Conservation and sustainable use of tropical trees in the genus Aquilaria II. The impact of gaharu harvesting in Indonesia Original Research Article

Aquilaria spp. are the main source of gaharu, one of the most valuable non-timber products harvested from tropical forests. In order to assess the impact of gaharu harvesting on populations of Aquilaria spp. in Indonesia, the activities of gaharu collectors were assessed by accompanying them on collecting expeditions. Gaharu harvesting generally involved felling the tree, with 31–92% of trees encountered being felled. The quantity of gaharu obtained from each felling was extremely low, mean values ranging from 0.10–0.18 kg tree −1 to 0.19–2.13 kg tree−1 for high and low grade gaharu respectively. A combination of these values with gaharu trade figures for the early 1990s enabled the number of Aquilaria spp. trees harvested annually in Indonesia to be estimated. Values ranged from less than 30,000 to more than 100,000 trees felled, depending on the year. Field assessments of two populations of Aquilaria spp. were used to parameterize a matrix model, which was used to predict the impact of different harvesting regimes on population dynamics of the species. Values of the dominant latent root (λ) were 1.22 and 1.20 for A. malaccensis in West Kalimantan and A microcarpa in East Kalimantan respectively, indicating that both populations are self-sustaining in the absence of harvesting. Analysis of λ under different harvesting scenarios indicated that for A. malaccensis, population expansion will continue if harvesting is set at a minimum diameter at breast height (dbh) of above 10 cm, but for A. microcarpa, a population decline will occur if trees with a dbh of less than 30 cm are harvested. Given current harvesting practices, it is, therefore, unlikely that gaharu is being sustainably harvested at present. These results suggest that the gaharu trade may have had a substantial impact on the population size of Aquilaria spp. in Indonesia, and their implications are discussed in the context of setting harvest quotas for regulation of trade, as required by CITES.