Logjams and Beaver Dams: How different landforms affect the amount of carbon in an ecosystem

Question

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 statement BEST expresses a central idea developed over the course of the article?
Responses
A Scientists want to know where carbon is found, how it affects climate change, and what can be done about it.Scientists want to know where carbon is found, how it affects climate change, and what can be done about it.
B Scientists who are trying to better understand carbon cycle have concluded that carbon storage is also influenced by many natural processes.Scientists who are trying to better understand carbon cycle have concluded that carbon storage is also influenced by many natural processes.
C Beaver dams force water onto the floodplain where soil particles that are carbon-heavy are deposited and cause decay.Beaver dams force water onto the floodplain where soil particles that are carbon-heavy are deposited and cause decay.
D There is little humans know about their role in climate change due to carbon in the air and what can be done about it.

Answer 1

The statement that BEST expresses a central idea developed over the course of the article is:

B Scientists who are trying to better understand the carbon cycle have concluded that carbon storage is also influenced by many natural processes.

This statement encapsulates the main focus of the article, which discusses research aimed at understanding the movement and storage of carbon in different ecosystems, particularly the influence of specific natural phenomena like beaver dams and logjams on carbon storage.