In Bwindi Impenetrable National Park (BINP) local community members are allowed to extract certain forest products in a controlled way, within so called ‘multiple use zones’ (MUZs). Since 2004, ITFC monitors the impact of harvesting on three selected species, by comparing their populations inside and outside MUZs. Ocotea usambarensis Engl. (Lauraceae), a tree from which bark is harvested to treat stomach aches and used for deworming, is one of them. It has remained unclear, however, whether current harvest levels for this species are sustainable.

In this study the impact of Ocotea bark harvesting was addressed with so called ‘matrix model analysis’, used for modelling population dynamics. For the construction of these models, annual growth of the trees’ diameter, survival probabilities and reproduction rates need to be assessed. ITFC’s monitoring data for 5 MUZs and 1 non-MUZ, collected over 5 years, were kindly availed for this analysis and monitoring data for 2010 and additional data to run the model were collected by me.

The goal of the study was to analyse the impacts of bark harvest on population structures and dynamics of Ocotea and to model the availability of the resource in the future. This will give an indication whether current harvest levels are sustainable.

A general description of the current harvest levels and impacts showed that almost all harvest took place on trees with a diameter thicker than 110 mm, which contain more bark than smaller trees. In most cases, only a small portion of the bark had been removed. Also, only a small percentage of the trees (3.4% of the total population) was harvested over the last years. There were no evidences that bark harvest had led to mortality of a single individual in our study and most wounds were healing. Harvest had taken place in MUZs as well as outside, and in fact was highest in non-MUZ areas. Probably the distance from the road and the occurrence of alternative species determine more where people harvest than regulations.

The population structure of Ocotea within each MUZ showed that the size class distribution was generally stable over the years. Most populations showed a distribution with more juveniles than adults, indicating natural, density-stable and self-replacing populations. Population structures differed between the MUZs, but this is probably due to differences in environment rather than in harvest levels.

For the construction of matrix population models ‘vital rates’ (diameter growth rates, survival probabilities and reproduction rates) were calculated. Unharvested and harvested individuals were separated in each area to assess and compare their growth rates. Currently, only a few individual stems have been tagged. To improve the estimations of the vital rates in these models, it is recommended to tag all stems for monitoring.

A matrix model was constructed for the harvested and unharvested population. Diameter growth rates of the harvested population were significantly higher than those of the unharvested population, which may be due to a non-random selection of harvestable trees. The survival probability was assumed to be equal in both populations. Reproduction rates were calculated in an alternative way, which resulted in higher reproduction rates of the harvested population. Both populations showed a positive population growth rate, but the population growth rate of the harvested population was higher, contradictory to findings in the literature. Simulation of stem availability in harvested and unharvested population showed an increase in total number of stems in the next twenty years for both. However, assumptions had to be made which may be questionable (e.g. constant environmental factors). We recognise that ecological differences across sites may have influenced the population growth rates as well.