lustrate the concepts of ecosystem rehabilitation of damaged, degraded, and derelict lands with examples of case studies from dry to humid life zones in island.
Rehabilitation of Tropical Lands: A Key to Sustaining Development Sandra Brown^ Ariel E. Lugo^ Abstract
Land rehabilitation is proposed as a management strategy to reverse the negative consequences of tropical deforestation and land degradation. We first define the concepts associated with ecosystem modificatjon—conversion, damage, and degradation—and those associated with ecosystem repair—restoration, rehabilitation, and reclamation. We then present a scheme of sustainable land use in the tropics, with illustrations of how rehabilitation and restoration activities fit into the overall scheme of the use of land. Because damaged lands cannot contribute effectively to sustained economic development, land rehabilitation is a necessary step for increasing the chances of attaining sustainability. Approaches for rehabilitating ecosystems are discussed, including the management of stressors and subsidies in relation to their point of interaction in the ecosystem. Finally, we illustrate the concepts of ecosystem rehabilitation of damaged, degraded, and derelict lands with examples of case studies from dry to humid life zones in island and continental situations throughout the tropics. The case studies demonstrate that opportunities for success exist, even with severely degraded lands, but a considerable amount of research remains to be done before we have a full understanding of the complexity of the task facing us.
Introduction
le increasing worldwide affenfion given fo fhe ropical biome usually focuses on fhe global impli1 Department of Forestry, University of Illinois, W-503 Turner Hall, 1102 S. Goodwin, Urbana, IL 61801, U.S.A. ^Infernational Institute of Tropical Forestry, USDA Forest Service, Call Box 25000, Rio Piedras, Puerto Rico 00928, U.S.A. © 1994 Society for Ecological Restoration
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cafions of deforesfafion, such as greenhouse gas emissions, loss of biodiversify, and global climafe change. Liffle affenfion is given fo a more fundamenfal and immediafe problem of susfaining economic acfivify for abouf half of fhe world's populafion. In fhis paper we discuss fhe issue of fropical land-use changes and fheir consequences, and we propose an approach fo curb rafes of forest loss, namely the rehabilifafion of damaged lands. Land rehabilifafion provides a fundamenfal solufion fo currenf problems of fropical foresf loss and a means fo slow fhe process. We focus on the tropics but believe fhaf fhe principles apply to all lands. We first review the concepts and definitions applied to the topic of land conversion and repair. We fhen discuss the fypes of human activities that produce damaged or degraded lands in the tropics, discuss fhe ecological and socioeconomic sfrafegies for rehabilifating fhese lands, provide some case studies that are examples of successful rehabilifafions, and conclude wifh a discussion of fhe basic requirements for success in any rehabilitation project. Concepts and Definitions
The topic of land degradafion and rehabilifafion is fraughf wifh many concepfs and definifions. Here we define most of fhe terms we use in fhis paper based on our own understanding and thaf of Bradshaw (1987), Lamb (1988), Lugo (1988, 1991), Berger (1990), Schreckenberg et al. (1990) and Barrow (1991). The use of fhese definifions and concepfs is illustrated in Figure 1. Lands damaged by nafural phenomena or human acfivifies and fhaf can be repaired by nafural succession fo fheir original state (boxes and arrows above the line in Figure 1) will be excluded from furfher discussion in fhis paper. The arrows on fhe leff-hand side of Figure 1 represenf fhe ferms we use fo describe fhe changes lands undergo generally by human acfivifies. Conversion is a more generic ferm fhaf represenfs fhe whole specfrum of changes. We view conversion as any modification of a mafure nafural ecosystem, from as simple as removing bark or branches from frees fo as severe as complefe deforesfafion and mining of fhe subsfrafe on which a foresf once sfood. Damage, on fhe ofher hand, is a more resfricfive ferm fhaf we apply fo converted lands incapable of normal recovery fo original condifions (mafure ecosysfems) because one or more of fheir key affribufes (soils, biofa, land morphology, hydrology, efc.) have been modified. However, damaged lands sfill have fhe capabilify fo produce goods and services fo safisfy human needs. Lands are described as degraded when fheir edaphic conditions and/ or biofic richness have been reduced by human aefivity 97
Rehabilitation of Tropical Lands
Types of ecosyslems
Secondary forests/ (oresl fallows
No human intervention required
Fulfy funcfional ecosystem Requires iiuman intervention
Arrested succession
Productivity and biota restored
Dereiict fands
Figure 1. An illustration of the definifions and common terms used to discuss the changes lands undergo, generally caused by human activities, and their modes of repair by human-directed succession.
