FORT COLLINS, Colo. -- State Highway 14 cuts a tightly winding channel through the canyons of the Cache la Poudre Wilderness, a section of national forest in this state's northwest corner and the site of last year's High Park fire. Each hairpin turn opens onto a different landscape: around one, a hillside flush with lodgepole pine; around the next, a blackened slope where bare trunks stand out like so many toothpicks against the sky.
Fire is as much a part of Western ecology as rain, thinning forests and purging old growth so that new trees can take root. But in the past several decades, something has shifted in that balance of destruction and regeneration, and trees are not always returning as quickly as expected, forest scientists say. In much of the Southwest, as well as select parts of the Northwest and the central Rocky Mountains, a fundamental ecological shift is taking place with new tree seedlings being outcompeted by grasses and shrubs.
"Taking the long-term view, in terms of a general narrative, we're looking at potential landscape change," said Tom Veblen, a professor of biogeography and conservation at the University of Colorado, Boulder. "If you look at what's happening in the Southwest, many areas that burned in the last few years are not experiencing good regeneration. It appears that Colorado is just a few years behind that in terms of drought."
Scientists are still working to understand the causes of this change, as it takes close to a decade to establish the pace of post-fire regeneration at any given site, and Western wildfires have been growing progressively more severe over the past 25 years. But two principal theories have emerged.
The first is that overgrowth and drought have resulted in hotter, more intense fires than those to which Western forests are adapted, clearing sections so completely that there are no trees left to reseed the soil. This theory has been borne out extensively in the Southwest, where fires over the past two decades have shifted large sections of landscape from coniferous forest to shrub and grassland (ClimateWire, June 28, 2012).
The second is that hotter, drier conditions are preventing new seedlings from gaining a foothold, giving the advantage to more drought-resistant species like prairie grasses. A recent study from the Metolius River watershed in Oregon found that regeneration occurred less often in lower-elevation sites, which tend to experience drought at more pronounced levels than upslope areas (ClimateWire, July 1). A separate survey of parts of the central Rockies in Colorado produced similar results (ClimateWire, Dec. 14, 2012).
Both theories hinge on the hotter, drier temperatures experienced in the West over the past two decades, conditions expected to deepen under climate change.
"It's not all sites and all places at all times, but if we have confidence in the climate model predictions, then according to these theories, we would expect the whole process to accelerate over next few decades," Veblen said.
Fire tips the balance
Global climate change moves slowly, advancing by parts per million of greenhouse gases in the atmosphere or fractions of a degree Celsius. But on the ground, the effects of climate change sometimes appear in pulses, or what scientists term "disturbance events." A disturbance event such as fire or flood or an insect outbreak sweeps through a given ecosystem, and when the flames have died down and new shoots appear, they're not necessarily the same species as populated the landscape before.
In areas where a particularly hot fire has cleared all the standing trees, "you might see a rapid shift in vegetation," said Monica Rother, a graduate researcher at CU Boulder who is conducting a survey of post-fire regeneration. "In areas without such a complete disturbance, mature trees wouldn't necessarily be killed by hotter, drier conditions, and regeneration would be more sensitive. In those areas, you'd see a more gradual shift" under climate change.
The mountain pine beetle epidemic that has ravaged much of the West, including large sections of British Columbia, could function as a similar disturbance event. It takes only a few years for younger, smaller beetle-killed trees to fall and begin to decompose, said Veblen.
Recent decades have seen the Western spring season lengthen, according to data from the U.S. Geological Survey. Snowpack melts one to four weeks earlier today than it did half a century ago, meaning less moisture for forests, going into hotter summers. Unsurprisingly, fire seasons begin earlier and burn longer.
Colorado set the record for its most destructive fire last year with the Waldo Canyon fire outside of Colorado Springs. That record was broken this year, again near Colorado Springs.
Should forest managers step in?
It will take years to know whether the High Park fire follows in the pattern of landscape change. If it does, the effects will likely not be dramatically apparent -- a few new high mountain meadows, a few areas where trees no longer grow. It would take many more years, and many more fires, for the landscapes to shift completely.
On parts of the burn area, however, landowners are already taking matters into their own hands, replanting saplings in places where the fire was severe enough to replace entire stands. Last week, the Forest Service announced that it, too, was formulating a proposal to replant areas of national parkland affected by the fire.
"The proposal is for a small fraction of the affected park land, specifically ponderosa stands that experienced high-severity burn," said Forest Service spokeswoman Reghan Cloudman. While higher-elevation lodgepole pines are historically adapted to stand-replacing fires -- their serotinous cones only open under high heat -- the Forest Service is concerned that ponderosas will not regenerate in those areas on their own, she said.
Leaving the landscape unforested increases the risk of flood-induced erosion, a potential threat to drinking water, she said.
But given the trend toward hotter, drier temperatures, the benefits of this effort may be short-lived. "From a management perspective, people need to be thinking outside of the box," Veblen said. "If it's true that the climate is no longer favorable to these tree species, then we need to do some assessment of where it's worthwhile to take actions like planting."