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July 10, 2009

Some longleaf pines shedding needles due to dry weather

The dry weather much of the Southeast has been suffering from recently has caused many longleaf pine trees to drop their needles earlier than normal this year.

Many folks become alarmed when they see brown needles on longleaf pines, since the trees are, after all, classified as evergreen. But the term evergreen can be a bit misleading. Although longleaf does retain needles year-round, in years with abundant rainfall, individual bundles of needles (called fascicles) generally remain on the tree for two growing seasons, and are shed in the fall. Recent rains that some areas have been fortunate to receive will help alleviate some of the stress pine trees are undergoing, but most areas have gotten little rainfall in the past month.

The two-year-old needles on longleaf pine trees are closer to the base of the branches than the younger needles, and so one easy way to tell if browning needles are a cause for concern or not is to note where they are found on the branch. If needles are browning at the base of branches, then likely this is a normal physiological response to drought. By dropping needles early, the tree reduces its transpirational demand, that is, it requires less water. Wilting of leaves in many other plants is a similar response to drought, but differs in that the leaves remain on the plant. By wilting, the leaves expose less surface to the sun and wind and so the plant requires less water. If the stress is not too severe or of too long duration, wilted leaves can recover when the plant receives additional water and/or less sunlight. But browned needles do not reverse to green. If the needles are browning at the extreme ends of the branches, the problem might be something other than drought stress.

"Needlefall tends to occur in waves or pulses," said Dr. Eric Hinesley of North Carolina State University. "The timing and size of the pulses vary mostly according to water stress during the growing season. In a year of abundant rainfall, the biggest pulse of needlefall is in late September or early October. In a dry summer, the first wave or pulse might occur in July or August, followed by one or more pulses later in the summer and fall. Loss of the two-year-old needles in this manner helps to conserve water and reduce stress. Loss of the two-year-old needles is a normal process. This year, the browning of pine needles began in late June in some locales, which is unusual."

Bob Franklin, Clemson University forestry and wildlife extension agent, said trees are efficient at taking up, conserving, and recycling nutrients. A high percentage of the nitrogen and phosphorus in pine needles moves back into the tree (translocates) before the needles turn brown and fall off. Nutrients such as calcium and magnesium do not translocate when needles abscise (shed). Consequently, these nutrients are lost from the site in substantial quantities when straw is raked on a regular basis. In those situations, it is beneficial to fertilize occasionally to offset the loss of nutrients, especially on poor land where longleaf pine often grows.

Individual trees of the same species may respond to drought differently, with some coping better than others. Trees on xeric sites—that is, those that are dry because of sandier soils, high elevation and/or significant exposure to wind and sunlight—suffer worse than trees on wetter sites.

Other species of Southern hard pines, such as loblolly pine, tend to react similarly to drought, although longleaf tolerates dry weather best of all. Besides being more drought-resistant as compared to other Southern pines, longleaf is also less susceptible to damage from wind, fire, insects and diseases. For more information on longleaf pine, visit the Longleaf Alliance’s Web site or contact the group in Andalusia, Ala., at (334) 222-7779.

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