fo such a degree that their ability to satisfy parficular uses has declined. In shorf, degradafion generally occurs when any of fhe ecosysfem's sforages (sfafe variables such as soil organic maffer, soil nufrienfs, seed pools, and biomass) have been reduced fo fhe poinf that nafural inpufs cannof replenish fhem fo fheir original state. Lands can also be degraded by contamination wifh foxic chemicals fhaf can render fhe ecosystem useless fo people. The poinf af which lands go from being damaged fo degraded is not always clear and would fend fo be sife-specific. In an advanced state, degraded lands are analogous to derelict lands, or lands thaf are of very limifed use to people or developmenf programs. On fhe righf-hand side of Figure 1 are fhe definifions of the terms commonly used to describe the modes of ecosystem repair by human-directed succession. By succession we mean all fhe changes fhaf occur in ecosystems over time (Odum 1969). Reclamation is the process by which derelict or very degraded lands are returned fo producfivify and by which some measure of biofic function and productivity is restored. This pro-
cess offen becomes arresfed, however, because of severe limifafions in the site or the biota. Rehabilitation is the return of any converfed ecosysfem, damaged or degraded, fo a fully functional ecosysfem, irrespective of ifs original or desired final state. The term restoration is used when any converfed ecosysfem is refurned fo ifs presumed or relafively original indigenous sfafe. Time is an imporfanf facfor in the use of fhe ferms reclamafion and rehabilifafion because some mighf argue fhaf given enough fime (cenfuries fo millenia), all damaged and degraded lands may be restored to their indigenous sfafe. In an ever-changing world, however, fhis amounf of time is nof likely fo be available, nor is if likely fhaf condifions of the past will be restored in the fufure. Insfead, we use fhese ferms in relafion fo shorfer fime periods on fhe order of a human generafion, which we inferpref fo mean abouf 100 years. Sustainabie and Unsustainabie Tropical Land Uses
The highesf rafes of foresf clearing during fhe past several decades have occurred in the tropics. In the late 1970s, abouf 11.3 million ha yr"^ of fropical foresfs were deforesfed and converfed fo nonforesfed lands (Lanly 1982). During fhe period 1980-1990, fhe rate was 15.4 million ha yr"' (Singh 1993). In addition to outright clearing, more than 4 million ha yr ' of mafure foresfs were selecfively logged during fhe 1970s (Lanly 1982). As wood producfion increased during fhe 1980s (Food and Agriculfure Organizafion 19801990), so mosf likely has the area of logged forests. These are the changes in fropical land use whose magnifude we know something abouf; fhere are ofher changes occurring, such as fhe formafion of damaged and degraded foresf and agriculfural lands, fhe magnifude of which we know liffle abouf (Grainger 1988; Brown ef al. 1991; Richards & Flinf 1994). For example, a major reason for deforestation in fhe tropics is to replace agricultural land that has become degraded and unproductive for growing crops (Brown 1993; Dale et al. 1993; National Research Council 1993). Much of the land currently being cleared of foresfs is replacing degraded agriculfural land, so fhaf fhe increase in agriculfural land does nof equal fhe decrease in foresf land. To undersfand fhe importance of land rehabilifafion, it is first necessary fo undersfand fhe processes fhaf lead to degraded and damaged lands and to do so in fhe confexf of susfainable land use in the tropical biome. Land conversion need not be defrimenfal fo forests or to people, if done properly (Figure 2). In fhe pasf, under low populafion pressure and in fhe absence of potent technological tools, people extracted forest products from complex fropical foresfs or conRestoration Ecology
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Rehabilitation of Tropical Lands
Available for re-use (8)
