Forest Health
Many folks visiting the Rio Grande National Forest see vast swaths of the high elevation forests where literally millions of trees have succumbed to the spruce bark beetle. A total of 617,000 acres of spruce-fir forest were infested by spruce beetles between 1996 and 2023. However, the Douglas-fir beetle is now the most active beetle on the Rio Grande in the mid-2020's. This bark beetle affects mid-elevation forests and is most noticeable between South Fork and Creede.
True to it's name, the Douglas-fir beetle attacks only Douglas-fir trees while the spruce beetle primarily attacks mature Engelmann spruce and it sometimes infests blue spruce. The tiny Douglas-fir beetles usually kill trees down to 12 inches in diameter. Luckily, small Douglas-fir trees are not affected by this particular insect, and they will provide the base for creating the next forest.
A visitor might choose to think of the beetle epidemic as sad. But it helps to remember that the forests have evolved throughout time from natural disturbances such as beetles, floods, landslides and, of course, wildfires. Our human perspective is valid to us, but in terms of the landscape over time, it doesn't have much bearing. An interesting perspective to consider is one from which we are witnessing one of the major changes we'll see on this landscape, perhaps for another five hundred years!
Forest Health Related Informational Links
The Spruce Beetle
The spruce beetle infestation has subsided on the Rio Grande National Forest (RGNF), and beetle populations have returned to endemic levels. Each year, aerial surveys detect the spread of spruce beetle infestations across the state, along with other insect and disease agents. Over 617,000 acres were infested here on the RGNF, from 1996 to 2023. The beetle populations diminished on the RGNF because most of the spruce-fir forests, and the Engelmann spruce components of mixed conifer forests, had already been infested and available food source was depleted. In the Rocky Mountains, spruce beetles primarily infest Engelmann spruce which grow in our high elevation forests. Spruce beetles can infest blue spruce, but it is not common.
Not all areas were affected equally. In some areas, all the spruce trees larger than 5 inches in diameter were killed by spruce beetles. In other areas, there is a mixture of dead and living spruce trees of all sizes.
Subalpine fir and many of the young spruce were not affected by the infestation and are now thriving. These trees will provide the base population for creating the next spruce-fir forests. Additionally, seeds collected from live Engelmann spruce on the RGNF are being used to grow seedlings to be planted in the West Fork Complex burned areas and in harvested areas where there is not adequate natural regeneration.
The RGNF continues to offer timber sales in designated timber management areas to salvage dead and dying spruce while the trees can still be used for commercial sawtimber.
The large number of dead trees in the forest may lead people to believe that there is an increased chance of a severe wildland fire. A closer look reveals that is probably not the case. Large fires are somewhat rare in the high elevation subalpine forest where Engelmann spruce and subalpine fir grow. Some research indicates that there is no difference in the chance of a wildland fire starting in a green or beetle-killed subalpine forest. Different fuel arrangements and fuel moisture content complicate how wildfires burn in areas with beetle-killed trees.
Initially, when the dry, dead needles of beetle-infested trees are still on the branches, there may be an increased risk of fire burning through the crowns of the trees. When needles fall off the trees, they quickly absorb moisture from the ground and decompose within a few years. In dry conditions, the fallen needles may help carry a ground fire, but the tops of standing bare trees are less likely to carry a crown fire than those of living trees. Once the trees fall to the ground, a fire burning through the area may burn very hot and damage the soil, but the risk of a fire starting is no greater than when the trees were still alive.
Many aspects contribute to wildfire behavior. The arrangement and moisture content of fuels as well as wind conditions affect wildfire behavior.
Safety is a concern in the beetle-killed forests because dead trees eventually fall. Although spruce trees may stand for decades after they die, many have blown down. There are a number of factors that affect the risk of trees falling, such as soil type, soil moisture, topography, and wind. In some situations, green, living trees may be more susceptible to blowing down than the dead trees because their needles catch more wind. It is difficult to predict when specific trees will fall, so it is highly recommended that people take extra precautions when visiting beetle-killed forests.
Precautions include:
- Do not hike, ride, or drive in beetle-killed forests on windy days.
- Do not park or set up a tent near tall trees in a beetle-killed forest.
- Carry a saw or an axe when driving on roads or trails that go through beetle-killed forest.
Other Forest Pests
There are several naturally occurring insects that the Rio Grande National Forest provides habitat for. These insects have been part of the ecosystem for millennia and typically only cause minor damage to small groups of trees. Here are a couple of common occurrences on the RGNF.
Aspen Defoliators
The two most common defoliators of aspen on the Rio Grande National Forest are tent caterpillar and large aspen tortrix. Defoliators are insects that eat the leaves of trees and shrubs. Tent caterpillars larvae eat the leaves and build silken tent-like structures in trees and shrubs. The large aspen tortrix larvae also eat the leaves of aspen trees but roll the leaves around them selves to pupate.
When populations of defoliators are large enough, entire stands of aspen trees may be stripped of their leaves. As long as the aspen have sugar reserves in their root systems though, the trees will sprout new leaves following defoliation. The new leaves are generally smaller than those that grew in the spring, but they can still make for magnificent fall colors.
Individuals and clones of aspen trees may begin to die after several years of repeated defoliation as the sugar reserves are used up or from other insects and diseases attacking the trees. Typically, populations of the defoliators crash before there is significant aspen mortality.
Photo: Western tent caterpillar, Colorado State Forest Service
Western Spruce Budworm
While spruce budworm activity can vary greatly from year to year, infestations tend to be chronic on the Rio Grande National Forest; that is, there are always areas on the Forest with budworm activity.
The larvae of spruce budworms are defoliators that feed on the new needles of white fir, Douglas-fir, spruce, and subalpine fir. Brown, needleless branch tips on green trees are a telltale characteristic of a spruce budworm infestation. Individual coniferous trees may die within stands that have been infested by spruce budworm for several years.
Photo: Western Spruce Budworm, Dan West, Colorado State Forest Service