To get answers to their questions, scientists often collect and record new pieces of information. These pieces
of information are called data. Although data can be in the form of words or even pictures, most data are numeric. Using technology, some data can be collected and recorded automatically. For example, equipment placed in a particular location might collect and record the hourly air temperature over months and even years.

In this research, the scientists were interested in how water moves through a tree over a 24-hour period. The scientists used technology to measure and record the inside temperature of a tree’s trunk. The inside temperature was used to estimate how much water is moving from a tree’s roots to its leaves. The equipment measured and recorded the temperature every 30 seconds. Every 15 minutes, the equipment calculated the average temperature and recorded it. The equipment did this continuously from April 2004 until November 2005.

You know that all living things, including trees, need water to survive. Trees take water in through their roots and send it throughout the tree, all the way to the leaves. When the water reaches the leaves, it is released into the air. Water is released in the form of water vapor, and it exits the leaves through stomata in the leaves (figure 1). Stomata are like the pores in your own skin, and they allow carbon dioxide to enter the leaf and water vapor to exit. They also allow oxygen to exit the leaf. When water vapor exits a tree through its stomata, scientists call this transpiration. The stomata close at night and transpiration stops. In turn, water movement into the tree’s roots from the soil slows down. Think about using a straw to drink liquid. If you stop sucking, the liquid does not flow into the straw. As you can see, a tree has a daily cycle, just like other living things. How is a daily cycle evident in humans? In what way is this like a tree’s daily cycle?

In this study, the scientists were interested in how much water a certain kind of tree species transpires every day and over a period of time. They know that when trees take up water from the ground and then transpire, trees are playing a role in the ecosystem. In this instance, trees are contributing to the ecosystem by taking water out of the ground during the day and releasing it back into the air as water vapor.

Water is important to all life. Water is constantly moving throughout the natural environment. Think of rain and snowfall, of streams and rivers, and of evaporation. You probably have studied the water cycle (figure 2).

As you know, some water is held underground, and this is called groundwater. Groundwater flows through soil, sand, and between stone underground. Eventually, some groundwater flows into streams and rivers. Some scientists study the flow of water into, out of, and held within particular natural areas (figure 3). They are interested in all of the things that might affect water’s normal flow. If the normal flow of water is disrupted, the plants and animals living in the area and downstream will be affected.

Trees play a role in the flow of water in forested ecosystems. During the day, trees absorb groundwater through their roots. This water is pulled up the tree, and the tree releases the water as water vapor through stomata in its leaves (see “Thinking About the Environment” (figure 1)). At night, these processes slow down or stop (figures 4 and 5).

In the forests of the southern Appalachian mountains, eastern hemlock trees grow near mountain streams from north Georgia and northeastward (figures 6a and 6b).

There are so many of these trees, they make up about half of the living plant material near mountain streams (figure 7). Eastern hemlock trees are evergreen, unlike most of the other trees growing in the southern Appalachian mountains. The needles of hemlock trees only live for 3 years, then they die and fall off of the tree.

Hemlock trees are now threatened by a nonnative invasive insect called the hemlock woolly adelgid (figure 8). Nonnative means that it is not naturally found in the area. Hemlock trees in the southern part of the Appalachian Mountains are in more danger than hemlock trees growing farther north. This is because colder winters in the north slow the reproduction of the adelgid.

The hemlock woolly adelgid was accidently brought on ships from Asia to the Northwestern United States in 1924. It has been spreading ever since. This insect’s larva feed at the base of hemlock needles, eating sugars stored in the twigs where the needles are attached. Sugar is a carbohydrate. Trees store carbohydrates for energy. Carbohydrates are used by trees to protect new buds over the winter. Carbohydrates are also used by trees to grow leaves and needles in the spring. When the larva feeds, it takes carbohydrates from the tree. In the fall and winter, therefore, buds cannot be protected. In the spring, new needles cannot be formed. In the third year after being attacked by the adelgid, the hemlock tree begins to die (figures 9 and 10).

Remember, all trees are a part of the water cycle. They absorb groundwater and release water vapor. Their leaves and needles also slow rainfall by intercepting raindrops. Some of the raindrops evaporate off the leaves and bark. This reduces the amount of rainfall falling on the soil. When this happens, the soil absorbs rain more slowly and this reduces runoff. As you can see, trees can have a big impact on the flow of water through a forest. The question the scientists in this study wanted to answer was: How might the flow of water change in areas near mountain streams if eastern hemlock trees are killed by the hemlock woolly adelgid?

The scientists studied an area along a stream in the mountains of western North Carolina (figure 11). At least 50 percent of the vegetation in the area was eastern hemlock. This area receives an average of 1821 mm (millimeters) of rain every year.

Recall that trees take in water through their roots, and the water flows up to the leaves. Water also flows downward through a tree. The water in trees, when combined with other substances, is called sap. The sap that flows upward is mostly made up of water. This is called xylem sap. The sap that flows downward also contains sugars, salts, and minerals. This is called phloem sap. Not surprisingly, the woody material through which xylem sap flows is called sapwood (figure 12).

The scientists placed two probes in the trunks of 16 different hemlock trees, so that both probes went into
the xylem sapwood (figures 13a and 13b). The 16 trees were a variety of sizes. One probe was placed 5 cm (centimeters) directly above the other. The top probe was heated. The temperature of the sapwood was measured at both probes. The difference in the temperatures enabled the scientists to estimate the amount of xylem sap flowing up the tree. Recall from “Thinking About Science” that the temperatures were automatically measured and recorded every 30 seconds, and everyv15 minutes an average was calculated and recorded. The scientists collected these data from April 2004 until November 2005.

Think about the air around you. You know that air holds some water, because some days are more humid than others. When the relative humidity is high, there is more water in the air. If the air around a tree has close to the maximum amount of water it can hold, then trees will not transpire very much.

The scientists measured weather data, such as temperature and relative humidity. The scientists used these data to calculate the amount of water in the air at different temperatures. They also identified the total amount of water the air was able to hold at each temperature. The temperature and relative humidity were recorded every minute (figure 14). The equipment calculated and recorded the average temperature and relative humidity every 15 minutes.

Recall that the scientists measured the amount of xylem sap flowing upward in each of the 16 trees. Once the scientists had this information for the 16 trees, they estimated how much water all of the eastern hemlock trees in the study area would transpire in total, depending on the weather conditions for each day and season. They were then able to estimate how much groundwater the roots pulled in and the trees transpired every 15 minutes.

The scientists found that eastern hemlock trees used water all year, but they used more water in the spring than in any other season. Since trees do not transpire at night, more water flows from the ground into the streams at night, raising the streamflow at night. Less water flows into the streams during the day, because groundwater is taken up by the trees’ roots. The scientists found that hemlocks were quick to respond to changes in relative humidity. For example, if relative humidity decreased, the amount of water flowing up the tree increased after about 15 minutes.

If all of the eastern hemlock trees in the study area were killed by the hemlock woolly adelgid, the scientists predicted the following would happen to the water flow in the area near the stream.

The amount of transpiration in the area would be reduced, especially in the winter and spring, thus increasing the amount of water in the soil. This would lead to more groundwater flowing into streams during the day, leading to a more even flow of groundwater into streams both day and night (figure 15).

If the hemlock woolly adelgid were to kill all of the hemlock trees in the southern Appalachian mountains, the flow of water in that area would be different than it is now. This would probably bring many other changes to the natural area and to areas downstream. For example, more water in streams might affect animals that live in the streams. There may be more soil washed into the streams. This would reduce the quality of the soil left behind and add soil particles to the stream water. These changes would affect plants as well as animals that
live in the area and the areas downstream.