Most people are probably aware that mountain pine beetles have been decimating Colorado's lodgepole pine forests, affecting over 2 million acres of woodlands. Google the subject and you can find dozens of articles blaming the beetle infestation on global warming - the absence of cold winters has allowed the beetle population to thrive, the story goes. Maybe.
A hypothetical posed on the TV series Law and Order went something like this: a man jumps off the top of a 100 story building and as he's falling past the 50th floor, someone fires a gun. The bullet lodges in the man's brain and he dies almost instantly. Given that he wouldn't have survived the fall anyway, is the shooter responsible for his death?
The condition of Colorado's forests is like the man who jumped off the top of that building. Global warming is the bullet lodged in his brain, but likely fired from the 1st or 2nd floor, rather than the 50th.
It's always good to start with a little history. Colorado was settled in the 1870s, largely due to gold and silver mining, which continued large-scale into the early 1900s. Colorado mining was hard-rock mining - underground tunnels shored up by mine timbers. Mine timbers were cut from trees in the mine vicinity. Enough timbers were cut to completely denude Colorado hillsides in the mining districts. These are the areas most affected by beetles now.
Colorado's forests re-generated, probably with the help of fires, since they're mostly lodgepole pine forests. Lodgepole pine cones require temperatures of 110F to 140F to open and release their seeds, and they thrive on burned-over mineral soils where they have little initial competition - the situation found after a fire. According to one of the slides in this presentation (which also has year-by-year maps of beetle spread), about 87% of Colorado's lodgepole pine are over 80 years old, and more than half over 100 years old - most date from the mining era. The term "even-aged stand" applies here, as it does to most lodgepole forests historically.
Lodgepole pine are prolific seed producers. Some re-generation sites have recorded as many as 300,000 seedlings per acre, and there are records of lodgepole "forests" of 70 year-old trees, 4 feet tall, with densities of over 100,000 trees/acre. If you can't picture an acre, picture the area of a 1 foot square floor tile with 3 trees growing on it. That, by any definition is high density forest, and even in more reasonable densities, lodgepole left to their own devices tend to form forests with a large number of trees per acre.
Lodgepole pine are thin-skinned trees, which means they have poor fire-resistance. While older lodgepole will sometimes withstand a low-intensity ground fire, more commonly lodgepole burn in "stand replacement" fires, where an entire stand of trees burns and is replaced. Most other pines have thicker bark and withstand fire well, so those pine forests can grow to great ages (thousands of years in the case of sequioia, although they're not pines, strictly speaking). On the pine scale, lodgepole are relatively short-lived. Individual trees can live to 600 years old, but most forests, on average, live 100-200 years.
What kills lodgepole pine? Meet the beetles, most often mountain pine beetles (MPBs). MPBs are little guys, about 1/4 of an inch long. Beetles have a one-year life cycle, so let's begin at the point where the adult beetles leave the trees they grew up in. They search for the nearest tree to attack, and when they find a suitable tree - usually one that's weakened in some way - they emit a pheromone that attracts other beetles to the tree. Then:
Female beetles chew through the bark and deposit eggs, a fungus and a bacterium.
While the eggs develop, the fungus grows into the tree tissues, turning the wood a dull gray-blue color, hence the name "blue stain fungus."
Contrary to popular belief and even some academic descriptions, beetle larvae do not eat tree cells or tissues. As with other animals, they cannot digest the cellulose. Instead, they eat the fungus.
Other fungi impalatable to the beetle larvae can invade the tree through the beetle tunnels and compete with the palatable fungus. The bacterium left by the female beetle afflicts and suppresses the undesirable competing fungi without impairing the vital food fungus.
Look closely to see the forest for the connections
- Kevin Cook
Beetles will also attack trees that have been cut down, and the blue stain fungus will spread through cut logs as rapidly as in a living tree (personal observation).
Trees do have defenses against beetles. As the beetles burrow through the bark into the cambium to create an egg gallery, they will cut into the phloem - the layer which carries nutrients between the foliage and roots. Pines have some larger tubes in this layer that, when cut by the beetle, produce enough sap flow to eject the beetle from the tree. Beetles also have natural enemies - woodpeckers and some other insectivorous birds at nearly any stage, and carpenter ants in the larval stage. During outbreaks, natural controls have little effect on beetle activity. There are insecticides for MPBs - they have to be applied annually and with near perfection over most of the bark surface to be effective.
One creative possibility:
Herbal tea may soothe your nerves, but it’s stinky stuff if you’re a bark beetle. So say U.S. Forest Service scientists who have found that sprinkling tiny flakes of tea containing the pheromone verbenone over lodgepole pine forests reduces the number of beetle-killed trees by two-thirds, according to The Washington Post. The pheromone tricks the beetles into thinking that a tree is crowded, so they move on.
The Holistic Way to Kill Bark Beetles
One other thing beetle larvae do is make their own glycol anti-freeze. A cold snap in late fall, before the beetles have produced enough glycol, can kill them, as can a cold snap in early spring as they're reaching the adult stage. But in mid-winter, it requires about 5 days at -30F to kill wintering beetle larvae.
So warmer winters do encourage beetles, but:
The mountain pine beetle has played an historic role in the dynamics of lodgepole pine ecosystems. By periodically invading stands and creating large amounts of fuels, which are eventually consumed by fire, creating favorable conditions for regeneration, the beetle has increased the probability that lodgepole pine will reoccupy the site at the expense of other species.
