The vibrant shimmering leaves of Southwest Colorado’s aspen stands have begun to transition, once again, from summer’s green hues to fall’s trademark yellows, oranges and velvet reds.
As days fall shorter and the trees’ chlorophyll production dips, the green that shades the leaves fades, revealing a sequential panoply of colors.
Colors are expected to hit their peak in Southwest Colorado at the end of September and into the first week of October, although stands at higher elevations will generally turn sooner.
Despite climate-related threats to the health of aspen stands, it is shaping up to be an excellent season for leaf peepers, foresters say.
“I think we’ll have a very good color change this year, based on some conditions, as far as a wet growing season, wet spring and summer, particularly,” said Ryan Cox, lead forester with the Colorado State Forest Service’s Durango Field Office.
The recent cool nights followed by warmer days is expected to produce vibrant colors, he added.
In the early 2000s, researchers noticed landscape-scale dieback of aspen trees, a phenomenon named sudden aspen decline, or SAD. The trend was attributed to the trees’ heightened susceptibility to insects and pathogens brought about by drought conditions.
Although trees in certain pockets of the Southwest are still suffering, aspens are, by and large, in a relatively healthy state, said San Juan National Forest Forester Travis Bruch.
“For the most part, we don’t really see any SAD, or it’s in really small places at this point in time,” he said.
In stands near Purgatory Resort, Western tent caterpillars that cling to aspen branches have caused defoliation in places – but Bruch said the species population tends to swell and die off before causing widespread harm.
The U.S. Forest Service’s annual regional report on forest insect and disease conditions, released every March, noted that heavy defoliation was observed near Purgatory in 2023.
In general, trees stricken by drought are less resilient and cannot defend against pathogens and insects as well as they otherwise might.
But stands are considered a single organism, and a hardy one at that. The trunks of trees in a single stand grow from one root system, making each a clone with the same genetic composition. And even when pathogens or insects wipe out a clone’s trunks, the root systems will sometimes still survive and regenerate with force if given adequate moisture.
“It’s not something that, you know, we expect to kill tens of thousands of acres of aspen,” he said. “But I mean, all in all, when you go around and look at the aspen, it’s in decent health.”
The SJNF maintains an online tracker of where colors are peaking for visitors who flood the region’s highways to get a glimpse.
Stephanie Weber, the executive director of the San Juan Mountains Association, is urging visitors to be smart.
“Please don’t stop in the middle of the highway,” she said. “Please find a pull off. Don’t impede traffic.”