In the past 20 years, Southwest Colorado forests have been in the line of fire of insect epidemic and disease.
The pattern is a clue of a drying climate that could produce a much different landscape 60 years from now in the Dolores watershed, said Jim Worrall, a forest pathologist with the U.S. Forest Service.
He presented the results of a new Forest Service study during a recent meeting of the Dolores Watershed and Resilient Forest Collaborative in Cortez.
First, it was the ongoing spruce beetle epidemic in the eastern San Juan Mountains, Worrall said, then a sudden aspen decline, which focused in the La Plata Mountains.
About the same time, the round-headed pine beetle moved farther north then before, ravaging forests in northwestern Montezuma County. Add to that an emerging budworm infestation in the Dolores Valley.
Many of these forest problems can be attributed to a drying trend from the mid-1980s that culminated in a turn-of-the-century drought with record temperatures in 2001-2003, Worrall said.
“It was a climate change-type drought, and it occurred across the interior West,” he said. “Put in context, you have to go back 800 years in the tree ring data to find a drought that severe.”
Climate studies forecast that the severe dry conditions of 2002 could be the norm by mid-century, Worrall said. Other research shows they could be the norm by the 2030s.
“We want to try and anticipate what kind of impacts that will have on our forests,” he said. “The models are projecting some pretty big changes that you can’t candy coat.”
Worrall and his team developed bioclimate models with data from 14 tree species in Southwest Colorado to tease out what the landscape might look like for the future of the Dolores River watershed.
Computer modeling of the watershed used 850,000 tree-location data points and accounted for tree type-location or absence, topography, historic climate and climate variables. It then used algorithms to predict the likelihood of the species being present there in the future.
The modeling shows significant future changes. It predicts that by 2060, a drying climate will have eliminated ponderosa pines from current locations. They will move to higher elevations, possibly replacing spruce-fir stands. Oak brush is also shown moving into higher elevations.
A lot of the stands of the Utah juniper variety would be lost to a drier climate, models show, but aspen stands will likely persist because of to their resistance to drought.
Worrall said the data is valuable for adaptive management strategies for the forest.
For example, if foresters have a good estimate of where a species will or not be suitable in the future, they can apply the best treatments.
“Let’s say you want to sustain threatened spruce stands: Bioclimate modeling can identify the best sites to focus resilient treatment efforts based on an anticipated future,” he said.
Some species such as Douglas fir and aspen have the potential for assisted migration to preserve the resource, the study shows. For example, it would make sense to manage persistent Douglas fir stands as a seed source to promote emergence in other areas.
Another scenario shows piñon pine suitability in some areas over ponderosa pine in the future.
That could guide future management decisions, Worrall said. For example, the round-headed pine beetle is impacting ponderosa forests in the Lake Canyon area of western Montezuma County, the farthest the beetle has ever been seen at that concentration north of New Mexico.
Local foresters are thinning the forest there to try and stop or slow down the beetle’s damaging progress.
“Long-term ponderosa may not be the future, so where there are good piñon stands, that component should be preserved instead of hydro-axing it,” Worrall said.
Protecting seed sources is an important aspect of planning for the future, agreed Bruce Short, a silviculturist from Mancos.
“We need to be proactive in thinking about how we manage what we have in the next 50 years,” he said. “Should we start taking seeds from one area to another based on future suitability?”
The forest crystal ball also reveals potential erosion problems in the future, added Mike Preston, general manager for the Dolores Water Conservancy District, which manages McPhee Reservoir.
“If the ponderosa pine zone comes apart, and there are no re-emergent species to replace it, it could impact water quality,” he said.
It used to be foresters did not have to think about the future of the local forest because it was understood that it was going to be like the past, Worrall said. But that is no longer the case.
“We have a different climate coming in the future and we have to think differently,” he said.