‘We are seeing the initiation of a retreat of forests to higher elevations’
Global warming is likely one of the main factors that’s preventing some Colorado forests from regenerating after wildfires.
When they started studying eight wildfire sites that burned across 162,000 acres of low-elevation forests along the Front Range, University of Colorado Boulder researchers said they expected to see young trees popping up all over the place, but that’s not what they found.
There were no seedlings at all in 59 percent of the study plots and 83 percent showed a very low density of seedlings. Future warming and associated drought may hinder significant further recovery, the researchers concluded. Only 2 to 38 percent of plots surveyed, depending on the fire site, were considered stocked, or on their way to recovery.
“It is alarming, but we were not surprised by the results given what you see when you hike through these areas,” said co-author Monica Rother, who earned her doctorate from CU Boulder in 2015 and works as a fire ecologist at Tall Timbers Research Station in Tallahassee, Florida. “”Fire severity is definitely relevant, but climate appeared to play the greatest role,” in limiting forest recovery, said Rother.
The study, published in the journal Ecosphere, looked at the Walker Ranch, Bobcat Gulch, Buffalo Creek, Hayman, High Meadows and Overland fires, which all occurred between 1996 and 2003. The data was collected at a total of 302 plots at various elevations and fire severities.
Fire intensity is definitely a factor, as suggested by a recent study in California that showed how big, severe fires are killing seed source trees. But the new CU study suggests that increasing Colorado temperatures are hampering forests’ ability to regenerate after a fire and moving the treeline of ponderosa pine forests upward.
“This should be a wake-up call, that under the warming trends associated with human-caused climate change, significant shifts in forest extent and vegetation types are already occurring,” said CU geographer Thomas Veblen. “We are seeing the initiation of a retreat of forests to higher elevations.”
The Hayman fire area, the largest fire in Colorado history, fared the best, with 40 percent of sites showing abundant regeneration. This could be due to its exposure to more summer monsoonal rains.
The study found that even after lower-intensity fires, which have less effect on mature trees and seed stock, seedlings were still scarce. Hotter, drier areas at lower elevations or on south-facing slopes had the fewest seedlings.
For a companion study, published in 2015, the authors planted ponderosa pine and Douglas fir seedlings at Heil Valley Ranch in Boulder County, placing half of them under warming chambers to raise the temperature by 2.7 degrees F. The warmer temperatures alone severely limited seedling survival and growth.
Statewide annual average temperatures have risen about 2 degrees Fahrenheit in 30 years and are expected to rise another 2.5 to 6.5 degrees Fahrenheit by 2050.
In terms of fire management, a transition from forest to grassland could be desirable in certain highly populated environments, he said. But such shifts could also have negative implications for some species of wildlife and for watershed management.
“I don’t want to present this as being entirely negative,” said Veblen. “For me, the negative aspect is what it indicates about the future.”
For now, the authors hope their research can help forest managers determine where to plant seedlings after a fire to give them the best chance of survival.