Figure 2. Conversion of tropical lands showing pathways that lead to (1) sustainable land uses; (2) damaged, degraded or derelict lands; (3) arrested succession; and (4) repaired lands. See text for further description. verfed fhem fo femporary agriculfural fields to produce food and fiber without degrading sifes irreversibly (pafhways # 1 , Figure 2). Nafural succession occurred affer abandonmenf, and fhese sifes reverfed to foresfs (pafhways #2, Figure 2). Af infermediafe stages of succession, secondary foresfs were produced that provided many valuable products and services for humans (Table 1). Technological innovafion and improved managemenf skills allowed people fo infensify agriculfural activities in certain localities where soil, climafe, and moisfure condifions were favorable for confinuous use. Vasf areas of fropical lands have been used fhis way for cenfuries, and fhey are sfill producfive areas, such as fhe rice-growing regions of fropical Asia (National Research Council 1993; Richards & Flinf 1994). Logging of foresfs also infensified as global demand increased and new fechnologies for exfracfing and processing fimber were developed (Whifmore 1984; Tucker 1992). Commercial logging can be a sustainable activity without long-term damage fo foresfs, if properly managed (Uhl & Vieira 1989; Uhl ef al. 1991; Johns 1992). Buf few foresfs in fhe fropics are currently managed properly (Schmidt 1987; Johnson & Cabarle 1993), with the resulf fhaf many of fhese lands become damaged or are converfed fo nonforesf uses (as in encroaching nonsusfainable agriculfure) and are in need of rehabilifation. JUNE 1994 Restoration
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Under mosf of fhe condifions jusf described, if is possible fo converf a foresfed landscape fo a landscape mosaic composed of mafure foresfs, logged foresfs, foresf fallows, shiffing agriculfural plots, and permanent agricultural lands (Brown & Lugo 1990). Humans derive multiple benefifs, bofh goods and services, from each of fhese land uses, as well as from fhe landuse conversion process. However, yields, services, and cosfs fo humans differ according fo fhe fype of land use. The use of such a landscape can be sustainable because natural successional processes are generally sufficient fo repair any shorf-ferm damage caused by human acfivify. There is little or no loss of natural productivity, and liffle fo no human infervenfion is needed fo repair fhe landscape. A cycle of damage fo lands begins wifh overuse of sifes. Excessive exfraction of maferials involving modificafion of fopography, compacfion of soils, unchecked erosion, confinuous cropping wifhouf fallow periods or crop rofafions, and oufrighf removal of vegefarion cover are examples of human acfivifies fhat result in damage and degradafion to the land (pafhways #3, Figure 2). For example, machine-clearing of foresfs is more damaging to sites than clearing by hand because machines remove or heavily compact soil, thus greatly refarding the regeneration cycle (Uhl ef al. 1988). Improper or excessive use of poisonous chemicals can also damage a sife and make if unsuifable for furfher use. In general, fhe greater the intensity of use of a site, the greater fhe probabilify fhaf it can be damaged and fhaf precaufions will need fo be faken.
Table 1. Values of secondary forests for human use.* Provide fruits, medicinal plants, construction materials, and animal browse. Produce valuable timber species because they are often dominated by a few species. Produce a uniform raw material with respect to wood density and species. Wood of secondary species tends to be low in resins, waxes, etc., which facilitate their use. Produce biomass at a fast rate. Relatively easy to regenerate. Species tend to have properties that foresters seek as suitable for plantations. Have an advantage as a raw material source because they are generally more accessible to markets than primary forests. Serve as foster ecosystems for late-secondary to mature forest species. Serve as useful templates for designing agroecosystems. Restore site productivity and reduce pest populations. May support higher animal production and serve as productive hunting grounds. Presence of a greater number of vertebrates may enhance ecotourism. 'Modified from Brown and Lugo 1990.
