My first Colorado summits in 1971 offered views of dark green pine, spruce, and fir forests stretched across the mountains as far as the eye could see, broken only by snow fields, red and gray rock outcrops, and the grayish green tundra of the highest peaks. I thought the forests always had and always would look dense and green. Now the mountain traveler in the Rocky Mountains of the United States, Canada and Mexico sees the green forests broken by vast swaths of red pines or ghost grey pines bereft of all needles. The immediate cause: bark beetles. The primary cause: warming and drying of forest lands. The drivers of this change are greenhouse gas emissions coupled with changes in land uses.
The most notable infestations involve the native mountain pine bark beetle (Dendroctonus ponderosae). This tiny critter, the size of a single rice grain, moves through lodgepole, ponderosa, or limber pine forests in three predictable stages:
1. Green needle – During mid- to late-summer dispersal, beetles bore into the inner bark of a living tree and lay their eggs, usually selecting trees over five inches in diameter and avoiding those where beetle pheromone secretions establish that a tree is already infested. Yellowish-white pitch tubes show that an infested tree is attempting to flush out the beetles.
2. Red needle – The red needle stage, highly visible during the second to fifth year of infestation, is evidence of the beetle’s double whammy. Eggs have matured into larvae, which eat the inner bark, while a fungus brought in by the burrowing adults has spread in a blue-gray stain, cutting off the flow of water and nutrients. In late summer, the new adult beetles emerge from the bark and fly off to infest near-by trees.
3. Gray tree – Following red needles drop, the stem and branches of the once-infested tree stand out as gray in the landscape for up to 20 years before dropping to the forest floor.
Mountain pine beetle infestations, both negligible and wide-spread, have moved through lodgepole forests over thousands of years of tree and beetle co-existence, and indeed play a role in forest health by creating a patchwork of old and young trees. Joining the mountain pine beetles in the forest dynamics are a host of other native beetles, each generally specific to one tree species: Douglas-fir beetle, spruce beetle, pinyon ips, and fir engraver. Native western spruce budworm, wood borers, white pine blister, and Douglas-fir tussock moth cause additional tree damage and mortality. Splashes of red highlight Colorado areas of dead and dying trees in recent mapping of infestations. The loss of coniferous forest is compounded by Sudden Aspen Death, a syndrome of factors such as cankers, stem borers, and grazing pressure, which diminish the green and golden aspen groves that spring up following forest fire.
Climate change makes this infestation particularly rapid and unpredictable. Severe drought and warmer temperatures are stressing already, crowded even-age tree stands. In response to a lengthening growing season, mountain pine beetles can produce two generations a year instead of one generation every one or two years. Episodes of fall and spring temperatures low enough to kill larvae (around minus 40 F) have become less frequent. Mountain pine beetles have been attacking smaller diameter trees as well as trees at higher elevations. Normally limited by their short flight capacity, beetles have lately been observed on radar, high above the canopy, boosted by high winds many miles into un-infested forests. Some researchers report that mountain pine beetles have been observed crossing into previously untouched conifer species. To make matters worse, in killing the trees, these little critters turn what was a carbon sink into a carbon source likely to last more than a decade.
The large tracts of dying and dead trees raise concerns of mountain residents – conversion of majestic, deep green forest into unattractive red and gray trees, wind throw damage to power lines and buildings, and the threat of uncontrollable wildfires, to name a few. Some ski towns, such as Breckenridge, require landowners to cut and remove all beetle infested trees. The Breckenridge Ski Resort is working in partnership with the U.S. Forest Service to revegetate old lift lines and trails with a diverse selection of conifer species. Some property owners are investing in treating healthy trees with a preventative spray, trying out pheromone pouches to discourage beetle attack, or thinning forests to allow more water and nutrients for the remaining trees. To encourage use of tree thinning and beetle-killed trees, the Colorado State Forest Service is promoting use of local wood for landscape timbers, animal bedding and landscape timbers. Boulder County, on the other hand, is letting the mountain pine beetle infestation run its course.
One might fear that the increased dead and dying trees might increase the risk of wild fires, but in a recent wild fire literature review, western forest researchers concluded that bark beetle infestations do not set the stage for catastrophic crown fires. High-intensity fires are the result of very dry conditions which allow the spread of flames through both living and dead trees.
How will the pine bark beetle impact Colorado’s future? Plant communities of classical ecology over time reset to a stable climax system of foreseeable species and interactions with the environment. In our time, change has become far less predictable. Eventually, the epidemics of mountain pine beetle and other insects will collapse from overuse of tree food sources. Depending on future climate, the now emerging woody species will develop into new tracts of dark green pines or possibly a new combination of mountain vegetation. We may see more aspen coloring the mountain sides and a higher proportion of hardy shrubs and grasses. Trees such as pinyon and ponderosa pines, adapted for the warmer, low elevation forests are likely to become more common at higher elevations. Mountain plant communities will adapt, and we will adapt along with them, changing our expectations of forestry functions and mountain beauty.