Community Agriculture Alliance: Subalpine fir declining in Routt County
November 7, 2013
Steamboat Springs — The local Colorado State Forest Service office has received several calls this past spring and summer about trees turning red on our landscape. Red trees on the south side of Emerald Mountain, on Rabbit Ears Pass and in North Routt had landowners concerned: Something was killing groups of subalpine fir, and they wanted to know what it was and what they could do about it.
The increase in subalpine fir decline fits the pattern we have been seeing in Routt County and throughout the state. The acreage affected in Routt County went from 2,000 acres in 2011 to 4,500 acres in 2012, according to a state aerial survey. Results of the current aerial forest health survey will be available in the spring, but we expect the number of acres affected to increase even further.
In forest health terms, a "decline" produces mortality or loss of vigor in species from a variety of causal agents, and it might be difficult to pinpoint the exact cause. The western balsam bark beetle (Dryocoetes confusus) clearly is the main culprit in our fir mortality, but a number of factors probably also contribute. These include several species of fungi, most notably Armallaria root rot, which is common in many of our fir stands in Routt County. The beetle also introduces a fungi into the tree that can help kill the tree, much like the mountain pine beetle introduces blue stain fungi into pines. Other factors include drought and windthrow, both of which we've had plenty of in recent years.
Fir decline creates patches or groups of bright red trees on the landscape. Again, this is similar to the mountain pine beetle epidemic we saw in our lodgepole pine but with some significant differences. Unlike the pine beetle, fir decline does not usually impact the whole stand in a few short years, and mortality tends not to be uniformly spread across the stand. Because the needles tend to stay on the fir longer than needles do on pine, the red patches we see on the hillsides might represent several years of mortality.
The western bark beetle typically has a two-year lifecycle, though that can be shortened to one year under the right weather conditions. In a two-year lifecycle, the beetle spends two winters developing under the bark, emerging in the second year's spring as an adult. It is the male beetle that attacks the tree, excavating a nuptial chamber under the bark to which he attracts several females. The resulting pattern seen under the bark is somewhat star shaped with a central chamber and galleries radiating out randomly. The external evidence of attack on the tree can be hard to see. There might be some pitch flow visible, but there might not. The attacked tree usually will turn yellowish-red within one year and can stay red for a year or two after death.
The best control method for the western balsam bark beetle is to remove beetle-attacked subalpine fir and subalpine fir windthrow, along with the logging slash, as soon as possible. The good news is that moisture we've received this summer and fall should relieve some of the drought stress that might be contributing to our fir decline. Although it is discouraging to lose any green trees after the significant change in our forests from the recent mountain pine epidemic, fir decline is not expected to create anywhere near the level of impact we saw from the mountain pine beetle.
John Twitchell is the Steamboat Springs district forester for the Colorado State Forest Service. For more information, call 970-879-0475.