99
Rehabilitation of Tropical Lands
Sometimes normal successional processes are insufficient or too slow (occuring over more than several decades) to rehabilitate lands fo producfive sysfems, or fhey are arresfed at an undesirable end-point; in fhese cases, infervention by humans is needed. Arresfed succession and lands degraded wifh overuse and poor managemenf require repair via rehabilifative actions (pathways, #4, Figure 2) or restoration (pathway #5). Intensive damage results in degradation towards a derelict state (pathway #6), and land reclamation is needed fo reverse fhis sfafe (pafhway #7). Rehabilifafed lands resulf in alfernafive foresf ecosysfems, which can then be reused to serve human needs (pathway #8). Causes of Damage to Ecosystems
Before discussing the specific causes of damage to tropical lands, we first provide a conceptual model fhaf illustrates how stressors impact ecosystems, and fhaf shows how fhe severity of impact depends on where in the ecosystem a stressor acts (Lugo 1978). A simplified model of an ecosystem with energy inputs (sunlight, rain, wind), major comparfmenfs (planfs, consumers, and soil organic maffer, nutrients, and associated decomposers), and funcfions (primary producfion, respirafion, and nufrienf cycling) is shown in Figure 3. Five fypes of ecosystem stressors are illustrated by the encircled numbers; for consistency, we refain the numbering system used originally by Lugo (1978, 1988). Stressors of types 1-2 (higher-impact) cause more damage than those of fypes 3-5 (lowerimpacf) because fhey remove low-qualify energy and
o Figure 3. Simplified model of an ecosystem with energy inputs, major compartments, and flows showing where sfressors (numbers in circles) act on an ecosystem to cause damage. Stressors 3-5 cause the least damage, whereas stressors 1-2 do the most damage (based on Lugo 1978, 1988; see text for further explanation).
100
Table 2. Examples of causes of damage and degradation to tropical forest lands.* Type of Stressor
Cause
Site alteration allowing unplanned invasions of exotics and pests Continuous use without proper fallow or crop rotation Excessive harvesting of any comparfment. including plant or animal populations Overgrazing Overfertilization Introduction of fires or changing natural fire frequencies Contamination with poisonous/radioactive
4-5
Inappropriate agricultural techniques Changes to hydroiogic cycle Soil salinization Soil compaction and erosidn Mining of soils or geologic substrate Consequences of poor resource analysis/ planning Poor land-use policy Use of inappropriate technology
2-5 1 1-3 1-4 1-5 1-5
War
4-5 3-5 3-5 3-5 3-5 2-5
1-5 1-5 1-5
*The type of stressor is keyed to the numbers in Figure 3.
materials before they can be fully used by the system or interfere with physiological processes (Lugo 1978, 1988). Sfressors fhaf harvesf planfs or consumers (fypes 3-5, Figure 3) generally have less impacf because they do not affecf fhe sysfem's ability to process energy and renew itself. On a long-ferm basis, however, even fhe lower-impacf sfressors can damage and degrade fhe ecosysfem. The causes of damage fo land are many, and fhey resulf from complex inferacfions of cultural and ecologic factors (Bainbridge 1990). Examples of specific causes and how fhey affect the ecosysfem are presenfed in Table 2. Generally, fhese causes affecf fhe land's physical characferisfics, nutrient status, foxicify, and sfrucfure of the biofic communify (Bradshaw 1989). Many of fhe causes acf in unison or synergisfically, making fhe rehabilifafion of such lands an expensive underfaking. For example, (1) poor land policy may lead fo use of inappropriafe agriculfural fechniques fhat result in unchecked soil erosion, and (2) over-harvesting of frees may change fhe hydroiogic cycle, causing fhe wafer fable fo rise and soils fo become salinized. Furthermore, although several causes affecf fhe same parf of fhe ecosysfem (Table 2), fheir net effect may differ. Infensity, frequency, seasonalify, and areal exfenf of fhe sfressor are also crifical in defermining fhe amount of damage. The degree of damage to forest lands can also range widely depending on the causal human action. A subRcstoration Ecology
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FORESTED LANDS Mature stands Damaged secondary forests Biomass reduction -wood/bark -litter -understory -peat Species impoverishment Frequently burned Exotic/pathogen-invaded NON-FORESTED LANDS Frequently burned Overgrazed Eroded Nutrient depleted soils Waterlogged soils Salinized soils NON-VEGETATED LANDS Highly eroded slopes Poisoned lands -chemical spills -industrial wastes air pollution War ravaged Strip-mined lands Derelict lands
Figure 4. Subjective ranking of tropical lands according to the degree of damage or degradation by human activities, from least damaged (relatively mature) to most degraded (derelict). Within each state there are gradients or degrees
of damage based on intensity of the stressor and natural environmental factors (clitnate and soils, for example). (See Fig. 5 for types of actions needed).
