Accelerated erosion and tree mortality in a primeval temperate forest dominated by large trees, Washington, USA

In some terrestrial ecosystems, trees may control soil evolution and hillslope processes, with their biogeomorphic influence closely linked to tree size and ecosystem productivity. This study investigates the biogeomorphic role of tree mortality in a temperate old-growth forest characterized by large trees (Wind River, USA). We hypothesized that mortality, the biomechanical impact of trees on soils, and erosion rates have recently increased as the partial factors controlling these complex processes are strengthened. By combining repeated extensive tree censuses with radionuclide-based estimates of recent (^239+240Pu) and long-term (¹⁰Be) soil redistribution, we show the value of a multi-proxy approach for inferring coupled biogeomorphic processes shaping hillslope and soil evolution. Tree mortality affected 3.95 t ha⁻¹ yr⁻¹ of soil, indicating a globally above-average geomorphic impact. Although only 28% of dead trees were uprooted, uprooting remained the main driver of slope dynamics, contributing 2.06 t ha⁻¹ yr⁻¹. The radiometric analyses showed that recent erosion rates (1.92–2.93 t ha⁻¹ yr⁻¹) were either in a similar range or higher than the long-term rates (0.79–2.36 t ha⁻¹ yr⁻¹), consistent with an increasing tree mortality rate. Extremely high recent erosion rates were registered on a mound with values between 9.8 and 14.7 t ha⁻¹ yr⁻¹. Integrating biotic and geomorphic data revealed that different mortality mechanisms (uprooting vs. breakage) have contrasting effects on soil mixing, downslope transport, and pedogenic trajectories. Slope dynamics were only weakly sensitive to species composition, reflecting similar functional traits of the dominant taxa. These results show that combining ecological monitoring with geochemical tracers provides a robust framework for understanding feedbacks among vegetation dynamics, soil redistribution, and long-term landscape evolution under changing environmental conditions.