Can timber production be compatible with conservation of forest biodiversity? -- Two case studies of plant biodiversity in managed neotropical forests.

摘要

A variety of practices can greatly diminish the impacts of forest management on biodiversity. A case study from a subtropical humid forest reserve in the province of Misiones, Argentina, is presented to illustrate a uniform spacing method of forestharvest, which is contrasted with the prevalently used minimum diameter cutting method (this extracts commercial species only if they have a minimum diameter specified per species, in the range of 40-55 cm dbh -- diameter at breast height). In the uniform spacing method, trees are selected for extraction or marked for retention according to their scarcity, their horizontal distribution, and their characteristics to serve as seed trees. Three years after harvesting, the forest cut by uniform spacing hadthe highest density of seedlings of total as well as of commercial species, and it also exhibited high diversity of understorey plants other than trees. A case study of a silvicultural experiment carried out by CATIE in Costa Rica is also presented, focusing on the effects of logging followed by postharvest silvicultural treatments on forest species richness and composition during the first 6-7 yr following logging and the 5 yr following the silvicultural treatments. The logging operation was carried out under strict planning and control for sustainability, in 1989 and 1990; the total harvestable volume was 23 m3/ha, but actual harvest intensity was 42% of this. There were 3 postharvest silvicultural interventions: (1) a control, with no further treatment after logging; (2) a poison girdling applied in 1991 to eliminate most non-commercial trees ?0 cm dbh, and a subsequent liberation of potential crop trees; and (3) a shelterwood treatment applied in 1992, which maintained a high canopy while creating conditions for the regeneration of more light-demanding commercial species by chainsaw thinning the middle storeys. The forest studied exhibited marked compositional variation in relation to a topographical gradient after the implementation of the experiment; such beta- or ecosystem diversity should be taken into account in all future evaluations of the effect of forest management on plant biodiversity. Postharvest silvicultural treatments caused an immediate reduction in species-richness ?0 cm dbh in 1.0 ha due to the chance elimination of species represented by one or a small number of individuals in the plots, and may affect overall species composition in the medium term as they are directed to favouring commercial species by decreasing competition from non-commercial species. Species richness declined under both silvicultural treatments, although no changes of species-richness or composition as a result of silvicultural treatment were evident in the forest understorey (individuals 2.5-9.9 cm dbh). The only detectable changes in understorey plant biodiversity were caused by the felling and extraction of timber, and these changes were restricted to the very localized areas disturbed by these management operations. The application of low-intensity logging methods with selection of remnant trees requires detailed pre- and post-harvest forest inventories and follow up of treatments to ensure successful regeneration and growth of desired species. The Costa Rican study shows that different forest management operations (logging and silviculture in this case) may have different effects on plant biodiversity; this perspective should be applied to future studies so that the balance between production and conservation in tropical forests will be betterunderstood. Long-term follow up is needed to assess the true effects of forest management on biodiversity, including natural regeneration as well as growth dynamics of trees, until a whole harvest cycle is completed.