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Effects of Drought on Arizona's Pine ForestsExtension Bulletin of The Arboretum at Flagstaff, 1990, No. 3 By Dr. Jill L. Wilson, Entomologist
During the past year, northern Arizona suffered one of its worst droughts in many years. Precipitation was approximately 69 percent of average. [Ed. Note: Although Dr. Wilson wrote this in 1990, the information is still timely; precipitation this past year has been 13% of average.] Though the forest surrounding Flagstaff still looks green and healthy, it’s appearance may be deceptive. Extended periods of moisture deficit can impact forest health. This article will focus on the impact of drought on the northland’s predominant forest tree, the ponderosa pine. Fortunately, among conifers, the ponderosa pine is relatively drought tolerant. It’s unlikely that many trees will die outright from the reduced supply of water. Still, drought can affect trees in other ways. The most visible is reduced needle retention. Normally a healthy ponderosa pine retains 4-5 years worth of needles. Every fall the oldest needles will die and turn brown. Every spring a new compliment is produced. This is a normal process; however, when the trees are severely stressed by drought, disease, competition, etc. fewer years of needles are retained. The crown takes on a thinner appearance. Other effects of moisture stress are less visible. Growth, both height and diameter, is reduced. Production of natural defensive chemicals, which normally confer resistance to insect attack, is also reduced. A number of insects feed upon different parts of a ponderosa pine tree, including foliage, cones, branches, trunks, and roots. Most of the time we don’t notice them or their effects on the tree. However, drought reduces the tree’s natural resistance to attack by some insects, particularly a group known as the bark beetles. Bark beetles are small cylindrically shaped insects, less than 1 centimeter in length. Most feed in the inner bark, in an area referred to as the phloemcambial region. They specialize in feeding in certain parts of the tree such as the branches, upper bole, lower bole, or roots. Some are very aggressive and attack living trees, others breed in recently dead material. The adults create tunnels called egg galleries under the bark. Each species has its own characteristic gallery shape. On the sides of these galleries, the female lays eggs. Small white grublike larvae hatch from these eggs and feed in their own tunnels. Later they undergo metamorphosis and become adult beetles. At that point the cycle is just about complete. The new adults emerge, fly and attack new hosts. The insects are active during late spring, summer and early fall at high elevations such as around Flagstaff. Interestingly, it takes more than just the beetles to kill a tree. The beetles carry on their bodies a secret weapon, certain fungi that are capable of rapidly colonizing the tree’s phloem and sapwood. The combined activity of the fungi and the beetles shuts down water and nutrient transport within the host. Some species of bark beetles even carry the spores of these fungi in special structures, called mycangia, on their bodies. These structures secrete nutrients which protect and nurture the spores until the beetles attack host trees and infect them with the fungi. In addition to the fungi, beetles have still another weapon. They produce airborne chemical messengers called pheromones, which regulate colonization behavior. These pheromones can serve two functions; They may attract more beetles if needed to kill the tree, or they may repel new beetles if enough beetles are already present. The latter adaptation is present in some but not all bark beetle species and is designed to reduce competition between beetles for a limited resource and therefore maximize brood production. The concentration of many beetles all introducing fungi is necessary to overcome the tree’s natural resistance to invasion. This is a highly evolved, complex system, involving a symbiotic association between the bark beetles and the fungi which is in turn mediated by the beetles’ chemical communication system. One would think that with such an effective system, no tree could survive. In most circumstances, pines are readily able to withstand the onslaught of beetles and fungi. Only when trees are weakened by drought, disease, old age, etc. do the beetles multiply to great numbers. So what can we expect in the coming year? We probably will see increased mortality of ponderosa pine in our forests. The extent of this mortality will likely depend on the amount of moisture received this winter and spring. A dry winter could mean more extensive mortality, but a good snow pack may dampen the effects of last year’s drought. What treatment may be done in anticipation of this event? In truth, the best treatments are preventative.. It is very difficult to prevent mortality from occurring once beetle populations have risen to epidemic levels. Healthy, vigorously growing trees will best withstand the effects of last year’s drought. The first trees to succumb will likely be those weakened by some of the many factors mentioned earlier. In town, trees damaged during recent construction, or by excessive foot or vehicle traffic (causing soil compaction and damage to the root crown) may be at risk. On the positive side, some of the trees killed will provide habitat for a variety of other organisms that depend upon snags, such as cavity nesting birds. Eventually the snags will rot and once broken down will return nutrients to the soil. If you have questions regarding insects and diseases on trees on private lands, please contact the Arizona State Land Department, located at 3650 Lake Mary Rd., Flagstaff, phone: 774-1425. They also have a forest pest specialist located in Phoenix. The USDA Forest Service also has a Forest Pest Management zone office in Flagstaff which works primarily with federal land management agencies in Arizona, including the National Forests, National Parks, and Indian Lands. |
Copyright © 2007 The Arboretum at Flagstaff
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