Changes in species composition following silvicultural treatment in Costa Rica

In Costa Rica, results of a silvicultural experiment carried out by CATIE in a primary forest focused on the effects of logging and post-harvest silvi-cultural treatments on forest species richness and composition during the first 6-7 years following logging and 5 years following the application of silvicultural treatments (liberation and refinement) (Montagnini et al. 2001). The forest studied exhibited marked compositional variation in relation to a topographical gradient after the implementation of the experiment; such ft- or "ecosystem diversity" should be taken into account in evaluations of the effect of forest management on plant biodiversity. In this forest, post-harvest silvicultural treatments caused an immediate reduction in species richness in individuals greater than 10 cm dbh. This was due to the chance elimination of species represented by one or a small number of individuals in the plots. It was concluded that post-harvest silvicultural treatments may affect overall species composition by favoring commercial species. However, no changes of species richness or composition were evident in the forest understory (individuals between 2.5-9.9 cm dbh). The direct felling and extraction of timber caused the only detectable changes in understory plant biodiversity and these changes were found only in the localized areas disturbed by these management operations.

Another study in Latin America reports that conventional low-intensity mahogany harvesting in lowland Bolivia has only a relatively mild physical effect on the forest. However, it is doubtful if this would hold true for the much more intensive harvesting characteristics of the eastern Amazon or the dip-terocarp forests of Southeast Asia (Pearce et al. 2003).

In the forests of Central Africa, Hall et al. (2003) compared forest structure and tree species composition between unlogged forest and forests that had been subjected to highly selective logging 6 months and 18 years prior to the study. While there was little difference in tree species composition and diversity between treatments, stem densities were significantly higher in the un-logged forests than those in the forest sampled after 18 years since logging. In the logged forest there was insufficient recruitment of the principal timber species Entandrophragma spp. (African mahogany). However, many other quality timber species remained after selective logging, making logging still attractive long after elimination of the preferred species.

Some species function as indicator species, meaning that their presence is indicative of high biodiversity and a well-functioning ecosystem (Lindenmayer et al. 2000). Such ecosystem functions can include stand structural complexity, plant species composition, connectivity, and heterogeneity. Carefully designed studies are needed to test the relationships between the pres ence and abundance of potential indicator species and the maintenance of ecosystem processes in forests.

In conclusion, SFM techniques can contribute to maintaining biodiversity in tropical forests managed for timber production. Application of silvicultural treatments following harvest may immediately reduce species richness by eliminating species represented by few individuals. The effects of forest management operations on plant biodiversity depend on the nature of the operation. Studies are clearly needed to assess the long-term effects of forest management on plant and animal biodiversity. Any systems designed to restore degraded ecosystems should be focused on recovering at least part of the lost biodiversity (Montagnini 2001).

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