Adapted from Natural Inquirer, 2019, Number 18, “Logjams and Beaver Dams: How different landforms affect the amount of carbon in an ecosystem,” produced by the U.S. Department of Agriculture, Forest Service, and the Cradle of Forestry in American Interpretive Association.
Carbon is an important part of our world. All living things contain carbon. It is found in water, soil, plants, animals, and the air. Even about 18% of the human body is carbon! However, too much carbon in the air contributes to climate change.
Carbon is released into the air in many ways, including wildland fires and the burning of fossil fuels. When animals and plants die and decay, they also release some carbon back into the air and deposit some in the Earth. This process of carbon moving through the air and the Earth is known as the carbon cycle.
Scientists are interested in this process, with some particularly focused on the movement of dead wood from forests to rivers in mountainous temperate ecosystems. Studying this movement can help them better understand the carbon cycle.
Scientists conducted a study to answer three questions about these mountainous forest and river ecosystems. First, where is carbon found? Secondly, how do the landforms near rivers affect how much carbon is stored or moved out? Thirdly, what natural processes in those areas affect the location and amount of carbon?
Many of the valleys in the mountainous Western United States were once forested. Before their trees were harvested, most of these forests were old-growth forests. For this study, the scientists wanted to compare old-growth forests with young-growth forests. Rocky Mountain National Park was a good place to study because some of the old-growth forests still remain.
The scientists identified six types of valley ecosystems along rivers in the park. Each of the six valley ecosystems was either confined or unconfined. Confined valleys have steep slopes and little vegetation. Unconfined valleys are flatter and contain a variety of features. These include beaver meadows, which are sediment-rich landforms created by abandoned beaver dams. Unconfined valleys also include multiple stream types. Streams in unconfined valleys may flow along a single path. Others, clogged with fallen trees that form logjams, may overflow their banks and split into multiple flows. The scientists collected samples from the six types of areas, and each sample was analyzed for organic carbon.
Overall, little carbon is stored in confined valleys. Their steep sides carry carbon into waterways and downstream, out of the valleys.
In unconfined valleys with old-growth forests, most carbon is stored in large pieces of wood, like logs and tree boles. Carbon is also stored in logjams, soil particles, living vegetation, and the sediments deposited in beaver meadows. Although valleys with abandoned beaver meadows covered less than 25% of their rivers' lengths, they held about 75% of the total carbon along these rivers.
The researchers concluded that natural processes encourage carbon storage in floodplains. Beaver dams and logjams force water onto the floodplain, where soil particles containing carbon are deposited. The water is then held on floodplains by logjams or dams, saturating the soil particles with water and slowing their decay. Finally, logjams themselves decay slowly and store a lot of carbon.
Question
Which sentence from the article BEST supports the conclusion that natural processes help accelerate the carbon cycle?
Responses
A Overall, little carbon is stored in confined valleys.Overall, little carbon is stored in confined valleys.
B Beaver dams and logjams force water onto the floodplain, where soil particles containing carbon are deposited.Beaver dams and logjams force water onto the floodplain, where soil particles containing carbon are deposited.
C The scientists collected samples from the six types of areas, and each sample was analyzed for organic carbon.The scientists collected samples from the six types of areas, and each sample was analyzed for organic carbon.
D Studying this movement can help them better understand the carbon cycle.
1 answer
The sentence that BEST supports the conclusion that natural processes help accelerate the carbon cycle is:
B Beaver dams and logjams force water onto the floodplain, where soil particles containing carbon are deposited.
This sentence directly highlights how natural processes (beaver dams and logjams) facilitate the deposition of carbon in the floodplain, indicating their role in the carbon cycle.
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