DENVER — Glenn Randall has noticed a trend throughout his 29 years in nature photography.

As the Boulder-based photographer treks to the San Juan Mountains for photos of its infamous crystal blue lakes, jagged, rocky peaks and rows of aspen and pine trees, the fall season seems different each year.

He said that what he remembers as months of bright gold and crimson and popping from the leaves of aspens on peaks in Colorado’s high country is now a period so short that he feels he could miss it with a blink.

“A lot of aspen trees also seem to be dying, and there don’t seem to be other trees replacing the dead ones,” Randall said.

Fall foliage in the state’s mountains draws thousands of tourists each year, creating a robust tourism industry around “leaf peeping.” But Colorado continues to see rising temperatures and less snowfall — results of human-caused climate change. Scientists say that while climate change’s impact on leaves can vary from year to year, the effects are apparent in the health and stress to trees.

Dan West, an entomologist with the Colorado State Forest Service, said trees changing color in September and October is largely dictated by the changing of light conditions; shorter days trigger trees into shutting down and leaves saying goodbye to their green color, revealing the golds and reds the Rocky Mountains are famous for.

While climate change has little impact on the length of daylight, West said, heat waves continuing into the end of summer and beginning of fall places added stress on the trees, causing them to either increase or delay shutting down the process of leaf production, meaning the season of vibrant foliage is likely to come at a later date, stick around for a short period of time and provide brighter colors.

“Warmer days typically equate to sunnier days, which means we’re probably in for a better show as we move into that climate change condition,” West said. 

West said optimal conditions for bright foliage are sunny days, cool-but-not-freezing nights, and a dry atmosphere in the weeks leading up to fall.

 

While warmer daylight can make for brighter colors, adequate rain in the spring and summer and snowfall in the winter also make for prettier fall colors. You might recall 2021's fall season was particularly beautiful when it came to the changing leaves; an arborist told us at the time that the picture-perfect foliage was due in large part to the wet spring Colorado experienced in 2021.

In years with little moisture, trees are more likely to shorten the color-changing process and leaves fall off sooner.

Carolina Manriquez, a forester with the Colorado State Forest Service, said colder seasons are undeniably shrinking. Any precipitation is better than none, Manriquez emphasized, but she said the state needs much more moisture to pull itself out of crisis. Denver just experienced its third-warmest September in recorded history, meanwhile most of the state is still in some form of drought, according to the US Drought Monitor.

“We’ve had 20 years of drought, so one or two really moist seasons is not going to make up for that,” Manriquez said. “This year, we had a ton of precipitation, but I’m still seeing a lot of stress in our forests.”

While light is the largest factor in fall foliage, scientists agreed that warming temperatures and drought certainly play a role, both in the color of the trees and the duration of the fall season as a whole.

Russ Schumacher, Colorado State Climatologist and director of the Colorado Climate Center, said September and November have seen the most dramatic warmth over the last 40 years, signaling a warming fall and a delayed winter.

“We’ve forgotten what a normal September is supposed to look like,” Schumacher said. “This year was a big example where it seemed like summer weather is stretching into September.”

Because of Colorado’s sporadic weather patterns, Schumacher said it's impossible to declare that future Septembers will continue to break heat records, but he added that general warming due to human-caused climate change will continue.

Alison Berg is a multimedia journalist at Rocky Mountain PBS. You can reach her at alisonberg@rmpbs.org.