Trees slow down erosion and allow soil progression in an extremely high-rainfall old-growth mixed dipterocarp forest of southwest Sri Lanka

Abstract

Recent studies have revealed that trees can significantly drive soil evolution and hillslope processes in some temperate forests through mechanical soil disturbances such as tree uprootings. Such tree-soil interactions have resulted in improved biogeomorphic state transition models for old-growth forests. However, the situation in humid tropical forests, with extreme precipitation and different soil-forming processes, is less explored. Here, we focus on the erosion rates and biogeomorphic roles of trees in the Sinharaja Old-Growth Mixed Dipterocarp Forest, Southwest Sri Lanka. The site is notable for the fact that, despite high rainfall in this tropical primary forest reaching 5674 mm per year, there are well-developed Alisols. We hypothesized that the erosion rate would still be similar to the expected soil production rate (ca 1 t ha⁻¹ yr⁻¹), despite the precipitation, and that there would be a significant bioprotective function of trees.

Total denudation was assessed using various radiometrical methods, resulting in a long-term erosion rate ranging from 0.24 to 0.9 t ha⁻¹ yr⁻¹, and a recent erosion rate of 10.8 t ha⁻¹ yr⁻¹. The ability of trees to protect and stabilize soil was indeed significant in the Sinharaja forest reserve, with 3.65 t ha⁻¹ of measurably protected soil. However, trees also contribute to erosion as well, and interestingly, tree uprootings had a lesser impact than tree breakages.

Such biotic-abiotic interactions can allow the progression of soils and may be important in protecting soils during ongoing climate change.