Demography of exploited tree species in the Bolivian Amazon

Abstract

Tropical forests are threatened world-wide. Therefore, there is a search for ways to use the forests in a sustainable way, as this could assist in the conservation of these special ecosystems. Non-timber products collected from trees in tropical forests are often mentioned as examples of sustainable exploitation. Their collection causes far less disturbance to forest structure and functioning than the felling of trees for timber. This PhD thesis reports on ecological studies on tree species exploited for non-timber products. Three tree species were investigated in the tropical forests of the Bolivian Amazon region. The demography of these species is analysed and the impact of exploitation on population dynamics and future productivity is assessed. Use is made of matrix population models to analyse the population dynamics of the study species. Two of the three investigated species yield products with good prospects for sustainable extraction (Brazil nuts and palm leaves), whereas the palm heart collected from the third species offers little potential for sustained use. Brazil nuts have been collected from the emergent tree Bertholletia excelsa for decades in the region, without a clear negative impact. In spite of the fact that around 90% of the seeds are harvested, seedlings can be found in reasonable densities and demographic models predict a stable size and a continued rejuvenation of exploited populations. The Brazil nut tree may reach an age of over 300 yr, and becomes reproductive only at the age of 120-140 yr. Therefore, if there would be an effect of nut collection on future nut production, this would only become apparent after several decades. Leaves of the small understorey palm Geonoma deversa are cut for high-quality thatching material. The collection of leaves does not infer permanent damage to the plant, and all harvested leaves are replaced by new ones in approximately three years. Reproductive activity (flowering, fruiting and cloning) is strongly reduced after leaf cutting, but defoliation did not cause an elevated mortality risk. Population growth was only moderately reduced after leaf cutting. Model results show that the availability of leaves at population level recovers rapidly after simulated harvests. When harvests are repeated once every 16 years, leaf availability may be sustained at reasonable levels. The third product, palm heart, is collected from the canopy palm Euterpe precatoria. Collecting palm heart - the meristem of the plant - involves the cutting of the entire reproductive individual, and thus causes the death of the individual. As reproductive individuals of this species are approximately 90 yr in age, exploitation has a strongly negative impact on the population. Cut palms have to be replaced from seeds, as the species does not reproduce by clonal propagation. Demographic models show that the survival of adult palms is crucial to sustain population growth, and model simulations show that the availability of palm heart can only be sustained when a low proportion of reproductive individuals is cut and populations are left undisturbed for long periods between harvests. The current harvest practice - cutting around 90% of the adults and repeating this as soon as possible - clearly can not be sustained for longer periods. In addition, the thesis contains two chapters focusing on technical aspects related to the type of demographic models used for the case studies - matrix population models. An additional type of sensitivity analysis for matrix models is presented, which takes into account the inherent variability in demographic rates. Furthermore, the application of matrix models to analyse the demography and sustainable use of woody plants is reviewed