Restoration of Areas Invaded by Aggressive Vegetation

In several cases a previously forested area is invaded by aggressive grass, as in the case of Imperata cylindrica in Indonesia, Imperata brasiliensis in Brazil, Sacharum spontaneum in Panama, and Pennisetum purpureum in Africa, or by ferns. The competitive advantage of grasses, combined with degraded soils and lack of nutrients, often prevents the germination and initiation of tree seedlings (Kuusipalo et al. 1995). These grassland areas are also often maintained by fires which inhibit natural colonization by tree species (Chapman and Chapman 1996). In these cases, if the area is left for natural regeneration to take place, it may stay in an arrested stage of succession for many years. Often the best alternative in order to restore these areas and revert

Fig. 6.16. Enterolobium contortisiliquum (timbo), a nitrogen-fixing tree, in a 9-year-old mixed plantation with other native species on a degraded site in Eldorado, Misiones, Argentina. This species is also used in agroforestry combinations with annual and perennial crops in the region. (Photo: F. Mon-tagnini)

Fig. 6.16. Enterolobium contortisiliquum (timbo), a nitrogen-fixing tree, in a 9-year-old mixed plantation with other native species on a degraded site in Eldorado, Misiones, Argentina. This species is also used in agroforestry combinations with annual and perennial crops in the region. (Photo: F. Mon-tagnini)

them back to forest or convert them to plantations is planting fast-growing trees, often exotics.

In many cases it is not feasible to plant tree seedlings without first removing the invasive vegetation. In such cases the use of herbicides, fire, or mechanical weeding may be necessary to clear the area and allow tree seedlings to establish successfully. In some cases treatments may involve tillage, or root removal of the invasive plant, as done in experimental settings in Sri Lanka to eliminate the invasive fern Dicranopteris linearis (Cohen et al. 1995).

Following treatment to eliminate the invasive vegetation, planting of fast-growing tree species, in many cases exotics, can be a first step to initiate tree cover, suppress the grass, and ameliorate the environment, thus facilitating the establishment of other tree seedlings that may be brought later to restore the original forest, or to give origin to a mixed or a monospecific plantation, depending on the objectives of the restoration project. In Indonesia, successful experiences have been reported on the use of species of Acacia to restore sites dominated by Imperata cylindrica (Kuusipalo et al. 1995; Otsamo et al. 1999). In the Kibale forest in western Uganda, exotic softwoods (Pinus cari-baea, Cupressus lusitanica) were planted in the 1960s and 1970s to convert grassland areas into wood fiber-producing sites. In the mid-1990s, tree regeneration was assessed under plantations and compared with adjacent forest. Diversity and abundance of natural woody regeneration were higher under pine than under cypress: the pines apparently provided quick shading which suppressed the grass earlier than the cypress. The composition of the advanced regeneration under plantations differed from that under natural forest. Plantations had similar numbers of species under their canopies compared to natural forests, but these were less valuable species, especially within the commercial classes (Fimbel and Fimbel 1996). However, the application of silvicultural treatments, including enrichment planting, was suggested as an alternative to improve the restoration value of these plantings. Another study at the same site evaluated the role of Pinus caribaea, Pinus patula and Cupressus lusitanica, and found that although the exotic species had been effective in suppressing the grass, and were an economically attractive alternative, lack of enough regeneration of valuable timber species under their canopies made the exotic plantations not as convenient in the long term, and recommended more research to test the possibility of planting indigenous tree species instead of exotics (Chapman and Chapman 1996).

In areas of the Panama Canal watershed invaded by Sacharum spontaneum, PRORENA is testing the effectiveness of native tree species to suppress the grass and restore natural forest, using mixed plantation designs (PRORENA 2003; Fig. 6.17). Prior to planting the trees, a series of treatments to combat the grass include cutting the grass, letting it dry for 1-2 weeks, burning to remove accumulated organic material, spraying herbicide (usually Roundup) after the grass resprouts, and cutting again if necessary (M. Wishnie, pers. comm.).

Other alternatives to initiate ecosystem restoration in areas invaded by aggressive vegetation may involve planting herbaceous species to protect the soil surface, retain soil moisture, improve soil fertility, and retard ground fires. For example, in the central hills of Sri Lanka, four species of nitrogen-fixing leguminous species were used to initiate site amelioration in succes-sionally arrested grasslands: Calapogum mucunoides, Centrosema pubescens, Desmodium ovalifolium, and Cymbopogon nardus (Ashton et al. 1997b). D. ovalifolium was the most successful ground cover at sites with high levels of herbivory, but under conditions of low herbivory the other species were most adequate due to higher biomass production. Establishment of these species was also affected by low soil fertility and pH, and therefore these conditions should be remedied to ensure their success.

Fig. 6.17. An area of the Panama Canal watershed invaded by the exotic grass Sacharum sponta-neum, where PRORENA is testing the effectivenes of native tree species to suppress the grass and restore natural forest, using mixed plantation designs (see text for details). (Photo: F. Montagnini)
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