Samenvatting

The number of primary rainforests in South-East Asia is in rapid decline since many formerly continuous forests become splintered as a result of human activities like mining, agriculture and silviculture. This study examined the effects of forest fragment size and logging on the tree carbon stocks and dead biomass proportions in lowland dipterocarp forests of Sabah, a Malaysian state on Borneo. Forest inventories were completed using plots along transects in 12 forest fragments (12–3529 ha) with different management histories; six locations were selectively logged before fragmentation, where the other six were undisturbed pre-fragmentation. Two continuous forest sites (~8000km2) were included and measured as a baseline for unfragmented logged forest. Using field data from 1266 trees the total tree biomass was calculated, which was then used for carbon stock estimations per forest fragment. This resulted in a high variability of carbon stocks, ranging from 13,8 t C/ha in one of the smaller unlogged fragments (120 ha) to 111,2 t C/ha in the continuous forest. Significant differences in aboveground carbon stocks were encountered when comparing the forest fragment sizes with a multiple linear regression analysis, proving that both logged and unlogged forest fragments contained significantly lower carbon stocks as their size decreased. This analysis also showed a significant difference in aboveground carbon stocks between forests that were logged and forests that had stayed undisturbed, the latter containing higher carbon stock values. Although the same analysis concluded almost no difference in dead biomass throughout the different locations, the relative dead biomass ratios compared to living biomass (range 1,0%–30,3%) showed very clear connections for both logging impacts and fragment size. Differences in carbon stock proportions of dead wood were highly significant (p > 0,01) with respect to both tested factors, with increasing dead/living biomass ratios as forest fragments became smaller or logging had occurred. Findings like these are important steps towards an improved understanding of forest dynamics, especially with human-induced disturbances. The results of this study call for alterations in future forest management as creation of additional small forest fragments should be avoided, wherever possible. The parties responsible for future forest management should attempt to abstain from timber extraction or implement less intensive logging techniques (e.g. Reduced Impact Logging). Although the effect of fragmentation and logging was measured in this study, additional sampling will be invaluable for identifying more accurate carbon stock models and formulating concrete management guidelines accordingly.