For Loblolly Pines, A Fertilization and Water Scarcity Paradox

Fertilized trees use water more efficiently, but may suffer the most in drought

An exclusion structure allowed researchers to simulate a 30 percent rainfall reduction in the loblolly plots. Photo by Andy Laviner, Virginia Tech.
An exclusion structure allowed researchers to simulate a 30 percent rainfall reduction in the loblolly plots. Photo by Andy Laviner, Virginia Tech.

Driving down a country road or even an interstate highway in many areas of the South, one cannot help but notice them: the straight rows of pine trees, sometimes as far as the eye can see, that make up the 20 million acres of planted pines in the region. Eighty percent of these trees are loblolly pines, which even in plantations perform important ecological services such as storing carbon and filtering water. These forests also provide abundant wood to support local economies and provide 16 percent of industrial wood supplies of the entire globe.

As in natural forests, the growth of loblolly pines in plantations is limited by essential resources: sunlight, nutrients, and water. Fertilization — an increasingly common practice for adding nutrients to boost wood production — allows trees to shift their growth away from roots to leaves and stems. Since a tree is dependent on its roots to provide water, how does a fertilized tree respond when water supplies are limited in times of drought? Does increased productivity intensify water stress?

To explore these questions, university and U.S. Forest Service researchers experimented with fertilization treatments and simulated drought during a two-year study in a loblolly pine plantation located in central Virginia. Their findings were recently published in the journal Forest Ecology and Management.

The plantation was in its eighth growing season when the researchers began applying a fertilizer of essential nutrients in four study plots, while diverting rainfall away to simulate a drought of 30 percent  water reduction in four other plots. They also combined fertilization and drought simulation in four additional plots. Finally, they left four additional plots untouched to study as controls.

Despite the researchers’ best-laid plans, the plantation received unusually high amounts of precipitation during the study period. Even the plots that received 30 percent less water through rainfall diversion still received near historic amounts of precipitation. In the second year of study, the stems of fertilized trees were 21 percent larger than the control plot stems. However, in plots with diverted rainfall, stem growth was not reduced as expected, either with or without fertilization.  It is likely that the unusually wet conditions nullified the water diversion treatment impacts on stem growth.

But in addition to measuring stem growth, the researchers also continuously monitored environmental conditions and tree water use with a network of more than 100 automated sensors located in stands throughout the plantation. As expected, they observed a relative decrease in water use in the stands where rainfall was diverted (a 19 percent reduction in the second year of study). Contrary to their expectations, researchers also observed decreased water use in fertilized plots (a 13 percent decrease in the second study year), and concluded that fertilization could reduce water use almost as much as drought conditions. The greatest decrease in water use (29 percent) was seen in the stands with combined fertilization and rainfall diversion treatments.

Researchers say the cause of water use reductions associated with fertilization remains uncertain. “We suspect the reason may lie belowground, either in the number of roots or their ability to transport water,” says Eric Ward, a North Carolina State University scientist cooperating with the Eastern Forest Environmental Threat Assessment Center and the study’s lead author.

“In the short term, fertilized trees that achieve more wood production with less water could benefit both the world wood supply and the local water supply. However, if fertilization reduces the ability of roots to supply water to trees, we are concerned that vulnerability of plantations to severe drought will increase with fertilization,” says Ward.

This is especially a concern in the changing climate of the Southeast, where variable precipitation is expected to be the norm. If more trees die as a result of more frequent droughts, ecological services and forest products from these plantations could be damaged.

“We know that management of loblolly depends on the site and we expect our results to be site-specific as well,” says Ward, whose research team will continue to investigate the effects of fertilization over the coming year. “By understanding how trees move water from root to leaf, we hope that we can explain why fertilization might increase drought stress at some sites, but not others. This is the kind of research practicing foresters can use.”

This study is one of four identical experiments also being conducted in Georgia, Oklahoma, and Florida as part of the PINEMAP project. The next step is to begin integrating findings across this network of sites.

Read the full text of the article.

For more information, email Eric Ward at ejward3@ncsu.edu.

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