The 2017 forest health aerial survey results were recently released and once again they showed a decline in spruce beetle activity on the Rio Grande National Forest from the previous year. In contrast, the area impacted by aspen defoliators and spruce budworm increased.
The spruce beetle infestation spread another 7,000 acres in 2017, which was down from 22,000 acres in 2016. In total, spruce beetles have infested about 617,000 acres of spruce-fir and mixed conifer forests on the Rio Grande National Forest. That is a bit more than one third of the Forest. The rice-sized beetles and their larva were active on 47,000 acres on the Forest in 2017 compared to 93,000 acres in 2016.
The decline in the spread is a result of the spruce beetles running out of food. The primary food source for the spruce beetles in this area is our high elevation Engelmann spruce and there aren’t many stands left to infest. The beetles will sometimes infest blue spruce too, but mostly they prefer Engelmann spruce in the Rocky Mountains.
When spruce beetles move into an area, they don’t infest and kill all the trees at once. The active acreage identified in the aerial survey is an indication of the areas where the beetles haven’t killed all the spruce and are still present and attacking trees. In many areas, the beetles have killed all the spruce trees five inches in diameter and larger. Sometimes they will leave an area before killing all the large spruce trees and then come back a few years later to clean up their plates. The drop in the active acres from the previous year indicates there are fewer areas with live mature spruce trees.
There was never anything the Rio Grande National Forest could do to stop the spruce beetle epidemic, but in response, the Forest has sold a lot of timber sales to salvage the dead trees for commercial sawtimber. And thanks to additional funding, which will continue for the next two to four years, the Forest has tripled the amount of timber volume offered annually in salvage sales from just five years ago.
Some people are concerned about the increase in cutting timber, while others think the Forest isn’t doing enough. The primary purpose of the increased harvest is to use the dead timber while it still has value as sawtimber.
Logging beetle-killed spruce only occurs in areas identified in the forest plan as suitable for timber harvesting. These areas typically have an existing road systems, but sometimes require construction of short segments of temporary roads. Following timber harvest, spruce seedlings are planted in those areas that do not have adequate natural regeneration.
Spruce beetles aren’t the only insects affecting Rio Grande National Forest trees. Spruce budworm activity increased in 2017 impacting 50,000 acres compared to 25,000 acres in 2016. Spruce budworms eat the new needles on coniferous trees such as Douglas fir, white fir, subalpine fir and spruce. They typically don’t kill trees unless they stay active in an area for several years.
There has also been an uptick in the area impacted by aspen defoliators. Approximately, 5,700 acres of aspen were detected to be infested by defoliators during the 2017 aerial surveys. This is down significantly from the 31,000 acres defoliated in 2014, but up from the 3,500 acres defoliated in 2016. There are approximately 260,000 acres of aspen forests on the RGNF.
The two most common insects munching on aspen leaves are tent caterpillars and aspen tortrix. These defoliators don’t typically kill aspen, as the aspen will put out new leaves later in the summer after being defoliated. The loss of leaves can weaken aspen, though, opening the door for other insects and diseases to attack the trees.
All of the insect species mentioned above are native to Colorado. Often, they will have temporary localized impacts to the forest, while other times, such as the spruce beetle epidemic, their affect will be noticeable for many decades.
Mike Blakeman is the public affairs officer for the Rio Grande National Forest. He spends much of his free time scrambling around the mountains with a camera in his hand.