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THE DEAD TREES
Have you seen the dead trees on Cedar Mountain?
Read articles about Beetle Kill in the newspapers?
What’s causing it? How long will it last?
The Dixie National Forest provides the following information on the spruce beetle epidemic to answer your questions.
The spruce beetle, Dendroctonus rufipennis is a bark beetle that attacks and infests all species of spruce trees.
They do not infest other species of conifer trees like pines and firs. The spruce beetle, as with all bark beetles, is always present in forested environments at what is called endemic levels. At these population levels the beetles typically attack individual weakened trees, or trees that have been blown over or cut down and do not generally present a threat to healthy standing green trees. They bore through the bark and lay their eggs in the phloem layer of the tree which is between the bark and the wood. The phloem layer is the living tissue under the bark that transports water and nutrients between the needles and the trees root system. The eggs hatch and as the small white larvae mature, they eat the phloem layer which girdles and kills the tree. They emerge from the trees as adults either the following spring or the second spring after hatching from the egg stage. As the adults disperse in the spring, they seek new green trees to attack and the cycle continues.
Effects to Adjacent Ecosystems The spruce beetle will not attack other species of trees. This includes ponderosa pine, subalpine fir, white fir, Douglas-fir, limber pine, and aspen. It also appears that blue spruce is not as susceptible to spruce beetle infestation, although some proportion of the blue spruce component will be lost due to spruce beetle attacks. With the exception of aspen, the other tree species on the Ranger District also have different bark beetle species that will attack and kill them. The spruce beetle will not affect the conifer component in the private land subdivisions around Duck Creek Village and Mammoth Creek, because little to no Engelmann spruce are present in these areas.
The bark beetle that infests ponderosa pine in this area is primarily mountain pine beetle, Dendroctonus ponderosae. This beetle only attacks some species of pine, such as ponderosa, limber, and lodgepole pine. The ponderosa pine forest on the Cedar City Ranger District did have high populations of mountain pine beetle in the mid-1990’s, however beetle populations have since declined to endemic levels. Due to the high densities of ponderosa pine forests and susceptible tree diameters, another increase in mountain pine beetle populations will occur again. In stands of ponderosa pine, mountain pine beetle generally kills trees greater than six inches in diameter in the denser pockets of trees, rather than killing all the larger trees like spruce beetle does in stands of Engelmann spruce. Historically, fire played a frequent role in the ecological succession of ponderosa pine dominated forests with a fire return interval of about 10 to 20 years. These fires served as a thinning agent that typically maintained the stands in low-density, "park like" conditions. Low-density ponderosa pine forests do not experience significant bark beetle mortality because the trees are more vigorous and stand structure is less susceptible to bark beetle attacks. Depending on resource objectives, thinning or maintaining a diversity in age class structure or species diversity will reduce susceptibility to bark beetle disturbance.
When bark beetle population’s increase to levels that cause high levels of mortality to healthy green trees it is called an epidemic. This is what is occurring on the Cedar City Ranger District and on Mt. Dutton on the Powell Ranger District of the Dixie National Forest. The Manti LaSal and Fishlake National Forests are also experiencing epidemic outbreaks of spruce beetle.
Because of the high elevation and moist conditions that occur in the spruce/fir forests, (Engelmann spruce/subalpine fir) fire does not play a significant role in ecological succession like it does in the ponderosa pine forest type. Fires are typically small (less than 5 acres) and burn on warmer sites, for example sites adjacent to meadows and south facing slopes. When fires do burn in this forest type they tend to be replacement events that occur about once every 300 years. It is likely that the fires are preceded by a spruce beetle epidemic which provides large amounts of dead/down wood and opens the canopy so that the sun can dry out the understory. It has been approximately 300 years since a large-scale fire occurred in this area. In these high elevation forest types, occasionally fire or spruce beetle epidemics are the major disturbance agents causing large-scale changes in the landscape.
Prior to the spruce beetle epidemic, the spruce/fir forest on the Cedar City Ranger District was over 250 years old and had matured into dense, old growth conditions. These are the conditions the spruce beetle finds ideal to its growth, development and spread. Since old growth forests are the last stage of ecological succession, the mortality caused by this epidemic serves as an excellent example of how old forests are replaced by young forests.
Trees can be sprayed with an insecticide to prevent attack by bark beetles but it can be expensive and requires application every two years until bark beetle population’s collapse. Once the trees are attacked spraying trees is ineffective since the insecticide treatment is only a preventative measure and does not penetrate the bark to the phloem layer where the insects are feeding.
Private Land Assistance Individuals who own land within the Dixie National Forest may contact the State Department of Forestry, Fire and Lands at 435-586-4408 for information regarding bark beetles, tree spraying, and other forest health issues or website http://www.ffsl.utah.gov/. Contact a forester with Dixie National Forest for information regarding activities occurring on National Forest System Lands at 435-865-3200.
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