Soil erosion affected by trees in a tropical primary rain forest, Papua New Guinea

Trees have the ability to distinctly determine soil evolution and hillslope processes through mechanical soil disturbances such as tree uprooting. Recent findings have resulted in new biogeomorphic state transition models for old-growth forests in the temperate climate zone. The situation in tropical forests, however, is mostly unknown. Here, we focus on determining short- (decades) and long-term (millennia) soil erosion rates and the biogeomorphic role of trees on hillslope dynamics in the species-rich Wanang primeval tropical forest, Papua New Guinea. We hypothesized that trees play a significant role in hillslope dynamics on a background of powerful soil erosion dominated by abiotic factors, and that this role has potentially intensified over the last decades. The long-term soil erosion was assessed using the meteoric ¹⁰Be levels in three soil profiles along the whole depth gradient along with one natural outcrop. The recent soil erosion rate was determined using ^239+240Pu levels in the uppermost soil layers. The specific role of trees in hillslope processes was evaluated based on repeated tree censuses of 65,535 tree individuals, calculating the tree mortality rate and modeling the soil volume affected by uprooted trees and by the in situ decay of the root systems of broken trees. Soil erosion was 6.25 m³ ha⁻¹ yr⁻¹ over the long term and indeed did increase during the last decades. While this rate was significantly affected by trees, they were not the main factor, with circa 0.95 m³ ha⁻¹ influenced annually by uprooting, and an additional 0.61 m³ ha⁻¹ by the in situ decay of root systems. These results reflect a forest ecosystem that is currently in the biogeomorphic stage of biotic/abiotic feedbacks.