The mixing ratio of tree functional groups as a new index for biodiversity monitoring in Bornean production forests

Abstract

While tree community composition can be a robust index for biologically indicating impacts of forest managements on tropical rain forests, the need of taxonomic expertise and accompanying costs may become a burden for foresters. The mixing ratio of two distinct functional groups (i.e., indicator genera for mature and disturbed forests) could be an alternative to the previous index that requires the identification of all species/genera. However, tree taxa indicative of logging intensity in a wide geographic range have not fully been determined. In this study, we conducted a large-scale vegetation survey in six forest management units (FMUs) spanning geographically distinct regions of Borneo (East Kalimantan in Indonesia, and Sarawak and Sabah in Malaysia) to detect a set of common indicator genera for logging intensity. A total of fifty 20-m radius plots were established in forests with a wide range of disturbance regimes in each FMU. A cluster analysis that was applied to all data combining the six FMUs identified two main vegetation types; one was the mature forest (with greater above-ground biomass, AGB) and the other was the disturbed forest (with lower AGB). Subsequently, 92 genera were derived as indicators for the mature forest by an indicator analysis, whereas 8 genera were derived for the disturbed forest. The mixing ratios of the two indicator groups per plot [(densities of mature-forest indicators – densities of disturbed-forest indicators)/densities of all trees] significantly correlated with another index of forest degradation (i.e., remaining AGB). When the number of indicator genera was greatly reduced by removing all genera other than those of Dipterocapaceae for the mature-forest indicators and Macaranga and Neolamarkia for the disturbed-forest indicators, the mixing ratios of indicator groups still significantly correlated with the index of forest degradation (remaining AGB). Because the two indicator groups with a minimal number of genera can be easily identified by local foresters, we conclude that the mixing ratio is a robust and practical index for the responses of tree communities to logging disturbances in Bornean tropical production forests.