Tree diversity and forest structure in northern Siberut, Mentawai islands, Indonesia

We assessed tree diversity, species composition and forest structure of 3.4 km(2) of primary rainforest in an important conservation area in North Siberut, Indonesia. We used floristic data from 34 20 m x 20 m plots for trees >= 20 cm diameter at breast height (dbh), 34 10 m x 10 m plots for poles (10-19 cm dbh) and 34 5 x 5 m plots for saplings (<= 9cm dbh). In total, 139 species were recorded from 93 trees. 57 poles and 86 saplings, with an estimated total species richness of 180 (Michaelis Menten species richness estimator). Overall, 70 genera and 35 families were represented, with Euphorbiaceae, Myrtaceae, Lauraceae and Moraceae being most important in terms of species diversity and Myristicaceae in terms of total individuals recorded. At species level, species of the genera Mallotus and Knema, as well as Baccaurea sumatrana were most dominant in all dbh classes based on the Importance Value Index (IVI), but only six species (none of which a dipterocarp) reached IV indices of >= 15, reflecting the high eveness and low dominance of dipterocarps in the tree community. Among trees >= 20 cm dbh, >50% were within the 20-39 cm dbh class and 73% of trees formed a canopy between 6 and 20 m in height, whereas emergent dipterocarps reached heights above 55 m (Dipterocarpus sp.) and diameters above 230 cm (Shorea pauciflora). The mixed composition of this forest emphasizes its significance for the biodiversity of the region.