Natural resource scientists sometimes face a special challenge. If these scientists want to understand natural processes, they must find ecosystems that have not been changed much by human activities. In the world today, many ecosystems have been changed by human activities.

The scientists in this study wanted to understand a particular process that happens in temperate mountainous forest and river ecosystems. Temperate ecosystems are those that lie between Earth’s tropical and polar regions (figure 1). Temperate ecosystems are not extremely hot or extremely cold, and they have four seasons every year.

The scientists identified a temperate mountainous forest and river ecosystem that had not been greatly affected by humans. This ecosystem was in Rocky Mountain National Park. National parks are established by the U.S. Congress and are protected by law (figure 2). National parks often include lands and waters kept in their natural condition, making them good places to study natural processes. When an ecosystem is left in its natural condition, scientists can study what happens in an ecosystem that has not had much human impact.

Rivers, under gravity’s influence, flow from higher land areas towards the oceans. In mountainous land areas, rain and snowmelt flow into rivers (figure 3).

If the area has forests and other vegetation, the rain and snowmelt carry soil, leaves, and dead wood into rivers and downstream. Leaves and other material from the forest floor are called litter. Dead wood includes dead trees, including boles and branches that have fallen to the forest floor.

Rivers in mountainous areas also flow through places that are not steep. In these places, the land beside the rivers is flatter, and the rivers flow more slowly. Old-growth forests are found in many of these places, and particularly in national parks (figure 4). When large trees fall, they may become lodged in the river, causing the water to overflow the river’s banks and create multiple channels (figure 5).

Recall that streams flowing through flatter places move more slowly than streams flowing down steep hillsides. Beavers build dams in the streams of these flatter areas (figure 6).

Beaver dams back up water in the stream channels, creating beaver ponds (figure 7).

In national parks and other places with limited human impact, slow-moving streams sometimes flow past old-growth forests. In some of these places, beavers may still be found. The scientists in this study were curious about how old-growth forests and beavers affect river-related carbon movement and storage.

All living and once-living things contain the element carbon. Much of Earth’s carbon is found in forest plants, trees, and soils. When animals, plants, and trees die, the carbon remains. As these once-living organisms decay, the carbon that they contained goes into the ecosystem. Some scientists are interested in how carbon moves throughout and is stored in ecosystems (figure 8).

Scientists know little about the movement of litter and dead wood from forests to rivers in mountainous temperate ecosystems (figure 9). Studying this movement is one way that scientists can better understand the carbon cycle.

The scientists in this study asked three questions about mountainous temperate forest and river ecosystems:

(1) Where is carbon found in these ecosystems?

(2) How do the landforms beside rivers affect how much carbon is stored in or moved out of these ecosystems? and

(3) What natural processes affect the location and amount of carbon in these ecosystems?

Many of the valleys in the mountainous Western United States have been forested at some point in history. Before many of the trees were harvested, most of the forests were old-growth forests. The scientists identified six types of areas along rivers in Rocky Mountain National Park (table 1).

They wanted to compare old-growth forests with younger forests (figure 10). As discussed in “Thinking About Science,” Rocky Mountain National Park was a good place to study because some of the old-growth forests still remain.

The scientists also studied two types of valleys along rivers in Rocky Mountain National Park (figure 11).

Confined valleys have steep slopes and often have little vegetation (figure 12). Unconfined valleys, which are flatter places, might have young or old forests. Unconfined valleys might also be areas where beavers built beaver dams (figure 13). After beaver dams are abandoned, the beaver ponds created by the dams slowly fill in with sediment and become beaver meadows (figure 14).

To conduct their study, the scientists collected samples of the vegetation, sediment, litter, and dead wood found in each of the six types of areas they studied:

(1) old growth forests with a single stream,

(2) old growth forests with multiple streams,

(3) recently abandoned beaver meadows,

(4) long abandoned beaver meadows,

(5) old growth forests, and

(6) newer forests.

Each sample was analyzed for organic carbon using a special machine (figure 15).

The scientists found that less carbon is stored in confined valleys than in unconfined valleys.

Confined valleys with steep sides carried carbon into waterways and downstream.

Unconfined valleys with old-growth forest held most of their carbon in large pieces of wood, like logs and tree boles. When trees fell in the stream or river or were otherwise pushed into the channel by the water current, the logs sometimes created logjams. The logjams caused the river to overflow its banks and split into multiple channels (figure 16).

Many of the unconfined valleys had abandoned beaver dams. These unconfined valleys held most of their carbon in sediment that was carried into them when rivers overflowed. The scientists found that these unconfined valleys with beaver dams covered less than 25 percent of the river’s total length. However, these valleys held about 75 percent of the total carbon found along the entire river ecosystem (figure 17).

The scientists identified four places where carbon is stored in temperate mountain and river valleys. Carbon is stored in old-growth single-thread, old-growth multi-thread, old-growth forests, and young-growth forests. Carbon is also stored in logjams or deposited on the floodplain. Carbon is stored in small pieces of matter, such as soil particles, and in living vegetation.

The scientists found that three natural processes supported carbon storage in these floodplain ecosystems:

(1) beaver dams and logjams force water onto the floodplain, where soil particles with carbon content are deposited;

(2) when water is held on floodplains by logjams or beaver dams, the decay of the soil particles is slowed, because the soil is saturated with water; and

(3) logjams decay slowly and therefore hold a lot of carbon in the waterways and on the floodplain.

The scientists found that old-growth forests, logjams, and beaver dams contribute to holding carbon within mountainous temperate forest and river ecosystems. Old-growth forests, logjams, and beaver dams may be found in unconfined valleys, which make up just a small proportion of a mountain river’s length. These landforms and ecological processes play an important role, however, in holding carbon within mountainous Western U.S. ecosystems.