Large portions of previously forested land in the tropics suffer economic depreciation through erosional loss of topsoil because they are cleared and applied to inapproproate forms of agricultural, such as pasture for livestock. Soils are often initially acid and infertile, and incapable of sustaining productivity for more than a few years or decades. Erosion exacerbates infertility and ultimately may lead to farm abandonment. This process has already occurred in the Amazon, and its early signs are now seen in Costa Rica.
We have begun experiments on a degraded farm in southern Costa Rica to test hypotheses about the use of native species of timber trees and nitrogen-fixing plants to restore soil fertility and income to farms of 8 ha or larger. Although the present price of lumber in Costa Rica is high, the future market of tropical hardwoods is uncertain, and the time to first harvest is not economically viable for small landholders. Therefore, we are proposing and testing an economic cost-benefit model involving timber trees, leguminous cover crops, and beans on a rotational basis that will allow the farmer to continue to raise some livestock. The plan is predicted to decrease soil erosion, increase soil fertility and production sustainability, and yield income in the first years from a combinaiton of reforestation incentives and farm productivity.
The unique aspects of the model and its test are the use of native trees and cover crops adapted to the soils and climate, and the application of the test to a farm already in an advanced stage of ecological and economic degradation.
Native trees have potential for regenerating tropical eroded cattle pasture. Several tree species native to Costa Rica have proven capable of growth in these degraded soils. Local farmers are beginning to show interest in reforesting with native trees. However, most nurseries rarely are able to record the provenances of the seedlings and may use seeds from a single individual. We outplanted nursery-raised seedlings from 14 seed trees of Terminalia amazonia (Combretaceae) into degraded pasture. Survival and growth rates varied at least 7-fold, depending on seed tree identity. Such strong effects have serious repercussions for the social acceptance of native species for reforestation. Nurseries should carry seedlings of a wide range of individuals from the region, keep track of their success once outplanted, and use these data to eliminate weak mother trees from their seed sources.
Early successional tree species may be useful tools for regenerating highly eroded tropical soils. These soils are often acidic, nutrient-poor, and either bare or have a high incidence of non-beneficial weeds or pasture grasses. We investigated how to establish the native tree species Terminalia amazonia in degraded Costa Rican cattle pasture by transplanting small seedlings in a randomized block design with eight treatments with five replicates. The treatments consisted of direct outplanting, using combinations of inorganic fertilizers, and using various leguminous plants as nurse species. First-year results showed both significant block and treatment effects as well as a strong influence of the seed tree. Growth and survival were better in the fertilizer treatments and in blocks with better baseline fertility(1).
(1) Both of these results changed in later years.
Early successional native tree species may hold potential for the restoration of eroded and degraded cattle pasture to productive capacity by regenerating top soil. To learn how to facilitate tree growth and establishment under eroded conditions, T erminalia amazonia ("Amarillon", Fam. Combretaceae) was outplanted at the seedling stage into a degraded pasture (Ultisol) in south Costa Rica near Las Cruces Biological Station. A long term experiment of eight treatments with five repetitions of 93- tree plots tests whether Terminalia establishment and growth might be improved by fertilization or by interplanting with native nitrogen-fixing "nurse" trees to improve soil conditions and provide mulch. First and second-year results showed influence of the seed mother tree. Growth and survival of Terminalia were better in blocks with better baseline fertility and in the fertilizer treatment(1). In the first two years after outplanting, the nitrogen-fixing trees did not grow well and did not influence Terminalia growth(2).
(1) Both of these results changed in later years.
(2) This result changed in later years.
The exposed subsoils of deeply eroded pastures on humid tropical Ultisols are acidic, nutrient-deficient, phosphorus-fixing, and high in toxic aluminum ions. On one such farm in southern Costa Rica, we tested the idea that legume mulches would help a native tree (Terminalia amazonia, Combretaceae) grow better on such soils. Mulches from three species of aboreal legumes (Inga edulis and Calliandra calothrysus, Mimosaceae; Gliricidia sepium, Papilionaceae) outperformed mulches from three species of grasses and one species of beneficial weed. However, when we interplanted various herbaceous and arboreal legumes on these soils with T. amazonia seedlings to provide mulch for the T. amazonia, most of the legumes died. One aboreal species (I. edulis) survived but grew slowly for two years. Only in the third year did it begin to grow quickly and produce large amounts of biomass for mulch, as hoped. Our research now seeks to determine, first, why legumes usually don't grow in these soils, and second, why I. edulis grew so slowly for two years. Our ultimate goal is to facilitate legume growth, which in turn may hasten restoration of topsoil and trees to these wasted lands.
Costa Rica has many species of aboreal and herbaceous legumes whose mulch has potential for restoring fertility to eroded lands. However, legumes have grown poorly at our site, a typically overgrazed old pasture with acidic, nutrient-poor Ultisols. Of six species of herbaceous legumes (fam. Papilionaceae) only two species survived and grew slowly. Of three species of tree legumes (fams. Papilionaceae and Mimosaceae), only one (Inga edulis) showed potential for production of large quantities of biomass, yet it grew only slowly for the first two years. We are investigating several possible causes for poor legume performance: low levels of soil phosphorus (P), lack of appropriate strains of Rhizobium, lack of mycorrhizal innoculae, and in the case of Inga, allocation of early growth below ground. Our results with respect to P conflict, and we are just begining to work on the microbes and underground growth of Inga.
Deforestation on steep lands in the tropics has resulted in high rates of erosion and associated soil degradation. Many forests were cleared for pastures, which are now losing their productiivity. Because deforestation in Latin America reached extremely high levels in the 1980s and many of these soils are thought to have a useful lifespan of only 20-40 years, we are facing a growing crisis in soil degradation. Various methods of soil restoration have been considered and tried, including planting native trees. However, some pasturelands are resistant to such restoration attempts. We believe that mycorrhizal fungi (AMF) are likely to be important in the attempts to restore soil fertility and/or trees to eroded pastures. As a first step to testing this idea on our degraded site in southern Costa Rica, we examined a gradient of land degradation from severe to mild and measured how soil chemistry and mycorrhizal communities changed along that gradient. In fact, available P, soil organic matter, and AMF density tended to increase along the gradient, and the fungal community composition changed. These trends yield promising clues about the use of AMF in restoration of tropical trees to pasture in the future.
Deforestation causes soil erosion especially in the humid tropics where rainfall is heavy and terrain is often steep. Land uses such as overgrazing and planting annual crops on slopes exacerbate the resultant land degradation. Consequent loss of productivity in this area of the world is on a collision course with increasing human population density and food demand. Because of the serious nature of erosion, its effects on tropical soil, especially biological characteristics that help re-establish soil fertility, need more study. Here we used apparent erosion intensity, land use history, and soil color to find eight sites representing an a priori spatial gradient of soil degradation on an overgrazed Costa Rican farm. We tested the gradient by measuring several chemical factors that indicate fertility of these tropical Ultisols. These factors decreased with increasing degree of soil degradation. Next we assessed spore density and diversity of arbuscular mycorrhizal fungi (AMF) along the gradient. We found that the diversity and composition of AMF changed across the gradient although not in the same pattern as the chemical factors. Finally, three years of vegetative regeneration after cattle exclusion had not improved the soils chemically but some improvement in AMF status was suggested for the less damaged sites. These results show that local farmers can use common sense cues to determine the chemical and biological status of their soils, that they can use these cues in future land use decisions such as planting hardy trees in the most degraded sites, and that they must expect severely degraded sites to require many years for recuperation. It is possible that intervention to improve the AMF status of soils could hasten recovery, since this process seems to be the first to occur.