Estimated Amounts and Rates of Carbon Mobilized by Landsliding in Old‐Growth Temperate Forests of SE Alaska

Abstract

Landslides occur on sloped terrain around the world, causing fatalities, damaging property, disrupting infrastructure networks, and altering ecosystems (Hovius et al., 1997;Jensen et al., 2009;Restrepo & Walker, 2009). Regional landslide rates are influenced by climatic and tectonic conditions, with wet climates initiating rainfall-induced landslides and tectonically active areas initiating earthquake-triggered landslides. Landslides serve as mechanisms for carbon (C) mobilization in densely forested regions by stripping vegetation and soil from hillslopes. Those materials are deposited elsewhere in the landscape and can be exported to the oceans via river systems (Hilton & West, 2020). If landslide-mobilized carbon is efficiently buried in offshore sediments, landslides may then be an important geologic carbon sink, especially if landslide occurs frequently or with large-magnitude events. The amount and the depositional fate of carbon transported Abstract Landslides, a forest disturbance, mobilize carbon (C) sequestered in vegetation and soils.Mobilized C is deposited either onto hillslopes or into the water, sequestering C from and releasing C to the atmosphere at different time scales. The C-dense old-growth temperate forests of SE Alaska are a unique location to quantify C mobilization rate by frequent landslides that often evolve into saturated moving masses known as debris flows. In this study, the amount of C mobilized by debris flows over historic time scales was estimated by combining a landslide inventory with maps of modeled biomass and soil carbon. We analyzed SE Alaskan landslides over a 55-year period where a total of 4.69 ± 0.21 MtC was mobilized, an average rate of 2.5 tC km −2 yr −1 . A single event in August 2015 mobilized 57,651 ± 3,266 tC, an average of 63 tC km −2 . Depositional fate was inferred using two methods, a standard stream intersection analysis and a second novel approach using simulated debris flow deposition modeling calibrated to the study area. Approximately 60% of debris flow deposits intersected the stream network (9% into mainstem channels, 91% into small tributaries), consistent with long-term modeled connectivity, suggesting that debris flows are likely to contribute to globally significant amounts of C buried in local fjord sediments. Our results are consistent with an emerging consensus that landslide disturbances that mobilize organic carbon may play an important role in the global carbon cycle over geologic time, with coastal temperate forests being hotspots of potential carbon sequestration. Plain Language SummaryThe amount of carbon stored in forests and soils on Earth plays an important role in the global climate by storing carbon away from the atmosphere. Landslides displace carbon in forests, and there is a need for estimating how much carbon is being relocated in heavily forested areas. Our study estimates the amount of carbon removed from hillslopes by landslides in SE Alaska, one of the most carbon-rich forests in the United States, and determines where in the l...

Effects of Landslides on Terrestrial Carbon Stocks With a Coupled Geomorphic‐Biologic Model: Southeast Alaska, United States

“…Within our study site lidar extent, area‐based landslide frequency in a 4 km 2 moving window ranged from 0 to 6 × 10 −4 yr −1 . More broadly, previously documented landslide frequencies at sites where landslide‐mobilized C rates have also been determined range from about 10 −5 to 10 −3 yr −1 (Clark et al., 2016; Hilton et al., 2011; Korup et al., 2019; Ramos Scharrón et al., 2012; Vascik et al., 2021). For the landslide size distribution observed in our study area, this corresponds to a number‐based frequency of roughly 10 −4 to 10 −1 landslides km −2 yr −1 .…”

“…Southeast Alaska experiences ∼0.002 landslides km −2 yr −1 (Vascik et al., 2021) in steep, glacially sculpted terrain that is widely covered by old‐growth temperate rainforests, making it an ideal site to investigate interactions between landslides and the terrestrial C cycle in a setting that has been minimally modified by humans. This study is focused on the area surrounding Sitka, Alaska (Figure 1), where the necessary data for calibrating the model are available (airborne lidar, a landslide inventory, and maps of terrestrial C density).…”

“…Contacts between bedrock and till, till and tephra, and layers within the tephra serve as failure planes for shallow landslides, which often mobilize into debris flows, the most common type of mass‐movement in the study area (Booth et al., 2020; Patton et al., 2022). These shallow landslide‐initiated debris flows strip soil and biomass from steep initiation sites high on hillslopes and deposit that material in valley bottoms or low order streams, where C is accumulated in lowland soils or exported to the Pacific Ocean (Vascik et al., 2021). Debris flows in the region tend to have limited runout, likely caused by interactions between large woody debris in the flow front with standing trees in the runout zone (Booth et al., 2020; Johnson et al., 2000).…”

“…Sitka has a mild, wet, maritime climate with average high temperatures above freezing at sea level year‐round and >2 m of annual precipitation (Shulski & Wendler, 2007). Much of that precipitation occurs as storms with high wind speeds and rainfall rates, which are fed by atmospheric rivers in the northern Pacific Ocean and often trigger landslides (Booth et al., 2020; Gomi et al., 2004; Johnson et al., 2000; Sharma & Déry, 2020; Swanston et al., 1991; Vascik et al., 2021). The climate of the region has been relatively stable for the past 7,500 years, and fire has been rare (Gavin et al., 2003; Hansen & Engstrom, 1996; Veblen & Alaback, 1996).…”

“…Most of this previous work has focused on surficial soil erosion, especially of agricultural lands, and little has focused on landslides in natural, forested settings (Billings et al., 2019; Dialynas et al., 2016). We hypothesize that landslides in forests should be an effective C sink because detachment and transport happen quickly (∼minutes), they bury C more deeply than surficial erosion, and they mobilize and bury biomass in addition to soil (Clark et al., 2016; Hilton et al., 2011; Johnson et al., 2000; Lancaster et al., 2003; Ramos Scharrón et al., 2012; Vascik et al., 2021; Wang et al., 2016; West et al., 2011).…”