jective ranking of fropical lands according fo degree of damage is illusfrafed in Figure 4. Lands are grouped into three categories (forested, nonforested, and nonvegetated), with lands subject to increasing degrees of damage ranked under fhese fhree cafegories. We assume fhat each land sfafe is subject to the same environmenfal condifions as fhe mafure sysfem. Therefore, fhe listing generally reflecfs progressive damage or degradafion of the original system. Trends in mode of repair, types, costs, and probability of success of rehabilifafion are shown by fhe arrows on the left-hand side of the diagram. Damaged but still-forested lands are subject fo lower-impacf sfressors (fypes 3-5 at low to moderate intensity. Figure 3). Lands in fhis cafegory can be rehabilifafed wifh nafive species af low cosf and wifh a high probabilify of success. Nonforesfed lands resulf when sfressors of fype 2 and high-infensify 3 are added. Rehabilifafing fhese lands requires a mix of nafive and exofic species and some human infervenfion; cosfs will be higher and fhe probabilify of success lower fhan in rehabilifafion of lands sfill foresfed. When sfressors of fype 1 are infroduced, lands become mosf degraded. The cosf fo rehabilitate or reclaim fhese lands is the highest, has the lowest probability of success, and requires fhe mosf research and technological efforf. Objecfive criferia of
degree of damage or land degradafion are needed fo facilifafe research, mifigafion, and collecfion of sfafisfical dafa. A criferion based on fhe fype and magnifude of sfressor fhaf causes fhe damage (Figure 3) is, we believe, a step in the right direcfion.
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The Role of Rehabilitation
Rehabilifation is a special form of succession characferized by human intervention and expenditure of energy (Figure 5). Rehabilifafion of damaged or degraded ecosysfems involves a reversal of fhe acfions in Figure 3, shown by a reversal of fhe numbering system. If consisfs of five types of acfions whose cosfs increase as fhe numbers in Figure 5 become higher. The simplest and cheapest action is (1) fo remove or confrol fhe sfressors acfing on fhe sysfem, such as high fire frequency, over-grazing, or biomass removal (poinfs 3-5, Figure 3). A second acf ion is (2) fo add species (planfs or animals) or maferials (ferfilizer, organic residues, or wafer). More expense and efforf are involved in regulafing fhe speed of ecosysfem processes (3), such as reversing soil compacfion or managing fhe quanfity and quality of soil organic maffer inpufs fo confrol synchronizafion of nutrient release and plant uptake. The most difficulf rehabilifative strategies (poinfs 4 and 5 in 101
Rehabilitation of Tropical Lands
fits: (1) it converts unproductive lands fo self-perpefuafing ecosysfems; (2) if prevenfs furfher damage fo downsfream ecosysfems; (3) it reverses a worldwide negative trend of land degradation; and (4) if removes pressure on undisfurbed lands and thus contributes to a reduction in furfher deforesfation. Beyond these benefits, land rehabilitation helps people manage change and correct or adjust the path of developmenf so that it remains on a positive track relative fo both people and the environment.
