture. Rooted cuttings remain the most popular of these. Once the technique for a particular species is developed, the production cost is modest.
Vegetative propagation has the advantage of hastening massive reproduction of genetically superior plants, ensuring that all are of the desired genetic type. It has the disadvantage of increasing plantation risks due to lack of genetic diversity.
Seedling survival and growth rates in the nursery and at the planting site sometimes can be improved by using special kinds of fungi and bacteria. For most tropical trees, associations between tree roots and mycorrhizal fungi are essential for healthy growth. The fungi are active in the transport of nutrients and water to plant roots, and in some cases are important for the release of nutrient elements from mineral and organic soil particles. Trials have shown that seedlings inoculated with fungi show improved growth and survival over uninoculated controls. Populations of mycorrhizae are found naturally in soils, but these can be depressed after long-term clearing and/or topsoil removal, making reestablishment of vegetation on degraded lands difficult. Various methods for reinoculating damaged soils with mycorrhizal fungi are being developed.
Legume trees can grow well on degraded land because their roots can be a symbiotic host for Rhizobium bacteria which produce nitrogen fertilizer, an essential nutrient for plant growth. The bacteria convert nitrogen gas in the soil into a form the plant can use. Most soils contain Rhizobium, but degraded soils probably contain fewer types and lesser amounts of the bacteria. Thus, the appropriate type of Rhizobium may not be present at the site of a reforestation effort, or present in enough quantity to infect the tree roots.
Inoculants are living organisms that must be transported and stored carefully and used correctly to retain their viability. These requirements can be difficult to satisfy, especially at remote tropical sites needing reforestation.
An alternative to using seedlings in nurseries is to plant or sow the seed directly at the reforestation site. This method is feasible where seed is plentiful and where seed and seedlings mortality is low. Direct sowing of drought-resistant species is sometimes preferred, especially for species that have long and fast-growing taproots that may be damaged in a nursery or in transfer to the field. The advantage is that no nursery is required and planting costs are low. On the other hand, seedling survival may be low because of weed competition, lack of tending, poor weather, or animal damage.
Tree Care and Maintenance
Proper care and maintenance of the planted site is essential to ensure that trees survive to maturity. Once grown, there is the problem of monitoring timber harvests and of systematic replanting. The main causes of reforestation failure, other than inappropriate technologies, are uncontrolled grazing and fires, competition from weeds, and uncontrolled cutting for fuel, fodder, poles, and lumber.
Direct protection through fencing or guards tends to be expensive. Other, less costly methods include planting unpalatable trees (e.g., Cassia siamea) or thorny trees (e.g., Parkinsonia) as barriers around the plantation. The use of living fences is becoming a more widespread practice because they provide a number of auxiliary benefits including shade, fodder, windbreak, fuel, and wildlife habitat. Another alternative is subsidizing farmers with livestock feed or with cash to purchase feed during the period when trees are most susceptible to animal damage. Grazing beneath the tree canopy sometimes can be beneficial as a means of weeding. However, livestock grazing on recently reforested watersheds can be harmful because animals compact the topsoil, leading to poor tree growth and increased runoff.
Weeding is an important aspect of plantation establishment and maintenance. Weeds compete directly with seedlings for light, soil nutrients, and water. Their shade can smother and eventually kill young trees. There are three main methods of weeding—manual, mechanical, and chemical. The manual method is the most common and requires little skill or capital. Mechanical methods may be used in large plantation projects but generally are not considered profitable in the Tropics. In many tropical countries, chemical weed control techniques have been tested and found successful, but because of safety and cost problems they seldom become the main means of weed control.
Whatever the type and location of tree planting, the cooperation of local people is essential if newly planted trees are to survive. Because most trees do not yield much benefit for several years, the options offered must demonstrate explicit benefits to the people. Tree planting programs are most successful when local communities are involved and when the people perceive clearly that success is in their self-interest.
In local communities, support can be generated through local involvement in project design, demonstration plantings, commercial plantings by entrepreneurs with larger land holdings, education of community leaders, extension and training programs working directly with farmers or laborers, and direct financial assistance or provision of substitutes.
Successful reforestation requires sufficient funds, strong political will, massive popular support, and cooperation among all involved parties. Foresters and policymakers must remember that "forestry is not, in essence, about trees. It is about people. It is only about trees so far as they serve the needs of the people" (Gribbin 1982).
Evans, J., Plantation Forestry in the Tropics (Oxford, England: Clarendon Press, 1982).
Gallegos, C. M., Davey, C. B., Kellison, R. L., Sanchez, P. A., and Zobel, B. J., "Technologies for Reforestation of Degraded Lands in the Tropics," OTA commissioned paper, 1982.
Gribbin, J., "The Other Face of Development," New Scientist 96(1334):489-495, 1982.
Wadsworth, F., "Secondary Forest Management and Plantation Forestry Technologies to Improve the Use of Converted Tropical Lands," OTA commissioned paper, 1982.
Wood, P. J., Burley, J., and Grainger, A., "Technologies and Technology Systems for Reforestation of Degraded Tropical Lands," OTA commissioned paper, 1982.
This article is excerpted from Technologies to Sustain Tropical Forest Resources (Washington, D.C.: U.S. Congress, Office of Technology Assessment, OTA-F-214, March 1984). Library of Congress Catalog Card Number 84-601018. This book is available for sale from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.