Long-term responses of populations and communities of trees to selective logging in tropical rain forests in Guyana

Abstract

Since only a small area of Guyana's forest can be effectively protected and because timber harvesting is an important source of income, logged forests will play an important role in the conservation of biodiversity in Guyana. Selective logging, in which only a few trees per hectare are harvested and after which forest remains available, is potentially a good way to utilise the forest without destroying it. In Guyana hard wood from selective logging is an important source of income. As in other tropical countries, sustainable forest management should result in sustained timber yields over long periods of time to provide lasting revenues and to secure livelihoods, while on the other hand also diversity should be conserved as much as possible. To be able to define criteria for sustainable forest management, information on the long-term effects of logging is needed.

Selective logging creates openings in the forest canopy, which results in increased light availability in the forest understorey. As a consequence of this increased light availability some tree species (the pioneers) are able to grow much faster. On the long term this may result in changes in species composition of the forest. The aim of the investigations described in this thesis was to determine the long-term effects of logging on tree population dynamics, forest composition and tree diversity and to evaluate the sustainability of alternative forest management scenarios for both future timber yields and biodiversity conservation. To investigate these long-term effects, a field study was done in logged and non-logged forests in Guyana and additionally a forest simulation model was developed to evaluate different management scenarios. This population dynamics model simulates growth, mortality and recruitment of trees and makes projections of forest composition and available hard wood in the course of decades.

The results of the field study showed that increased light availability after logging is especially advantageous for pioneer species. The abundance of inherently slow growing tree species decreased, but recovered again in the course of years after logging. Model simulations showed, however, that selective logging did not severely affect forest composition. Even in simulations of the most intensive way of logging (12 trees ha-1, every 25 years) forest composition remained rather intact. This is probably due to the fact that in forests in central Guyana, pioneer species are very rare and thus will not easily dominate the forest after logging. After logging once using high harvest intensities of 12 trees ha-1, it took, however, more than 100 years before harvestable timber volumes were comparable again with a baseline (non-logging) situation. For slow growing tree species it even took more than 160 years after logging before the abundance of stems was comparable again with the baseline situation. Projected recovery periods were, however, substantially longer than the currently in Guyana advised length of felling cycle of 60 years. Highest total timber yields were achieved if trees were harvested every 25 years using high harvest intensities. At the same time this approach also resulted in a fast depletion of the available commercial timber volumes in the forest and thus reduced timber yields. The results of the investigations in this thesis can be used to determine criteria for sustainable forest management in Guyana.