Chalienges in Tropical Forest Rehabilitation Figure 5. Simplified model of an ecosystem (see legend to Figure 3) that illustrates the actions (numbers in circles) needed to rehabilitate damaged, degraded, or derelict lands. The increasing numbers reflect the increasing cost of rehabilitation (based on Lugo 1988; see text for further explanation).
Figure 5) involve either removing fhe higher-impact stressors (points 1-2, Figure 3) or regulating fhe energy inpufs info a sysfem to control fhe condifions for ecosysfem developmenf. Because higher-impact stressors are the most damaging to an ecosysfem, high costs are generally incurred just to remove them. Changing the hydrology of a foresfed wefland (through reversing draining or damming) or modifying topography on hillsides (by terracing to control erosion) are examples of fhese sfrategies. Rehabilifation has several drawbacks: (1) it is a lasfdifch effort to reverse a negative sifuafion caused by poor managemenf of nafural resources; (2) if has a high energy and economic cosf—if is not free like nafural ecosysfem development; (3) it demands knowledge and understanding of ecosysfem repair processes, so research is needed; and (4) fhe resulfs of rehabilifafion are unpredicfable because fhe ecosysfem can recover in a mulfiplicify of ways, and if is offen impossible fo direcf fhe rehabilifation to a specific final sfafe (Figure 2). The costs of careless use of fhe land are very high and are offen paid back in ferms of fime, fhus generafing a "fime fax." The fime tax is the time thaf society must spend waiting for a resource fo heal, during which fhe resource cannof be used and musf be nursed. This implies cosfs in energy and resources without immediafe refurn on invesfment. The return is the rehabilitation itself. The fime fax is proportional fo the initial damage inflicted upon the resource. The fime fax—like income fax, deafh, and enfropy—is unavoidable and must be paid! Land rehabilifafion has at least four immediate bene102
The literature in tropical forest rehabilitation is sparse, and it provides more ideas and suggestions than actual examples of successful l-ehabilitafions of complex foresf ecosystems (see Bradshaw 1987; Lamb 1988; Berger 1990; Schreckenberg et al. 1990; Barrow 1991). The reasons for fhis lack of success are many. For example, it is only recently that the issue of rehabilifafing fropical forests became a goal of managers. Furthermore, fropical foresfs are complex ecosystems; we undersfand fheir structure and function insufficienfly fo be able to puf one together starting with an array of isolafed parts. In addition, because most fropical foresfs are already in some less-than-mature state (Brown & Lugo 1990; Wood 1993), the target we are frying fo reach is uncerfain. Such a primifive state of knowledge is not exclusive to tropical forests. A critical examination of the rehabilitation and resforafion liferature reveals that most successes have been with simple, highly-stressed ecosysfems such as mangroves, marshes, and sand dunes, or on derelicf lands where the establishment of any vegetation is considered a success (Lugo 1988). Highly-stressed ecosystems are relatively easy fo rehabilifafe because of their low sfrucfural complexify and relafively hardy biofa. Moreover, these systems usually respond rapidly to changes in environmental conditions. For example, prairies and wetlands respond rapidly fo fire and fo changes in its frequency or intensify. In confrasf, complex foresfs fax our abilify and knowledge because fheir successful rehabilifafion requires fhe building of highly diverse sfrucfures and interactions. Furthermore, it is easy for fhe sysfem fo evolve in directions fhaf are fofally unanficipafed by fhe manager. Therefore, fhe preferred sfrafegy of rehabilifafing complex ecosysfems involves whaf Lugo (1988) calls "rolling with the punch," in fhe sense fhaf the manager provides a set of condifions for fhe developmenf of fhe ecosysfem, allows natural processes to do most of fhe work, and is willing fo accepf what the natural succession offers. Humans become directors or care-fakers of fhe nafural process of succession, nudgRestoration Ecology
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Rehabilitation of Tropical Lands
