How Drought and Thinning Affect Water Balances in Southeastern Pine Plantations

loblolly pine
The loblolly pine plantation in the study is dominated by a humid subtropical climate. The long-term (1945 to 2014) annual precipitation in the region is 52 inches. Image by David Stephens, courtesy of Bugwood.

Evapotranspiration – the combination of water evaporation and plant transpiration – is an essential process for forests and water supply and climate. This is particularly true in the Southeastern U.S. where evapotranspiration from forested watersheds can return 50 to 90 percent of annual precipitation to the atmosphere.

Until now, there has been a gap in scientific knowledge about how drought and forest thinning affects water balance and evapotranspiration in trees and forests.

Ge Sun, USDA Forest Service research hydrologist, led a study about this relationship with collaborators from North Carolina State University. Another team, headed by Xiaodong Liu, a professor at the Chinese Academy of Sciences, used data from Sun’s team to further investigate the effects of droughts, thinning, and water use on evapotranspiration.

The resulting paper was published in Forest and Agricultural Meteorology. It followed up on two earlier papers based on data from Sun’s team that investigated energy and water balance in the same forest. Those studies were published in Forest Ecology and Management and Forest Science.

The most recent paper, and the entire project, had been in the works for over ten years. In 2006, the first team set up shop at a loblolly pine plantation on the lower coastal plain of North Carolina. There, they began collecting continuous measurements of the forest’s water vapor and carbon dioxide intake and outtake. With the second team taking over, data were collected up to 2015.

The loblolly pine plantation was chosen for a reason. Loblolly pine is the most commercial plantation tree of the South. Almost 2.5 million acres of loblolly pine plantations are found in eastern North Carolina alone. In addition, it provides many benefits to people, such as clean water and air. These plantations are actively managed, which provides a need to understand how forest thinning can affect evapotranspiration.

In addition, climate change models predict an intensifying hydrologic cycle, including more droughts. This puts forth a need to understand how climate change, and subsequent droughts, can affect evapotranspiration.

The 2009 thinning reduced annual mean leaf area index – the amount of green leaf area per unit ground surface – by 60 percent in 2011, compared with 2009 values. USDA image, courtesy of Flickr.

During 2006 to 2015 there were two consecutive years of severe drought and one year of tree thinning, with around 50 percent of the forest removed. This yielded useful data to the team of scientists trying to determine the relationship between these factors and evapotranspiration.

The drought occurred from 2007 to 2008. “It was a very severe drought … there was nearly 40 percent of normal annual precipitation, which is a huge drop,” says Sun. Even with that, though, an evapotranspiration decreased by only eight percent. This was relatively small for such a large drought.

“The forest used more water than the precipitation in a drought year. It used extra water from the ground. The water table was found far underground during 2007 and 2008, over 7 feet below. It is typically 20 inches underground for forests here,” adds Sun. “The groundwater appears to be supplying all of the water trees need during drought periods. Such a supply of water allows the trees to keep transpiring.”

The forest thinning also resulted in a temporary evapotranspiration drop. “When you remove the trees they stop using water, which reduces evapotranspiration. However, after a few years, the initial reduction is unnoticeable. The reduction is mainly in the first two years. The remaining trees have more water and sunlight to grow, so they eventually make up for the trees that were cut down,” says Sun.

The findings from the follow-up study suggest that a deep water table can be invaluable to forests during droughts. Without it, trees have little backup if they cannot get water from precipitation.

The other major finding was that forest thinning does not affect evapotranspiration in the long run. While it causes a short-term dip, it eventually leads to stronger trees that make up for those cut down.

“Soil water storage is important, especially since there will be more dry years due to climate change. As for thinning, it is not harmful to evapotranspiration, and water supply for that matter, and can help forests grow,” notes Sun.

Read the full text of the follow-up report.

For more information, email Ge Sun at ge.sun@usda.gov.

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