Tree species traits response to different canopy cover for 34 tree species in an enrichment planted tropical secondary rain forest in Sabah, Malaysia

Abstract: Tropical rain forests only cover around six percent of the world’s land area and contain around 70 % of the world´s animals and plants. Tropical rainforests have been, and still are, negatively affected by human activities. These activities lead to forest degradation which has negative impacts on production and biodiversity. Between 1970-1980 forests in Borneo, Sabah, Malaysia, was subjected to intense logging. In 1982 and 1983 Sabah was exposed to a prolonged drought followed by wildfires which further ruined the already damaged forests. In 1998 the Yayasan Sabah Group and IKEA started a forest rehabilitation project in the degraded forest in the Kalabakan region in Yayasan Sabah Concession Area in Malaysia. The main aim of the program is to improve the biodiversity status in the degraded tropical rain forest within the project area. To improve or accelerate the regeneration in the rehabilitation areas, enrichment plantings are used. Information about tropical tree species traits is limited, which complicate the rehabilitation work. Functional traits for plants are traits that affect growth, reproduction and survival and hence the plants fitness to the environment. Tree species used to be grouped after their demand for light and growth pattern. Light demanding pioneer species often have low wood densities and fast growth and shade tolerant climax species tend to have high wood densities and slow growth. However, the pioneer-climax classification is considered as a continuum. The objectives of this study were to investigate how different canopy cover affected tree species traits for 34 tree species in an enrichment planted tropical secondary rain forest, and if tree species growth response to increased light after a shade adjustment was related to tree species specific wood densities. The results indicated that tree individuals with least available light generally had smaller dbh (diameter in breast height) and lower tree heights compared to tree individuals with higher light availability. The maximum measured dbh and heights for tree individuals decreased with increasing wood density. Increased dbh growth and horizontal crown growth for the investigated tree species were observed after the shade adjustment. The increased growth was correlated with wood density which showed to be negatively correlated both with dbh growth and horizontal crown growth. A negative trend between wood density and height growth after the shade adjustment was found, but this was not statistically proven. The investigated tree species in this study indicated different adaptations to light depending on wood density even though they all are classified as climax species. Information from this study, on how species with different wood density, respond to light, can be used to adapt the choice of tree species for the current lighting conditions in the forest. This information can improve the rehabilitation work in the INIKEA Forest Rehabilitation Project and elsewhere in Malaysia.