Recovery of Biodiversity in Degraded Lands Plantations of Native or Exotic Species

In abandoned areas, natural forest regeneration is often significantly delayed by physical or biological barriers. The establishment of plantations may overcome some of these barriers by attracting seed dispersal agents into the landscape and by ameliorating local microclimatic conditions within the area, thereby accelerating the recovery of biodiversity (Parrotta 1992; Powers et al. 1997; Lamb 1998).

The establishment of tree plantations in degraded areas may facilitate regeneration of native species that could not otherwise establish in open microsites or in competition with herbaceous species. Several authors report on the role of tree plantations as a catalyst of natural succession in tropical and subtropical sites (Parrotta 1992; Keenan et al. 1999; Parrotta 1999). There are several examples where exotic tree species have been effective in suppressing invasive grass vegetation and recruiting native species under their canopies, as in some of the cases mentioned in the previous section. In Puerto Rico, in the understory of plantations of the exotic tree Albizia lebbek, 22 species of trees and shrubs were found, in comparison with just one species in control plots without trees (Par-rotta 1992). Even species that have been claimed to have harmful effects on un-derstory plants such as eucalypts have been used with relative success: for example, in highland Ethiopia, 83 plantations of exotic species and adjacent forest stands were analyzed for herbaceous plant cover and biomass, species richness and soil characteristics. The exotic plantation species evaluated included Eucalyptus grandis, E. globulus, E saligna, Pinus patula, and Cupressus lusitanica.

Fig. 6.18. Mixed plantations with native tree species had relatively high abundance and high numbers of regenerating species in their unders-tory, in comparison with pure plantations, in research plots at La Selva Biological Station in Costa Rica (see text for details). (Photo: F. Mon-tagnini)

Fig. 6.18. Mixed plantations with native tree species had relatively high abundance and high numbers of regenerating species in their unders-tory, in comparison with pure plantations, in research plots at La Selva Biological Station in Costa Rica (see text for details). (Photo: F. Mon-tagnini)

Plantations were over 9 years old and had been planted at 2 X 2 m distance. The richnesss and biomass of herbaceous species in plantations of eucalypts and pines were as high as in natural forest, although most of the species found under plantations were widespread species, mainly weeds or species invading from montane or wooded grassland (Michelsen et al. 1996). Natural regeneration was much less abundant under cypress than under eucalypts or pines; therefore the authors do not recommend planting cypress due to the risk of soil erosion.

However, native species can be as effective or even more effective than exotics in suppressing grass and recruiting native species under their canopies. For example, in north Queensland, Australia, a greater diversity of species was found in the understory of plantations of native species than in plantations of exotic species (Keenan et al. 1999). At La Selva Biological Station, Costa Rica, results of some studies also suggest that tree plantations of indigenous species have a good potential for accelerating the processes leading to recovery of biodiversity in degraded soils (Guariguata et al. 1995; Powers et al. 1997; Carnevale and Montagnini 2002; Cusack and Montagnini 2004; Box 6.14).

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