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Tracking carbon from forest to coast

How do landslides impact the carbon balance of Southeast Alaska?

The Tongass National Forest in Southeast Alaska is the greatest carbon reserve in North America, capturing carbon from the atmosphere and storing it in plants and soil. But natural and human disturbances, like erosion and logging, affect carbon stocks by moving organic material out of the forest, into streams and eventually coastal waters.

UAS researchers and ACRC affiliates Sonia Nagorski, Jason Fellman, and Eran Hood were awarded a grant from the National Science Foundation Division of Earth Sciences to study what role landslides play in the movement of carbon from land to sea.

Their efforts complement a larger project led by former UAS ecologist Brian Buma (now at the University of Colorado-Denver) to study how forest disturbances impact the storage of carbon in Southeast Alaska soils. Buma found that forests occasionally disturbed by wind or landslide events had higher long-term carbon storage than forests without any disturbance history.

Nagorski, Hood, and Fellman will assess another piece of the puzzle; how landslides, a frequent occurrence across Southeast Alaska’s steep and saturated terrain, influence the movement of carbon off the forest via streams.

“It’s not a dead-end when that carbon comes into the forest via photosynthesis and gets integrated into plants and soils,” said Nagorski.

“Carbon can continue to move through the ecosystem in a number of ways, such as streams. We are investigating whether landslide-affected watersheds differ from stable watersheds in the amount and quality of carbon carried in streams out to the ocean,” said Fellman.

Despite containing the largest U.S. forest carbon reservoir, there are still substantial gaps in our understanding of carbon movement between land and sea in the north Pacific coastal temperate rainforest of Southeast Alaska and British Columbia. Most carbon-balance studies focus on the exchange of carbon between the land and atmosphere, but the movement of carbon from the land to surface waters is often not accounted for. This lateral flow of carbon delivers bioavailable carbon (edible to microbes and other organisms) into coastal waters and high-latitude fjords, which are also hot spots for the storage of organic carbon via burial in fjord sediments.

In a landslide-prone terrain such as Southeast Alaska, forest disturbance may play a significant role in how and where carbon ends up. “Landslides are ubiquitous on the steep slopes of Southeast Alaska. They may be difficult to detect at first glance because they are often revegetated, but gravity-driven landslides and powerful streams are continuously wearing down the hillslopes,” said Nagorski.

Nagorski, Fellman, and Hood will be studying five paired stream sites around Sitka, Alaska, where there was a series of large landslides in 2015. They will analyze water samples from streams draining both recently landslide-disturbed watersheds and relatively stable drainages to determine if there is a significant difference in the amount and form of carbon they carry.

Preliminary research collected by Nagorski last summer shows that landslide-impacted streams do carry more organic carbon material than stable streams. “This indicates that landslides could enhance the export of terrestrial organic carbon that needs to be accounted for when trying to track carbon through the forest,” said Nagorski.