Sometimes when scientists conduct their research, they collect data over a short period of time and then they analyze the data and draw conclusions. The scientists use data from shorter time periods because those data are usually the most accessible. Obtaining data that span decades is often difficult because of the money and other resources needed to obtain data over a long time period. However, when data are available over long time periods, scientists are thrilled to use it. Scientists like to use these data because these data offer the opportunity to discover long-term trends as opposed to only having short snapshots of how a system is responding.

Think about if someone has a bad year in school and they are evaluated on just this one year for getting into college. Do you think it is better to look at one year or several years the student was in school to make a decision about getting into college? Looking at several years a student has been in school is a better way of determining whether to accept a person into college. It is a better way because it provides more details and history about the person. Similarly, this longer-term understanding provides scientists with more detail for them to draw their conclusions. In this study, the scientists analyzed streamflow data over a 75-year time period.

Have you ever heard the term “ecosystem services?” Ecosystem services are provided by healthy natural areas just because they are healthy natural areas. Examples include clean air, clean water, beautiful landscapes, healthy soil, places for wildlife to live, minerals, and even places to do outdoor activities. Ecosystem services are important because they provide goods and services that are vital to human health and quality of life. For example, many people rely on water from streams for drinking water and everyday use in their homes.

Ecosystem services can be influenced by many things. For example, climate change can influence ecosystem services because landscapes and ecosystems may change how they function or look due to the change in climate. In this study, the scientists were interested in ecosystem services, including streamflow, provided by forested watersheds. The scientists were interested in how climate change may impact the streamflow provided by these forests (fig. 1).

Climate change can have direct and indirect impacts on water resources. Direct impacts of climate change can be seen by the presence of more extreme weather events. Extreme weather events include things like heat waves and droughts (fig. 2).

Droughts have a direct impact on water and water supply. The indirect impacts of climate change on water resources relate to temperature and the amount of carbon dioxide in the atmosphere. For example, an increase in temperature could increase the amount of water plants use because of transpiration and evaporation (fig. 3). If plants increase their use of water, then there will be less water available for streamflow or groundwater.

Forests and how forests are managed also have impacts on water resources. For example, some types of forest management change the number of trees in an area. The number of trees that are in an area influence how much water is intercepted by these trees. The number of trees also influences how much water evaporates in this area. Forest management can change the way water flows. Some types of forest management can also create disturbances in the soil. The scientists hypothesized that climate impacts may either be made better or worse by forest management that changes the land cover.

The scientists in this study wanted to figure out how forest management, climate change, and streamflow interact. First, the scientists wanted to identify if forest management could affect streamflow. Second, the scientists wanted to identify types of forest management that would help protect against extreme precipitation changes that may occur as the climate changes.

The scientists obtained their data from an area called the Coweeta basin in the Southern Appalachian mountains (figs. 4a, 4b, and 5).

Air temperature and precipitation have been recorded at the main climate station there since 1934. Nine recording rain gauges and twelve standard rain gauges are located throughout the basin (fig. 6). Six of these gauges have been recording since 1936.

The scientists also used long-term streamflow records from six watersheds. The six watersheds had different management and land use histories (fig. 7).

The scientists were interested in the effect of the different management practices on these watersheds. The scientists compared closely located watersheds that were similar in size and pre-management land conditions. One watershed served as a reference watershed, and the other watersheds had a management treatment applied (See fig. 7).

The scientists created mathematical models to help them explain the interaction between forest management and changes in climate on streamflow. The scientists used future precipitation and temperature forecasts from general circulation models (GCMs) and management histories to estimate the streamflow up to the year 2050. The scientists determined management forecasts by assuming that in 2009, each watershed was managed as it had been in earlier years.

The scientists then predicted how streamflow from the different watersheds might respond to future extreme precipitation events. Extreme precipitation events include extremely dry, or drought, conditions and extremely wet conditions. The scientists’ predictions were based on what happened to these watersheds in the past. Instead of past weather and climate variables, however, the scientists used weather forecasted for the area from GCMs.

The mean annual air temperature has been increasing at Coweeta. Since 1982, the temperature has been increasing at 0.5 °C per decade.

The scientists identified 10 extreme drought years since 1936. Eight of these extreme drought years have occurred since 1980. The most extreme dry year was 2000. The frequency of extreme wet years did not increase with time. The scientists identified 6 extreme wet years. Three of these extreme wet years occurred in the 1970s. The most extreme wet year was 1989. The summer months are becoming drier over time. The fall months are becoming wetter.

The scientists found that predicted streamflow in different future weather conditions was affected by almost all of the management actions examined. This finding supported the scientists’ hypothesis that climate impacts may either be made better or worse by forest management that changes land cover. The streamflow in different possible climate conditions depended on what type of management action was taken. Converting areas of deciduous trees to pine trees reduced annual streamflow during both extreme wet and extreme dry years. Different tree species absorb different amounts of water through their roots. Different species emit different amounts of water from their leaves during transpiration. Because of these differences between tree species, more or less groundwater may be available to fill streams. The type of tree species being managed, therefore, affects the rate of streamflow during both extreme wet and extreme dry years.

The scientists found that both temperature and precipitation changed over the time period they studied the Coweeta basin. The scientists determined that management affected the relationship between precipitation and streamflow.

The scientists also determined that how an area is managed has an impact on streamflow. The change from deciduous trees to pine trees reduced annual streamflow during both extreme wet and extreme dry years. The scientists said that the reduced flow may worsen a drought during extreme dry years. However, the reduced flow may also help reduce flooding during extreme wet years.

In areas where pine trees were managed, the scientists found that there is a higher rate of evapotranspiration (ET). Greater ET means that soils have more room to store water during wet years. Using this management treatment would be useful, therefore, in a future climate where precipitation is increasing. If the climate becomes drier in the future, however, then this management treatment would not be a good option.

The scientists are not sure if the forest management actions they studied can reduce the effects of climate change. They found, however, that converting areas from deciduous trees to pine trees helps to reduce the impact of excessive precipitation. In a future climate with too much precipitation, therefore, this management action would be helpful.