Structure and diversity of dipterocarps in a lowland dipterocarp forest in East Kalimantan

Dipterocarps were studied in three permanent plots of four hectares each in Berau, East Kalimantan (Indonesia). The structure, species composition and the spatial distribution of dipterocarps were analyzed and compared between plots. Dipterocarps represented 25% of the total tree population (DBH > 10 cm) and about 50% of the total basal area. #Shorea# and #Dipterocarpus# species are the main emergent and upper canopy trees whereas #Vatica# and #Hopea# are mostly present in the lower strata and the understorey. Among the 61 species identified in the plots, #Shorea# and #Dipterocarpus#, with respectively 31 and 12 species, were dominant, amounting to 70% of the recorded dipterocarp species. The other genera were #Vatica# (8 species), #Hopea# (4 species), #Parashorea# (2 species), #Anisopiera# (2 species), #Cotylelobium# and #Dryobalanops# (1 species). The species composition in relation with topography and soil properties was also analyzed and the role of physical factors discussed. In this Berau area, the structure and density of dipterocarps are similar to those found in other parts of Borneo, in Sabah or Sarawak. However, this forest shows remarkably high dipterocarp richness although the Western part, particularly Sarawak, has always been regarded as richer in dipterocarps than the Eastern part of Borneo Island (Sabah). This could result from the scarcity of botanical forest inventories in large areas of East Kalimantan. The comparison of the dipterocarp composition of the three plots revealed local floristic variations. The commonest species (#Dipterocarpus confertus#, D. #humeraius#, #Shorea hepeifolia#, #S. parvifolia#, #S. parvistipulata#, #S. pauciflora#, #S. pinanga#, #S. smithiana#, #Vatica oblongifolia#, #V. vinosa#) were recorded on all types of soil and topography. #D. acutangulus# distribution seems to be linked to the combination of both soil drainage and texture. However, for most of the dipterocarps, correlation between species distribution and soil characteristics could not be clearly defined. (Résumé d'auteur