David Bartholomew, Lindsay Banin, Paulo Bittencourt, Mohd Aminur Faiz Suis, Lina Mercado, Reuben Nilus, David Burslem, Lucy Rowland
The tropical forests of Borneo in South-East Asia possess exceptionally high levels of biodiversity, act as important carbon stores and are home to the world’s tallest tropical trees. However, these characteristics are not evenly distributed across the island. Instead they vary widely according to the underlying soil. Forests found on soils richer in nutrients have taller trees, more complex multi-layered canopies and greater aboveground biomass stores. Different soils also support different tree species, with many confined to a narrow range of soil fertility. However, we still lack an understanding of how soils affect the physiology of Bornean trees and whether specialised adaptations drive the patterns of diversity we find across soil fertility gradients.
We investigated the role of soil properties on the physiology of 13 dipterocarp species in Northern Borneo. Dipterocarps are a dominant floristic component in these forests – trees belonging to this family comprise over 20% of all tree stems and over 50% of large, canopy trees. They are also highly threatened with extinction, meaning an improved understanding of their physiology can inform where we may be able to protect them and where they could be planted for restoration. We measured a range of leaf traits, including the capacity to assimilate and respire carbon in 218 trees ranging in height from 4 m to 66 m across a soil fertility gradient.
The capacity of trees to assimilate carbon increased with soil nutrient availability, but leaf respiration was higher in forests with lower soil water availability and those that experienced sporadic flooding. Soil fertility and tree height explained variation in leaf traits, suggesting both nutrients and height limit leaf metabolism in Bornean forests. On nutrient-rich soils, nitrogen and phosphorus limited carbon assimilation, but on nutrient-poor soils, calcium, potassium and magnesium were the main limitation. Our results suggest dipterocarps are highly specialised to soil conditions and that vertical changes in the environment drive changes in leaf physiology. Our findings reveal how soil environments are partitioned by dipterocarp species and that matching species to soil conditions is likely to be necessary for successful restoration of these forests.