What’s killing the great forests of the American West?

Large-scale die-offs have occurred in the past. Mountain pine beetles are native to the West and are part of the ecosystem. Lodgepole forests regenerate through large-scale “stand replacing events,” which include fire and insects. The die-offs now, though, are on a scale unprecedented since the West was settled and are so big that they are having unusual impacts on ecosystems. Diana Six contends that the whitebark pine, once largely protected from the beetles because it grew at high altitudes and was shielded by cold, is nearing functional extinction in large portions of its range and may no longer be able to feed grizzly bears and other species that love its high-fat nut. In Mexico, bark beetles are beginning to kill oyamel fir trees in a rare 139,000-acre biosphere preserve where the majority of North America’s monarch butterflies travel each fall to spend the winter. So far, about 100 acres in a core area of 33,000 acres have been killed by bark beetles.

Tree-killing bugs aren’t the only problem. In 2005 Colorado researchers noticed that aspens were suddenly dying in large numbers. That year they found 30,000 acres of dead aspen forest. The next year there were 150,000 acres, and in 2008 it had soared to 553,000. The die-off is called Sudden Aspen Death, or SAD. “It’s growing at an exponential rate,” said Wayne Shepperd, who researches aspen for the Forest Service. “It’s pretty sobering when you see a whole mountainside or whole drainage of aspen forest dead.”

Groves at low elevations and facing south are dying fastest, and scientists believe the cause is hotter temperatures and drier weather. It’s not only killing mature trees, but the root mass as well. An aspen grove is the offspring of a large single underground clonal mass, which sends up shoots. “The whole organism is disappearing and it has profound implications,” Shepperd said. “When the roots die, groves that are hundreds or thousands of years old aren’t going to be there anymore.”

If the die-offs continue, the loss of the aspen trees would be a blow to goshawks, songbirds, and a number of other species that find food and refuge in the groves.

Perhaps more than anyone, Craig Allen is familiar with these large-scale forest die-offs. A forest ecologist with the US Geological Survey’s Jemez Mountain Field Station in New Mexico, not only are his office and home surrounded by a pinyon die-off, he also is the lead author of the paper - with 19 other authors -published in Forest Ecology and Management, which sought to document and begin to understand what is happening to forests in North America and around the world as the result of climate change.

Coming up with a definitive understanding at this point is impossible, Allen says. Forests are complex, and unfortunately, woefully understudied, and there isn’t nearly enough data to draw a conclusion about the reasons behind forest die-offs globally. “There’s huge information gaps and uncertainties,” says Allen.

What contributors were able to do in the paper is collect anecdotal reports of broad-scale forest mortality from around the world. “The point of this paper is to connect the dots, at least the ones we can connect,” says Allen. “We can’t even tell you for sure if there’s more forest mortality. There’s not consistent monitoring.”

In 2005 a strong el Niño caused a dramatic drought in the Amazon. It killed forest across the region and is extremely well documented because so many researchers had existing plots there. “The heart of the biggest rainforest in the world turned from a carbon sink to a carbon source,” said Allen. “If you have long-term drought you can bleed a lot carbon into the atmosphere.”

A lot of beetles can also turn vast tracks of forest from carbon sinks to carbon sources. Take British Columbia, which is ground zero for the mountain pine beetle infestation in North America. Some 53,000 square miles of mature pine forest is dead and the province is projected to lose 80 percent of its mature lodgepole pine trees by 2013. The second largest known die-off there occurred in the 1980s and claimed just 2,300 square miles. Bill Wilson - the province’s director of Industry, Trade and Economics Research - said he has flown in a plane for hours over the province and seen nothing but dead forest the entire time.

In 2008, so much of British Columbia’s forests had died they also went from being a net carbon sink to carbon source.

Diana Six works in Africa where she has seen other die-offs first-hand. “In Africa where I work, suddenly whole hillsides are dropping dead,” she said. “It’s happening so fast people are in shock. It’s a tragedy.” Species include the quiver tree, camel-thorn, and the giant euphorbia, a 30-foot-tall succulent. The causes are not known, but the suspects are hotter and drier weather, or shifting rainfall patterns.

All told, the paper that Allen co-authored describes 88 well-documented forest die-offs around the world, going back as far as the 1960s and 1970s, although most are in the 1990s and 2000s.

If there was a way to predict die-offs, Allen said, land managers could take preemptive action, such as mechanical thinning or prescribed burning to increase the vigour of forests.

What gives researchers pause is that many of these large die-offs have occurred with minimal warming, in just a few years. In the West, for example, the average temperature has warmed on average 1.8 F over the past century. “This is before we put two to four degrees centigrade (3.6 F to 7.2 F) into the system,” said Allen, referring to forecasts for warming by the end of this century. Trees across the world are stressed already from fragmentation, air pollution, and other problems, he said. “I don’t know how much stress the forests of the world can take,” said Allen.

Correction, March 18, 2010: An earlier version of this article misstated the projected damage to British Columbia’s forests from infestations of mountain pine beetles. The insect outbreak is projected to kill 80 per cent of the province’s mature lodgepole pine trees by 2013.