When scientists work to solve a problem or answer a question, they often work on teams. As you can see from the scientists above, this research involved a team. This team included men and women with different skills, abilities, and interests.

Think about your experience of working on teams. Do you always agree with everyone on your team? At times during this research project, the scientists did not always agree either. It is normal for scientists to disagree with one another. They might disagree, for example, on how to collect their data. They might disagree on how to explain their findings. When scientists work together on a project, they must work out their differences. Because they respect each other’s talents, they often suggest new experiments that will help them to resolve their differences.

The atmosphere is made up of layers of gases surrounding Earth (figure 1). The troposphere is the layer closest to Earth. About 99 percent of the troposphere is made up of nitrogen and oxygen. Of the remaining one percent, about 0.036 percent is carbon dioxide, or CO2.

Trees capture some of this carbon dioxide from the atmosphere through the process of photosynthesis. One product of photosynthesis is glucose, from which trees obtain energy. Glucose is a carbohydrate and contains carbon, hydrogen, and oxygen. You can see that through photosynthesis, trees capture carbon and use it to grow and maintain their living tissues.

Largely from burning fossil fuels such as coal and oil, Earth’s troposphere is experiencing higher levels of carbon dioxide and other greenhouse gases. These gases trap the sun’s warmth and are causing changes in Earth’s climate. As the level of carbon dioxide rises, more carbon dioxide will be available to trees. This increase in carbon dioxide may not only change Earth’s climate. It may also change the amount of photosynthesis in trees and forests world-wide.

Can you see an entire tree just by looking at it? Of course not! A tree’s roots reach underground and cannot be seen. Roots absorb water and nutrients from the soil to be used by the tree to grow and survive (figure 2). Roots are necessary to keep a tree from falling over. Although most of a tree’s living tissue is visible above ground, its unseen root system is also alive.

In “Thinking About the Environment,” you learned that carbon is captured by trees during photosynthesis. Carbon is found aboveground in the trunk, branches, and leaves, and belowground in the roots of trees.

When a tree is cut down to be used for products, some of the carbon can last in solid form for many years. This is particularly true if the tree is used to build homes or furniture. If a tree is not cut down, most of the carbon stays in the tree until it dies and decays or is burned in a fire.  During decay or a fire, the tree’s aboveground carbon is released back into the troposphere. Because roots are better protected underground, they decay at a slower rate. Across a tree’s life span, a large amount of carbon is also released during respiration (figure 3). Respiration happens both aboveground and belowground.

Climate change is caused in part by a rising amount of carbon dioxide in the troposphere. Scientists are becoming increasingly interested in how global climate change might affect life on Earth. As a part of this, some scientists want to predict what might happen to trees as the climate changes. To do this, scientists need to understand how much carbon dioxide trees capture, how much carbon they hold, and how much they respire back to the troposphere. To understand the total amount of carbon captured, held, and respired by a tree, scientists need to know how much carbon is kept aboveground and how much is sent belowground to a tree’s root system.

Scientists can estimate the amount of carbon found aboveground in a tree. The amount of carbon is about equal to one-half of the tree’s weight, after all water has been removed. Belowground, however, it is a different story! Because roots are underground, it is difficult to measure the amount of carbon they hold and how much carbon is lost during respiration. If a scientist digs up a root, the roots may be destroyed and the tree may be damaged or killed. Some scientists are now using technology such as radar to estimate the amount of carbon in roots.

The scientists in this study wanted to know how rising levels of carbon dioxide in the troposphere might affect the amount of carbon sent belowground by trees and made available for tree root growth and maintenance.

The scientists used data from four special research areas. In these areas, trees were planted on an area of land and allowed to grow. When the trees were ten years old, large towers were constructed in circles within the trees (figure 4).

The towers emit carbon dioxide into the air surrounding trees in the forest (figure 5).

Using technology in this way, the scientists could simulate rising levels of carbon dioxide on trees in the out-of-doors. These areas and the research done within them are called FACE. FACE stands for Free-Air Carbon Enrichment. Data from trees growing in four FACE areas were used. These areas are in North Carolina, Wisconsin, Tennessee, and Italy (figure 6).

Remember that carbon is captured during the process of photosynthesis. The amount of photosynthesis happening in a tree is partly dependent on the amount of leaf area exposed to sunlight (figure 7). You can see that the amount of carbon a tree captures is related to its ability to photosynthesize. When carbon is captured during photosynthesis, it flows to different areas of the tree. Some of the carbon flows underground to the roots. Some of the carbon stays aboveground in the trunk, branches, and leaves of the tree. Aboveground and belowground, much of the carbon is released during respiration.

The amount of leaf area exposed to sunlight and the amount of carbon dioxide in the air control the amount of photosynthesis. The scientists reasoned, therefore, that the amount of leaf area in a tree might be related to the amount of carbon captured by the tree. It is not easy, but it is possible for scientists to estimate the amount of leaf area in a tree.

Scientists can also estimate the amount of carbon sent below ground, although this is much harder for them to do. The carbon sent to the roots as glucose is used by the tree to keep the root system alive. When roots respire, they release carbon dioxide into the soil as they use the energy in the carbohydrates. Therefore, some of the carbon is used to make new roots and some is released as carbon dioxide (figure 8).

The scientists selected trees at different points within the FACE area. For these trees, the scientists estimated the amount of carbon being sent belowground. This included carbon held in the roots, as well as the amount of carbon released during respiration. They did this at different times, and they did it when the trees were exposed to higher than our current levels of carbon dioxide.

The scientists compared the amount of carbon being sent underground with the amount of leaf area in the same trees. This included the amount of carbon held in the roots, as well as the amount released during respiration. They added these amounts to arrive at a total amount of carbon for each year, and they took measurements for four years.

The scientists, therefore, observed and recorded the relationship between the amount of a tree’s leaf area and the amount of carbon sent belowground when different levels of carbon dioxide were in the air.

The scientists found that under the current amount of carbon dioxide in the air, the amount of leaf area in the trees was inversely related to the proportion of carbon sent below-ground. In other words, as leaf area increased, the proportion of carbon sent belowground decreased. The same relationship was found under increasing levels of carbon dioxide (figure 9).

At some point the number of leaves on a tree exceed a certain amount. Then, the lower leaves begin to receive less sunlight. When the tree reaches this point, it does not photosynthesize as much as before. When photosynthesis is at its peak, scientists say a tree has reached saturation.

In this study, the scientists found that the amount of carbon sent belowground by a tree is inversely related to the amount of leaf area in the tree. As photosynthesis increases in a tree, therefore, the largest proportion of the carbon in the tree is kept aboveground.

Trees are dependent on a number of resources for health. They must have sunlight, water, and nutrients. Remember that through photosynthesis, carbon dioxide is converted to glucose. Trees use glucose for energy. Energy is created during the respiration process, when some of the carbon dioxide is released back to the troposphere.

When trees have enough water and nutrients, they put most of their energy into photosynthesis and aboveground growth. This means that they do not send a lot of carbon, and therefore energy, into their root systems. When trees do not have enough of the resources they need, they put less energy into aboveground growth and send more carbon (and energy) into their roots.

As the level of carbon dioxide in the atmosphere rises, the amount of carbon dioxide available for capture is increased. Under these conditions,
trees may experience an increase in leaf area and photosynthesis until they reach saturation.

This research helps scientists better understand the relationship between a tree’s leaf area, the rate of its photosynthesis, and its root system. It also helps scientists predict what might happen to trees as the climate changes. This is, however, just one study. For scientists to better understand these relationships, they must study a greater variety of tree species.