Vignettes > What Do Landslides Have to Do with Carbon Budgets?

What Do Landslides Have to Do with Carbon Budgets?

Mary Ann Madej
USGS
Author Profile

Shortcut URL: http://serc.carleton.edu/42683

Location

Continent: North America
Country: USA
State/Province:CA
City/Town:
UTM coordinates and datum: none

Setting

Climate Setting: Humid
Tectonic setting: Continental Collision Margin
Type: Process

Figure 1. Landslides in steep terrain commonly deliver soil, rock, and vegetation directly to stream channels. Details


Figure 2. Soil profiles from different watershed positions showing various degrees of organic (dark) material. Details


Figure 3. Pie chart showing the relative amounts of organic carbon from various components of a second-growth redwood forest. Details


Figure 4. Pie chart showing the contribution of carbon delivered to the Redwood Creek steam network through landslides generated in a flood in January, 1997 Details


Description

The development of carbon markets as one tool to reduce greenhouse gases and their role in global climate change has raised the interest of society in carbon sequestration and release. Much current research is now focused on changes in carbon storage through forest harvest and regeneration. Forests can absorb large amounts of atmospheric carbon dioxide (CO2) through photosysnthesis, but can release CO2 through decomposition or fire. In a forest, organic carbon accumulates in several pools: trees, understory vegetation, forest floor litter, roots, soil, and large wood in streams. Human activities that change patterns of erosion and sedimentation may affect the release and sequestration of carbon in the environment. For example, timber harvest and road construction change the cycling of carbon by accelerating hillslope erosion that increases loss of vegetation and soil. Landslides, both natural and human induced, can contribute to either carbon export or sequestration in a watershed. If landslides from upper hillslopes bury soils farther downslope, they can protect that soil carbon from entering the atmosphere. On the other hand, in steep terrains, landslides can strip soil and vegetation off a hillslope and deliver that carbon directly to streams (Figure 1). Studies in New Zealand have shown that landslides entering small rivers in steep terrain that was logged in the 19th and 20th centuries are significant sources of carbon to the ocean (Gomez and others, 2004; Hilton and others, 2008).

Case Study:
In the western United States, landslides are common in mountainous terrain. Carbon removal by landslides can be estimated using information on the organic carbon content of soils and vegetation. This study focuses on the redwood region of north coastal California. In an old-growth redwood forest, the carbon budget is dominated by the huge redwood trees, but landslides rarely occur in old-growth forests. In contrast, landslides are common on logged hillslopes with young trees. The above-ground biomass (vegetative material) consists of live standing trees, dead trees (snags), and forest floor litter. In this setting, soils also store a significant amount of carbon, depending on the type of soil and its location (Figure 2). (A simple method for determining the amount of organic carbon in soils is by the loss-on-ignition test, in which the soils are heated at very high temperatures, and the organic matter is essentially burnt off. The difference in weight of the soil sample before and after burning gives an estimate of how much organic matter was in the sample). For soils on hillslopes where landslides are common, soil carbon content is about 100 Mg per hectare (a Mg (megagram ) equals about 2200 pounds, and a hectare (ha) is about as large as 2½ football fields). Soils in second-growth forests represent about one fourth of the organic carbon pool (Figure 3). A second-growth redwood forest typically has a carbon mass of 290 Mg/ha in live standing trees, 24 Mg/ha in snags, and 17 Mg/ha in dead wood (Christensen et al., 2008), so total biogenic carbon is about 430 Mg/ha.

Using these estimates, we can estimate the amount of carbon that was removed from hillslopes by landslides during a moderate-sized storm. In January 1997, a 10-year storm initiated landslides covering a total of 45.4 ha in the 720 km2 Redwood Creek watershed, which removed roughly 20,000 Mg of carbon from the hillslopes and delivered it to Redwood Creek and its tributaries (Figure 4). This is equivalent to an export of 28 Mg/km2 of watershed area. Interestingly, this value of carbon export is similar to that found in river sediments deposited on the continental shelf off the mouth of the nearby Eel River after a moderate flood in 1995 (27 Mg/km2) (Leithold and Hope, 1999). Larger storms in 1964, 1972 and 1975 resulted in more than 1800 streamside landslides in the Redwood Creek basin, encompassing 630 ha of forested terrain, which would have contributed roughly 280,000 Mg of carbon to the river network. Perhaps we should consider prevention of landslides through careful land stewardship as a type of carbon sequestration!

Not all carbon delivered to streams from landslides gets transferred out of the watershed. In some environments, deposition of sediment on floodplains helps to sequester soil carbon that is delivered to rivers from landslides. In the steep, forested terrain of northwestern California, however, channels and hillslopes are tightly linked and floodplains are very narrow, if they are present at all. So in this region, carbon export from landslides is probably much greater than floodplain sequestration of carbon.

A full carbon budget for a watershed would require us to quantify all carbon inputs, storage and exports. This approach would include the examination of such factors as weathering and soil formation, microbial activities in the soil, litter input, and bioturbation (churning of the soil by animals, tree throw, etc.). Other major erosional processes (surface erosion, gullying, earthflows, and erosion from fires, floods, and windstorms) would also need to be included in a full carbon budget. "Carbon geomorphology" is in its infancy, but has the potential to help us understand carbon input into oceans and the atmosphere.

Associated References


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