Intensive research shows vigorous regrowth in beetle-killed tracts
By Bob Berwyn
SUMMIT COUNTY — After years of uncertainty over the future of Colorado’s forest landscapes, a new study by U.S. Forest Service scientists puts the recent pine epidemic into perspective.
The insect outbreak ultimately will result in more diverse and resilient forests in the long run, adding structural complexity and species diversity, researchers with the USFS Rocky Mountain Research Station concluded after carefully monitoring regrowth in beetle-killed stands.
New growth is surging under the dying lodgepole canopy with the vertical growth rate of lodgepole and fir doubling in beetle-killed areas that were left untreated after the epidemic. Harvested stands also showed strong lodgepole regrowth, with aspen gaining ground in some places.
“Forests come and go … It’s not a crisis, but this was an amazing synchronism,” Forest Service biogeochemist Chuck Rhoades said of the massive pine beetle outbreak that will alter the forest landscape of the Southern Rockies for generations to come.
The bugs swarmed across vast swaths of the Canadian Rockies; they’ve invaded the Front Range and moved east to the Dakotas, especially the forests of the Black Hills.
“This event is not over, but the fear part should be over,” said Rhoades, who, with a team of researchers from the Fort Collins-based Forest Service Rocky Mountain Research Station, has been carefully studying regeneration in beetle-killed areas. “But the idea of forest health and maintaining forest ecosystem processes is something we’ll always be thinking about,” he said.
After years of studying the state’s trees, Rhoades wants people to understand that, more than anything, forest ecosystems are always dynamic. The pine beetle epidemic bulls-eyed over north-central Colorado was a dramatic pulse of change in short period of time, but not necessarily unprecedented in scope on a historic timescale.
Along with immediate concerns about increased fire danger and hazard trees long campgrounds and trails, the insect infestation raised serious questions about the future of the state’s forests.
Spending weeks in the field, Rhoades and his colleagues have been watching the forest regrow. Part of the work involves painstaking counts of seedlings in test plots, but maintaining that data over years enables the scientists to make direct comparisons between side-by-side stands of beetle-killed lodgepole, one side logged after the epidemic, the other left untreated.
Part of the research is in the Fraser Experimental Forest in Grand County. In that setting, Rhoades said the researchers had the luxury of choosing the stands, matching slope aspect and elevation to ensure an accurate comparison. Some of the new trees, and understory vegetation, are also measured for nutrients and water uptake.
Consistently, the monitoring points toward abundant regeneration in both areas, with aspen and lodgepole thriving in the treated areas, and subalpine fir starting to take over the unharvested stands.
In some of the study sites, the team found declining understory plant cover where the beetle-killed trees were logged. Some of the shrubs, including berry bushes, don’t respond well to sudden mechanical disturbance and to the associated increase in direct sunlight.
A science bulletin issued by the research station summarized: “The bark beetle outbreaks promote diversity in … species composition, age, and structure of the forests they infest, which may benefit forest health through increased resilience following future disturbance.”
“The fir is growing like gangbusters … if you’ve still got guys on the bench, they’re growing faster now,” Rhoades said, describing observations from on of the untreated areas. The scientists measured dramatic increases in foiliar nutrients, signalling that any nitrogen made available by the dying lodgepoles was being used in spurt of compensatory regrowth.
“The size of that response is the biggest surprise I’ve seen,” Rhoades said, explaining that the overall regeneration was not unexpected, given previous research in stands of lodgepoles killed during a 1980s pine beetle outbreak.
A few more highlights from the research station bulletin:
- Aspen increases following mountain pine beetle outbreak and will benefit some wildlife species and possibly slow the spread of future wildfires
- Surviving understory trees respond vigorously as beetle-killed canopies open and more water and nutrients become available.
- Multiple independent studies have found that water quality changes in watersheds infested by mountain pine beetles are minor; the positive growth response of the remaining forest vegetation and increased demand for soil nutrients are the likely explanation.
The bulletin is based on paper published last year in the journal Forest Ecology and Management, authored by Rhoades and his team. It describes specifically the research done in eight paired harvested and untreated lodgepole pine stands in the Fraser Experimental Forest, where overstory mortality was 70 percent.
The results have enabled the researchers to project what the oft-discussed future forest will look like, based on present conditions. There are other wild cards in the deck, including potential insect pests or other pathogens, but in general, forest managers can use the information to guide forest management practices.
And those efforts should continue to be focused on protecting homes, high-value recreational areas and critical infrastructure, said Wilderness Workshop director Sloan Shoemaker.
“Despite the hand-waving, hair-pulling rhetoric, it’s not an ecological crisis. We should go back to what we do best, which is focusing on protecting communities,” Shoemaker said, referring to potential wildfire threats.
“What’s so stunning about it is … the forest is recovering toward our management goals … of more diversity,” he said. In the bigger picture, the research shows a path away from the old paradigm, according to Shoemaker.
“We managed the forests to adapt them to our vision of the way things should be … what we found is, that’s a losing proposition. We’re accepting the fact there are processes happening that are bigger than us,” he said. “The new paradigm is to adapt our communities to make them more resilient, and to make sure communities can sustain themselves in the face of that change,” he said.
Shomaker says the report outlines clear choices: “If you go in, salvage lodgepole pine, you’ll get a new carpet of dense lodgepole, along with weeds and roads. If you leave it alone, you get a vibrant understory of shrubs and forbs and greater species diversity,” he said.
But the contrast between the regrowth in treated and untreated stands presents land managers with other challenges. The paper explains that a middle-aged forest filled with subalpine fir could change fire behavior. Firs tend to keep a thick coat of branches from the ground up, which could act as a ladder fuel.
There may also be changes in hydrology based on different vegetation types, but Rhoades said it’s important to remember the total scope of treated areas in these discussions. Even in Grand County, at ground zero for the outbreak, the Forest Service is only treating about 10 percent of the affected areas, he said.
Meanwhile, attention is shifting to a growing outbreak of spruce-killing beetles in the San Juans of southwest Colorado, where, according to surveys, entire drainages are losing their spruce component, and in the dryer, lower elevation areas, piñon are still struggling to come back from a drought-driven wave of ips beetles that wiped out 80 percent of mature trees in some areas, part of the ever-changing mosaics of Colorado forests.