LANDSLIDE-FOREST INTERACTIONS GOVERNING THE POST-GLACIAL LANDSCAPE EVOLUTION OF SOUTHEAST ALASKA, UNITED STATES (Invited Presentation)

Landscapes evolve under interacting physical, chemical, and biological processes, with landslides dominating physical erosion of hillslopes in steep landscapes. Such landscapes are often forested because of the cooccurrence of high relief and high precipitation, but interactions between landslides and forests are rarely quantified or included in landscape evolution models. Here, we take a broad view of landscape evolution that encompasses not just topography, but the distribution of life and compounds on and in it. Specifically, we quantify effects of forests on landslides and landslides on forests, focusing on landslide runout and terrestrial carbon (C) stocks, respectively. Southeast Alaska (SEAK), a region with extensive old-growth temperate rainforests that are a globally significant C stock, provides several natural experiments to quantify these process interactions. Landslides in SEAK runout significantly less than landslides elsewhere. On average, their deposits cover 1/3 the area and travel 1/2 the distance compared to a generic global dataset. Within SEAK, landslide runout decreases by 30% from the youngest to the oldest forest stands. Ubiquitous log rims around deposit that abut standing trees suggest that frictional, granular processes, such as force chains and jamming of large woody debris, are the main mechanisms responsible for limited runout. By transporting large volumes of wood and soil, landslides also affect the terrestrial C cycle by (1) facilitating C export to depocenters where it can be sequestered for geologic timescales and (2) transferring C from pools with short C residence times, like biomass, to those with longer residence times, like soil. A landslide-driven C budget shows that just 10% of C mobilized by landslides in the past 50 yrs would need to be buried offshore to explain globally high C burial rates in nearby fjords. A novel landscape evolution model that couples landslides and the terrestrial C cycle suggests that landslides also increase the size of the terrestrial C reservoir itself by 0.5-5%. Therefore, landslides are likely a net C sink with respect to the atmosphere. Overall, quantifying the two-way interactions between landslides and forests is essential to better understand the coupled nature of natural hazards, biogeochemical cycles, and landscape evolution.