Tree diversity and forest composition of a Bornean heath forest, Indonesia

Heath Forest on Borneo is a forest type mostly found between the transitions from the peat-swamp forests in lowlands to the Dipterocarp Forest in hillier areas. Heath forest soil structure is characterized by a dominant sand layer with a thin organic horizon on top. Tropical Heath Forest has been understudied and undervalued for many years, considered as a low-diversity habitat due to a nutrient-poor sandy soil, unable to aggregate and bind nutrients, resulting in stunted, low canopy forest. Despite its reputation, our research in the KHDTK Mungku Baru(it is a Forest Area with Special Purpose (KHDTK)) in southern Central Kalimantan is revealing heath forest of remarkably high quality, with large trees, high canopy and high biodiversity, including important endemic and emblematic flora and fauna species. Fourteen 30 m x 30 m nested plots were established, where all the trees ≥ 10 cm diameter breast height (DBH) were identified and measured. Additional 5 x 5m and 2 x2 m nested plots were used to define the micro-habitat and soil composition, determining the pH, organic layer depth, water moisture, and canopy cover. A total of 1,007 trees were recorded, representing 87 species in 58 genus and 40 families. Soil layers exhibited visual differences and analysis of soil composition was used to define 2 sub-habitat types of Heath Forest, named as Black sand Heath Forest (BHF) and White sand heath forest (WHF). Significant differences were found on peat/organic layer depth (BHF 26.4 cm; WHF 6.1 cm) and soil moisture (BHF 18.3 %; WHF 11.1 %). Independent analysis for the tree community compositions was done for each sub-habitat type. Few differences were found between sub-habitat types on DBH (BHF 18.9 ± 11.7 cm; WHF 18.6 ± 9.7 cm) and tree height (BHF 18.9 ±5.5 m; WHF 17.9 ± 5.4 m). Tree species composition analysis reveals in both cases a high species richness, with no differences in distribution (Simpson's Diversity Index BHF=0.96; WHF=0.95) but significant differences in species abundance (Fisher's alpha index BHF=23.74; WHF=18.01). BHF is a more homogeneous sub-habitat type, dominated by species belonging to Dipterocarpaceae (19.5 %), Myrtaceae (19.2 %) and Sapotaceae (13 %) families, whereas WHF is largely dominated by Dipterocarpaceae species (43.4 %) including Dipterocarpus borneensis and Shorea teysmanniana. The preliminary results of this study indicate that the Central Kalimantan's Heath Forest presents high tree species diversity with high conservation value, with potentially 2 different sub-habitat types characterized by their soil and tree composition. This highlights the importance of protecting this rare and understudied habitat in the context of Borneo biodiversity conservation.