Tropical heath forests comprise a unique habitat prevalent in Borneo, covering approximately 10% of the land surface. Heath forests (HFs) are home to many rare species, and have specialised vegetation with high nutrient-use efficiency, suggesting that the highly acidic sandy soils (podzols) that underlie these forests cause vegetation stress. Indeed, the local name, kerangas, meaning ‘land that will not grow rice’ recognises the edaphic stresses posed by this fascinating forest ecosystem (refs 1, 2).
Despite the high conservation value of heath forests, their relative scarcity, and increasing risk of disturbance/degradation, little is known about how these forests form and what factors control their distribution, diversity and ecosystem functioning. The profusion of insectivorous plants in the forest, and the characteristic thick, small leaves of the vegetation, suggest that HFs are strongly limited by soil nutrients, primarily nitrogen (refs 3, 4). If this is correct, the large increases in nitrogen deposition projected for the region (ref 5) may alter the species composition and hence influence multiple ecosystem functions of these tropical heath forests. To understand the relative impact of nitrogen (N) and soil acidity on soil properties, tree growth and performance in heath forest, in 2016, we established a fertilisation/liming (to reduce soil acidity) experiment in the Sepilok forest reserve in Sabah (Malaysian Borneo) (ref 6, 7). Results suggest that while soil acidity is important in explaining tree species distributions in heath forests (ref 6) and influences soil mesofaunal activity (ref 7), N appears to play a greater role in litter decomposition (ref 7) and tree growth (ref 3).
Soil resource availability can affect both above- and below-ground biodiversity, with knock-on effects on multiple ecosystem functions (ref 8). Yet, it is unclear which aspects of forest biodiversity and their associated traits might affect particular ecosystem functions. Capitalising on the established experimental research infrastructure, and support from UK supervisors and Malaysian colleagues, this PhD project will further explore how soil properties influence the functioning of tropical heath forests. The PhD student will be encouraged to develop the project to address research questions in line with their particular interests. Questions will address how long-term (5+ years) N and lime addition affects the biodiversity of different forest-dwelling taxa and their traits, and link these to measurable aspects of forest functioning and ecosystem service delivery. The results will help to predict the impact of N deposition on heath forests’ biodiversity and organismal multi-functional relationships.