New York Times, Nov. 17, 2008–Montana campground.
Clint Kyhl, a Forest Service employee working out of Laramie, Wyoming directs an “incident management team” that focuses on managing fire threat in dead forests. Dead forests? Yes. Because more and more, up and down the Rockies, that’s what we’ve got: dead forests.
I don’t need a newspaper to show me this; I can see it when I look at the foothills of the Hyalite Mountains just south of Bozeman, and when I go skiing in the Bridgers to the north.
One beetle. NY Times, Nov. 17, 2008
If you round up all the usual suspects, you’ll only have one: the mountain pine beetle (Dendroctonus ponderosae Hopkins) an insect about the size of a grain of rice that appears to have laid a diabolically clever plan (along with millions of eggs). In the late summer adult females leave the trees they’ve killed over the summer and go searching for new victims, preferably mature, healthy pines. A female who finds a likely-looking tree bores into the bark and then releases pheromones which attract both males (potential mates) and more females, which release more pheromones, which results in a mass attack on the tree, which is one reason why the tree’s defenses can be overwhelmed.
Last spring I wrote about pine beetles but I focused on British Columbia, where the situation is worst. Three days ago (Nov. 20), the New York Times ran a story on the infestation here in the United States. It included Mr. Kyhl’s estimate that within three to five years, “virtually all of Colorado’s lodgepole pine trees over five inches in diameter will be lost, about five million acres.”
Blue stained trunk; Colorado State U. Ext.
The beetles do all the normal things, all of them under the bark: they mate, they tunnel, they lay eggs, and the eggs, all too often, hatch. All that happens in the fall. The larvae spend the winter in their tunnels, snuggled down into the frass (a mixture of sawdust and excrement) with which their beetley parents so thoughtfully lined their living quarters. Come spring, the larvae get down to business eating the layer of wood under the bark (the phloem) which carries nutrients to the foliage above. As just about everyone knows, all you have to do to kill a tree is to “girdle” it: damage or kill that nutrient-carrying layer all the way around the tree, and the tree will die. Enough larvae munching away under the bark of one tree can girdle it, and voila: one dead tree.
Pine beetle in sap; Colorado State U. Ext.
But it turns out that the beetles don’t operate strictly alone. When the adults colonize a new tree, they often carry with them the spore of Ophiostoma minus, better known as blue stain fungus. This charmer inhibits the flow of sap, and sap is, guess what, necessary to the tree. It’s also the tree’s main defense against the beetle, since a beetle swimming in sap is a stuck bug.
There are always a certain number of beetles around, so why have we suddenly got a crisis? Why are most of the forests throughout the west probably doomed? As usual, there’s more than one reason.
Drought: Drought weakens trees, leaving them more vulnerable to any onslaught. And guess what: most of the west has been drought-stricken for years. But drought plays a more specific role in relation to pine beetles. Since the tree’s primary defense against the beetle is sap, and drought decreases sap-flow, drought reduces the tree’s ability to fend off early invaders.
Warm winters: Extended periods of extreme cold—thirty or forty below for weeks at a time—used to be the norm in the high Rockies. Such spells kill many of the larvae, keeping the beetles in check. But such winters are now a thing of the past, which is why the beetles have proliferated and the forests are dying.
So though the direct, the immediate, the proximate cause is the beetle and its attendant fungus, the larger, indirect, more fundamental cause is—you guessed it—global warming.