Many tropical forests have been subject to low-intensity human management for centuries: this management generally preserves environmental functions and species diversity (Gómez-Pompa 1991). Because of increased awareness of the importance of sustainability and the preservation of biodiversity, many tropical countries have recently changed forest management regulations to make them compatible with the principles of sustained yield and biodiversity preservation (Boyle and Sayer 1995). The ITTO and Forest Stewardship Council principles, criteria, and indicators for SFM give special emphasis to conservation of biodiversity. Specific guidelines are needed to cover the vast array of forest conditions that are present in each situation: species, soils, environmental constraints, markets, and other factors. The management guidelines should be adjusted to suit the scale and objectives of management in each particular case. In addition, methods are needed to evaluate ecological indicators that can serve to verify effects of management on long-term forest productivity and maintenance of biodiversity (Lowe 1995).
Operational definitions of diversity are required to provide practical and consistent frameworks for measuring and monitoring biodiversity. Typically, four different levels of diversity have been considered for measuring and monitoring biodiversity: genetic, species, ecosystems, and landscape levels (Boyle and Sayer 1995). Species diversity is conceptually the simplest of the four levels of organization of biodiversity, both because many species are visually distinct and because species extinction is an emotionally dramatic event. Practical measures of biodiversity tend to focus on the species level. Studies of genetic diversity have been done for several tree species of commercial value, and in some cases they have led to regulations about species conservation (e.g., a ban on logging mahoganies in Costa Rica; Navarro et al. 2002).
There are many published studies of the effects of forest management on biodiversity. For example in a forest reserve in Misiones, Argentina, a "Uniform Spacing" (US) method of forest harvest was used, where trees were selected for extraction or marked for retention according to scarcity, horizontal distribution, and quality as seed trees (Fig. 5.5). The system is called Uniform Spacing because the trees remaining after its application tend to be uniformly spaced. Generally the cutting intensity in the US method is about half that of conventional logging methods. In this case, no silvicultural treatments were applied following timber harvest. Forest regeneration following timber harvest was compared between the US method and conventional logging methods to see the effects of the different harvest techniques on forest biodiversity. Three years after harvesting, the forest cut by uniform spacing had the highest total density of seedlings for commercial and non-commercial species, and also exhibited
Fig. 5.5. Ocotea puberula trees are generally extracted from forests in Misiones, Argentina, due to their high timber value. However, in the Guarani Forest Reserve, when cutting was done using the Uniform Spacing method, this individual was left standing, while a nearby tree of the same species was cut (note remaining stump in foreground). (Photo: F. Mon-tagnini)
higher diversity of understory plants compared with forest cut using conventional logging methods (Montagnini et al. 1998). In the US system the resulting forest had higher variability of microenvironments which led to the establishment of a greater variety of species. In contrast, in the conventional cutting method the intensity or extraction was higher, resulting in greater canopy openings, which led to the establishment of more bamboo, ferns, and grasses. Other studies in Costa Rica also report on changes in forest composition following post-harvest silvicultural treatments (Box 5.6).
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