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New Mexico State University

New Mexico State University

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Stressed-out Pines Help Determine Best Planting Methods

LAS CRUCES -- Before the first glimmer of sunup, New Mexico State University researchers are busy taking cuttings from pine trees to test their stress levels. No, it's not high blood pressure. The researchers are looking for transplant shock.

"We're trying to imitate transplant procedures common in the nursery and landscape industry," said John Mexal, tree physiologist with NMSU's Agricultural Experiment Station. "We hope to determine the best method that will help get the trees off to a strong start."

Mexal and Mike English, superintendent of NMSU's Agricultural Science Center at Los Lunas, are testing four varieties of pine -- piņon, Austrian pine, Scots pine and eldarica or Afghan pine. "We chose these pines for their varying growth rates. Elderica has a fast-growing, aggressive root system, while piņon has a slow-growing system," Mexal said.

The varieties underwent four treatments at the center. The control group was left in the ground undisturbed. The second group was dug with a tree spade then replaced in the planting holes. The root balls of the third group were wrapped in burlap bags before being replanted, and the root balls of the fourth group were wrapped in burlap bags and wire baskets. "Wire baskets are used in nurseries as an added measure to keep the root balls intact during transport," he said.

To test the trees for transplant stress, the researchers are plotting growth and water stress. "Initial measurements of height and trunk diameter were taken before the testing began," Mexal said. "The trees will be measured regularly throughout this first year and annually thereafter."

To determine water stress, shoots from the growing tips of branches are removed. "We have to do this while it's dark before the trees' cells open and begin transpiring water," Mexal said.

The bark is removed from around the cut end of the shoots. "The bark contains resin that can distort our tests," he explained. Each shoot is placed upside down in a "pressure bomb" -- an airtight box attached to a bottle of pressurized nitrogen gas.

"By exerting pressure on the shoot until water bubbles the end of the stem, we're able to estimate the amount of pressure the water system is encountering inside the tree," Mexal said.

A low reading of three to four atmospheres, or about 45 pounds per square inch of pressure, indicates the tree is not under serious stress. A high reading of 11 or 12 atmospheres, or several hundred pounds per square inch, indicates high water stress.

This water stress is linked to the various transplant methods. "The trees' roots are involved in water uptake. Once you cut the roots, you impede water transport throughout the trees," Mexal said.

The researchers have already seen substantial water stress in some of the larger trees. "The eldarica pines react rather quickly to water stress by shedding their older needles," he said. "Surprisingly, we're seeing some growth this year on a number of the trees, so it looks like they're handling the transplant shock fairly well."

Through his research, Mexal also hopes to determine when, and if, burlap bags around the trees' root balls decompose in New Mexico soils, and whether the burlap/wire basket combination prevents roots from establishing.