ing if one way or anofher af crifical fimes in fhe process. Despife fhe lack of know-how for rehabilifafing damaged and degraded lands, some general sfrafegies for improving chances of success have been proposed. These can be divided info ecological and socioeconomical sfrafegies (Lamb 1988; Lugo 1988); sfrafegies from both groups must be considered for any rehabilitative project fo be successful. Fundamenfal ecological requiremenfs (Table 3) are fo improve fhe sfrucfure, fauna, organic maffer, and ferfilify of fhe soil, and fo assure an ample supply of genefic maferial (Lugo 1988; Bainbridge 1990; Harrington & Howell 1990; Lavelle et al. 1993). Moreover, fhe selecfion of any suife of ecological sfrafegies musf consider the climatic, edaphic, and geomorphologic factors fhat characterize a given site because they regulate the speed of recovery (fhe fime fax) as well as defermine fhe amounf of human infervenfion needed. For example, fhe use of fallows may be appropriate in humid climates where plants offen re-esfablish quickly, assuming fhaf sufficienf seed sources are available fo fhe area fo be rehabilifated. Under very arid climates, however, planfing and irrigafion may be needed fo ensure fhaf vegefafion cover is restored. Some of the strategies in Table 3 may be controversial, particularly the careful use of exotic species. This is recommended because many exofic species are generally adapfed fo poor soil condifions, and fhey have been shown fo fosfer nafive species (Lugo 1992fl; Parrotta 1992, 1993; Lugo et al. 1993). Although rehabilitation is often thought to be an ecological problem, socioeconomic impediments also exist, which may be more important fhan fhe ecological obstacles (Lamb 1988). There are a variety of reasons why social and economic facfors are so infracfable; as a result, no universal solutions exist. Based on a variety of case studies, however, the most important socioeconomic requirements appear to be stable land-use pafferns, equifable land-fenure sysfems, homogeneous human populafion (wifh respecf fo efhnicify, economics, and so forfh), local public involvemenf, sfrong local leadership (fo enforce collecfive rules), and parficipation by governmental insfifufions, parficularly for large projects (Lamb 1988; Karki 1991). Examples of Rehabilitation Projects
The examples of rehabilitation projects that we discuss below are generally site-specific in response fo local environmenfal condifions and socioeconomic needs. They are presenfed not only to illustrate the diversity of problems and solutions in fhe field of rehabilifafion and resforation ecology, but also to serve as a basis on which to build and provide guidance for future projjUNE 1994 Restoration
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Table 3. Ecological requirements for rehabilitating damaged and degraded tropical forest lands.* General Strategies • Maintain flexibility in the approach. • Be alert to local environmental conditions. • Avoid specificify of the ultimate goal. • Couple systems that maximize their value and accelerate rehabilitation (for example, use treated sewage effluent for irrigafion when possible fo qrcelerafe planf growfh). • Create nuclei of biotic activity. • Maximize ecosystem complexity to optimize use of site resources. • Maximize protection from pests, fires, etc., and minimize risks. • Use stressors to arrest succession at desirable end-point. Strategies Targeted at the Soil ' Manipulate the soil as needed, either physically or biotically (for example rip the soil, introduce soil fauna, mycorrhizae, etc.). • Fertilize and irrigate when absolutely necessary (for example, on derelict lands or in arid areas). • Keep topsoil moist, cool, and shaded. • Be aware that nutrient cycling strategies may change during the rehabilitation process. Strategies Targeted at the Flora and Fauna • Maximize vegetation cover. • Restore tree cover. • Manipulate the existing vegetation before attempting substitution. • Use multiple seeding techniques when in doubt as to what to plant. • Let natural selection decide the best species combination for a site. • Develop species mixtures based on their ecological combining ability. • Use fallows to do most of the forest rehabilitation. • Use exotic species to foster native species. •Based on Lugo 1988.
