Restoring dipterocarp rainforest diversity

Abstract: Rainforest restoration is an important application in today’s multipurpose management of secondary forests. In the Malaysian state Sabah (Borneo) most forests have been selectively logged, but many still hold potential for high biodiversity and carbon sequestration, as well as future timber production. Restoration for biodiversity is increasing, but knowledge concerning tree species growth and adaptations are insufficient for foresters to make good decisions for sustainable management. The PhD research was mainly conducted in the Sow-a-Seed project (Sabah) that started in 1998 to restore rainforest biodiversity degraded from logging and wildfire. Within the project area a common garden experiment was established in 2008, involving 34 tree taxa. There, tree species responses to increased light were predicted with the help of species traits, and related to a pioneer-climax continuum of life history characteristics. Some of these traits, e.g. wood density, specific leaf area and foliar nutrient concentration were major factors in predicting the trees growth responses. Also in the common garden, young trees growth under different levels of canopy cover were studied to examine the potential increase in growth under higher levels of light, and the occurrence of species specific light optima for growth. The tree species showed varied growth performances under different levels of light, e.g. that species with climax properties had optima at lower light levels than species with pioneer properties; and life-history traits were used to model these variations among species. Further, diversity patterns among naturally regenerated trees (≥10 cm diameter at breast height) were studied over 10 years after selective timber harvesting, using a gradient of harvest intensities (0–16 trees cut ha⁻¹). Results showed that for optimum biodiversity harvest intensities should not exceed 8 trees ha⁻¹; since species richness was highest at an intermediate level of extraction, after which it started to decline. Also the broadest range of traits was achieved at intermediate levels of disturbance. Tree species richness influence on the diversity of foliar lichens and canopy arthropods was also studied. Results indicated that a foundation species approach could be useful for selecting trees that promote high biodiversity. The work in this thesis highlights that the biodiversity of regenerating forests can potentially be enhanced with suitable post-disturbance management.