Weak edge effects on trees in Bornean rainforest remnants bordering oil palm

Many tropical forests are dominated by edge habitat, with consequences for forest structure, carbon stocks and biodiversity. However, edge effects are highly variable and context-dependent, and are poorly quantified in oil palm landscapes. We studied edge effects in 10 lowland rainforest remnants bordering mature oil palm plantations on Borneo, by surveying 0.2 ha plots along transects running perpendicular to the forest edge (ten 1.6 km transects, 5-6 plots per transect; 57 plots in total). We examined how edge proximity affected plot-level forest structure (canopy cover, number and size of stems ⩾10 cm diameter), aboveground carbon stocks, microclimate (air temperature and light intensity), and tree community composition and richness. The largest trees were significantly smaller (up to 21% reduced diameter) in plots near edges, and plot-level carbon was up to 30% lower (model-fitted average = 64.7 Mg ha−1 at 50m from the edge, versus 92.3 Mg ha−1 at 1600 m), with the strongest effects within 300m of edges. However, these significant effects of edge proximity were relatively small in the context of existing variation, with distance-from-edge explaining <13% of the total variability in maximum tree size or carbon. Additionally, there were generally no effects of edge proximity on any other component of forest structure, composition or diversity, and only a weak effect on microclimate. We conclude that limited edge effects in this system may reflect low structural contrast between forest and mature oil palm, and limited invasion of pioneer trees from plantations, which diminished edge influence in highly heterogeneous forest remnants.

Methods
This dataset was collected in Sabah, Malaysian Borneo, between 13/07/2019 and 05/10/2019. Methodological details can be found in the associated publication in Biotropica.