ecfs, as well as fo femper expectations from any rehabilitation program. Rehabilitation Dominated by Removal of Lower-Impact Stressors. The best examples of fropical foresf rehabilifafion are fhose where fhe main human infervention is the removal of fhe lower-impacf sfressors (sfrategy 1, Figure 5). In this situation, ecosystems repair themselves mostly by natural succession affer fhe stressors have been removed. Buf fhese rehabilifafions are nof enfirely nafural because fhey offen involve exofic species (producfs of pasf human acfivify) or nafive species seiecfed and favored by people fo fhe exclusion of ofher nafive species. The final producfs are ecosysfems wifh differenf species composifion and producfivify than those existing prior to human activity. Guanica Forest, Puerto Rico. For 123 years, people lived
inside the Guanica Forest, now a Man and the Bio-
Rehabilitation of Tropical Lands
sphere (MAB) Reserve. This MAB Reserve encompasses a dry forest fhaf has been exfensively studied by ecoiogists for many years (Murphy & Lugo 1986). Human activities involved wood and fuelwood collection, grazing animals, farming, and construction of houses, schools, and recreational areas (stressor types 3-5, Figure 3). No machines were used, and deforesfation occurred by manual means. Studies are documenting what happened to the foresf lands used infensively by people for 128 years and lafer abandoned for 42-52 years. The resulfs fo dafe show fhaf: (1) previous land use made a difference in fhe direcfion of recovery (Figure 6); (2) fhe most intensively used lands (the recreation areas) recovered fhe leasf, while less infensively used foresf sfands (fhose used for fuelwood collecfion) had fhe most advanced recovery; (3) fhe harsh sife condifions (low wafer availabilify) seiecfed for adapfed native species rather fhan exofics, which in Guanica were more common in pioneer sfages of developmenf; and (4) human preference for cerfain free species was very imporfanf in defermining biomass, basal area, and wood volume of fhe foresf because those trees favored by people became the largest frees in fhe recovering areas (Figure 6b). For example, mahogany trees {Swietenia mahogani) on abandoned farmland had an importance value of only 6.3% buf confributed abouf 15% of fhe sfand's basal area. Human use of fhe Guanica forest was based on fhe slow consumption of fhe long-ferm sfores of fhe foresf, namely its rich soil organic horizons and high underground biomass, which supported rapid resproufing. This use slowly depleted site fertility, although if did so over a long fime (128 years). Half a cenfury later, the sifes are still in recovery phases in spite of nof being sfressed any furfher. This example underscores the time elemenf in rehabilifafion. Time fo complefe recovery can be very long, depending upon fwo independently operating facfors: fhe exfent of damage and the intrinsic capacify of a site to recover. Guanacaste National Park, Costa Rica. The goal of fhis
Figure 6. {a) Foresf recovery on both sides of the main street (still used as a forest trail) of a small town in the Guanica forest after about 50 years of abandonment. Here, the rehabilitation strategy was to remove the stressors (sfrategy 1, Figure 5). {b) Lands used for collection of wood producfs had the mosf advanced recovery because many species, such as Pisonia albida, are capable of coppicing; fherefore, a closed canopy quickly forms (photos by A. E. Lugo). 104
project is fo rehabilitate degraded farmland in fhe dry forest zone of Gosta Rica to create a new Guanacaste National Park (Janzen 1986, 1988). The lands have been subject to burning, overgrazing, over-cutting, or continuous farming over more fhan 400 years (sfressor fypes 3-5, Figure 3). Much of fhe area is now grassland buf sfill subjecf fo frequenf fires. Buf pafches of near original vegefafion fhaf confain original planf and animal species sfill exisf (Janzen 1986). Several steps (strategies 1-2, Figure 5) are involved: (1) excluding wildfires, anthropogenic in origin, by constructing fire lanes or confroUed burns fo reduce fuel loads; (2) helping fosfer recolonizafion by woody planfs eifher fhrough exclusion of fire and grazing or Restoration Ecology
JUNE I99
