Landslides often occur in mountainous, forested regions, but the two-way interactions between forests and landslides have rarely been measured.  Understanding how forests affect the distances landslides travel is important for accurate hazard assessment and for interpreting how landslides shape the landscape over longer periods of time.  On the other hand, understanding how landslides affect forests is important for determining how much carbon is sequestered from the atmosphere in vegetation and soils, and how sensitive that amount is to changing climate.  This study used intact temperate rainforests in southeast Alaska as a natural laboratory to document the nature of these interactions. 

In southeast Alaska, recent landslides travelled about half the distance and covered about one-third the area relative to typical landslides in other regions.  This low mobility was likely caused by interactions between logs transported by the landslides and standing trees in the surrounding forests.  Logs were often observed to form jams against standing trees, which stopped the landslide from travelling farther downslope.  This observation was supported by a simplified computer model in which adding more logs to a landslide reduced its mobility.  Analysis of thousands of historic landslides showed that forest age significantly reduced landslide mobility within southeast Alaska, such that landslides in the oldest forests with the largest trees were the least mobile.  Broader impacts of these findings were shared in Sitka, Alaska by developing a table-top scale landslide lab activity for middle schoolers through the Sitka Sound Science Center's Scientist-in-the-Schools program and sharing landslide runout maps with city professionals. 

Landslides in southeast Alaska also mobilize large amounts of carbon, which is subsequently incorporated into lowland soils, respired back to the atmosphere though decomposition, or exported through river systems to the Pacific Ocean.  This project developed a novel computer model to determine whether the net impact of these three main fates is to increase or decrease the amount of carbon in the atmosphere.  The modeled net effect of landslides was to systematically remove carbon from the atmosphere, increasing the carbon stored in soils and dead vegetation by 0.5-3%.  When roughly scaled to the entire globe, the model suggested that Earth has 18 gigatons more carbon in soils and vegetation than it would in the theoretical absence of landslides, equivalent to about 2 years of present-day carbon emissions from fossil fuel burning.  Landslides store more carbon as they occur more often, which is expected to happen with ongoing climate change, so they should act as a small buffer against future increases in atmospheric carbon.

Last Modified: 10/28/2022
Modified by: Adam M Booth