Lodgepole Pine - James E. Lotan and William B. Critchfield
What evidence is there for this kind activity in the past, pre-global warming? Some is buried in reports by Forest Service and other scientists that few people read, despite their being a surfeit of information online.
Curiously, people in the urban eastern US have experienced a similar epidemic - Dutch elm disease. In the early 1960s in Milwaukee, I used to walk about 6 blocks to junior high school through residential streets lined with huge elm trees that arched over the streets. By the end of the 1960s nearly all of the elm trees in Milwaukee (and Chicago, and Detroit, and elsewhere) were gone - victims of a beetle and fungus with the same dynamics as the mountain pine beetle.
If you're not old enough to remember elm trees, perhaps you remember the 1988 fires in Yellowstone National Park. Those fires, which covered over 700,000 acres, were largely in lodgepole ecosystems. One paper (actually published in Science) analyzed the period from 1970 to 1986 and 1987 to 2004 and concluded there were larger fires in the later period due to global warming. What they didn't note was that 95% of the acreage burned in Yellowstone over the entire period from 1970 to 2004, burned during the 1988 fire, and represented something like 20% of the acreage burned in that year nationally. Why is that a significant omission? Here's why:
A 750-year fire history was reconstructed for the Central Plateau of Yellowstone National Park from the deep-water sediments of five lakes. The charcoal record from a large lake provided a chronology of regional fires. ... Large areas of the region burned in AD 1988, c. 1700, c. 1560, and c. 1440. From c. 1220 to 1440 and c. 1700 to 1987, intermediate to small areas burned. The near-absence of fires in the twentieth century prior to the large fires of 1988 is evident in the charcoal record.
Again, parts of Yellowstone are largely a lodgepole pine ecosystem, and the interaction of beetles and fungus, mature lodgepole and fire is a repeating cycle that lodgepole have evolved to live with. Global warming may have accelerated the death of lodgepole pine in Colorado forests in this cycle, but the forests were nearing the end of their time anyway. Same as it ever was.
For those who believe that nature should be left to its own devices ("untrammeled" as the Wilderness Act says), the decimation of Colorado forests is a Good Thing. It's another turn of the wheel that's been spinning for eons, a natural response to conditions that were started a) when lodgepole pine became the dominant ecosystem in parts of Colorado sometime during the Holocene and b) when mining leveled Colorado forests in the late 19th century. Letting Colorado forests burn is the natural thing to do.
But wait --- there's more ... Could Colorado forests have been saved? Maybe. Start with how the beetles decide to attack a tree: MPBs like large trees - meaning older trees - and they're more successful at attacking weakened trees. What weakens trees? In the inland west there are a couple of major factors. One is the occurance of dwarf mistletoe, a vegetative parasite that affects conifers and slowly kills them. The other factor is water. The inland west has dry summers in most places - if not every year, than frequently enough to be significant. Remember all those westerns where the ranchers fought over water rights?
It turns out that density - the number of trees per acre - exacerbates both of those conditions. If there are more trees per acre, there's less water available per tree. Dwarf mistletoe spreads by shooting sticky seeds from little hydraulic cannons (I am not making this up) with a range of about 30 feet. If there are more trees within 30 feet, there are more places for the seeds to stick and take root. And it turns out that in a 46 year study, British Columbia Forestry found out that the fewer lodgepole per acre, the better their chances of surviving a beetle attack:
During the 46-year observation period, the plots were attacked by mountain pine beetle (Dendroctonus ponderosae Hopkins), and the results of the study tend to support the theory that heavy thinning may help to beetle-proof lodgepole pine stands.
Thinning Lodgepole Pine in Southeastern British Columbia: 46-year Results - W. D. Johnstone
You can read the entire study online. The title of that article gives the game away. If you knew a beetle attack was coming, you could thin a forest to make it more beetle resistant, and someplace like Colorado wouldn't lose nearly all of its forest in just a few years. And in the 1970s and 1980s, the Forest Service was well aware of that, and actually operated programs to reduce the threat of beetles (among other things).
In the 1970s, Colorado produced over 100 million board feet of lumber annually from lodgepole pine. By the 1980s that was down by 80%, and is probably less today. I have no idea of how that program was run - whether it was actually done in a way to improve forests. But selective cutting and some small clearcuts in infected areas over that period would have left Colorado forests in better shape.
What stopped that? Public outrage over trees being cut.
That outrage leaves Colorado facing basically two choices: let the forests burn, or cut down the forests before they burn and yes, that's spelled "c-l-e-a-r-c-u-t", because less drastic prevention wasn't carried out in time. The trees are already dead.
The logging option may have gone with the mortgage crisis. British Columbia (which has an even larger problem than Colorado with MPBs) fostered a logging/lumber boom in its dying forests in an area roughly from Quesnel and Fort St James north to Prince George. It was exceptionally good for the BC economy until the bottom fell out of the US construction market.
Had cutting and reforestation continued from the 1970s onward, Colorado would no longer have only the old, even-aged forests that beetles thrive on, that are timed to die almost all at the same time. And instead of the millions that will be spent in coming years defending from wildfire the homes of people who stopped the logging in Colorado, Colorado would have a sustainable forest products industry, along with growing forests.
Sometimes you can't see the forest for the trees. But ironically, the forest and trees and the beetles are doing what they've always done, and the trees are there because beetles and fire helped create the conditions that made it possible for the trees and beetles and fire to be there. If that sounds circular, it is.