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Tom and Gael Bittenbender, who visit Breckenridge from their home in Austin every fall, move through changing aspen trees on a hike, Sept. 28, 2023, toward Boreas Pass. (Hugh Carey, The Colorado Sun)

Every fall, Coloradans and visitors dust off their cameras and binoculars to take in the fall colors. But each year’s display has a lot to do with one key factor, water, and whether there’s enough of it.

Over the past two decades, many regions of Colorado have experienced dry or drought conditions. This prolonged drought has shaken communities’ certainty in their future water supply, impacted business for farmers and ranchers and changed aquatic conditions for wildlife. And, yes, even fall colors are impacted by drought, according to entomologist Dan West, the Colorado State Forest Service’s go-to guy for information about the fall colors. 

“When trees are in drought for several years, what ends up happening is they’re shutting down some of their processes and trying to dial back some of their growth,” West said.

The colorful fall foliage, which typically peaks from mid-September to early October, is part of Colorado’s multibillion-dollar tourism industry. People from around the world flock to the Million Dollar Highway between Ouray and Silverton, the Dallas Divide near Ridgway, Kebler Pass near Crested Butte and other Colorado forests to see spectacular views of the changing leaves.

For businesses like Alpine Scenic 4×4 Tours in Ouray, the leaf peepers offer a late-season boost after the August lull when kids go back to school, said Amber Oker, booking manager.

“Town was pretty busy last weekend, and I do think that that was when the colors peaked,” Oker said. “(On) most of our fall tours folks are interested in photography, so we help them get to those higher elevations where they can snap some amazing photos.”

Signs of autumn on Aspen Highlands and Buttermilk ski areas, Oct. 5, 2023, in Aspen. (Hugh Carey, The Colorado Sun)

What made this year so colorful?

This year, deciduous trees put on a great show: They grew dense foliage with colorful leaves that have stayed on trees for a long time, said West, who spends much of the year hiking, driving and flying around the state to assess forest conditions. 

That has a lot to do with temperature, precipitation and weather. Deciduous trees absorb about 70% of their water supply for the entire year from spring runoff, when the snowpack melts and flows through watersheds, he said. 

The deeper the winter snowpack, the more water trees have for their spring growth. Also, the slower snow melts, the more time runoff has to seep into deeper layers of the soil.

“In years past, we’ve had these kind of abrupt spring months that have just caused the snow to come off super quick,” West said. “That doesn’t allow the snowpack, or basically the water, to work its way down through the soil column and to be able to allow for capture from most of these trees.”

With enough water and nutrients, deciduous trees can produce more leaves, creating denser foliage that offers even more of a spectacle to enthusiastic leaf peepers in the fall. 

This year, winter precipitation blanketed Colorado in a deep snowpack, which acts as a vital natural reservoir for the state’s water supply. By May, most of Colorado mountains had an average to above-average snowpack compared with historical records from 1991 to 2020, according to the Natural Resources Conservation Service.

The summer was cooler overall, and some parts of the state even received record rainfall. The state hasn’t seen many of the windy days, cold temperatures and snowfall in aspen stands that all contribute to falling leaves.

“It’s just shaped up to be a fantastic year to get out and see some of the colors in Colorado, and it’s a good year for trees in general in Colorado,” West said.

How drought impacts fall colors

The impacts of drought are far-reaching, but when it comes to fall colors, drought has the ability to dampen the annual show.Parts of Colorado have been dry or in drought for much of the last 23 years. In 2002, 2012, 2018 and 2020 to name a few, portions of the state were in exceptional drought, the most severe drought category used by the U.S. Drought Monitor. As of Oct. 3, much of the Western Slope was experiencing abnormally dry, moderate or severe drought.

The western half of the state also lies within the Colorado River Basin, which is experiencing its driest period in the 1,200-year historical record, according to a 2022 tree ring study. The Colorado River provides water for wildlife and 40 million people in Colorado, six Western states, 30 Native American tribes and parts of Mexico.

When deciduous trees don’t have enough resources — when they are drought-stressed — the trees don’t grow as many leaves, and those leaves are often smaller in size, West said. Drought conditions can also impact the color of the leaves in the fall and how long the fall colors last. 

Each spring, chemical triggers in deciduous trees, called auxins, signal that it’s time for the tree to produce leaves. These new leaves grow with all the fall colors baked in: The chemical compound chlorophyll produces the green color, while other compounds, called carotenoids and flavonoids, produce the vivid yellows and oranges.

In the fall, the tree’s auxins respond to shorter days and cooler temperatures by telling the tree to stop replenishing chlorophyll. During Colorado’s sunny fall days, chlorophyll burns off more quickly than other compounds, leaving the orange, yellow and red colors behind, West said.

But when a tree is severely or exceptionally drought-stressed, its leaves will brown around the edges, a process called marginal necrosis, and prematurely drop off, West said.

“The tree says, ‘I’m losing more water than what I can take in. … It’s not worth my effort at this point. I have to save more water than I’m losing through transpiration, which is just basically how trees breathe,” West said.

Each fall, auxins also tell the tree to produce a barrier between the leaf and branch, which helps the tree retain water, protects against pathogens, and causes the leaves to fall from the tree. 

Drought-stressed trees tend to change color 10 to 14 days earlier in the year, which cuts their time to add growth and absorb nutrients, West said.

“That’s significant when you really think about it in a tree’s lifetime,” he said. “In the summer months, when you’re shortening things up by two weeks on the back-end, that can really make a big difference.”

Seasonal weather patterns can also influence how long the fall colors last. If temperatures drop quickly, snow is in the forecast or winds increase, the fall showing is typically shorter.

“The colors just turn quickly and then they (leaves) drop, and we just don’t get a nice, long season like we’re having this year,” he said. 

Will drought continue to stress trees out?

Climate experts can only gauge conditions into the near future, but they’re keeping a close eye on two factors: increasing temperatures and fluctuating precipitation. 

In Colorado, the average yearly temperature has increased by 2 degrees Fahrenheit, compared with the state’s long-term average, according to the 2023 Colorado Water Plan. As the climate changes, the peak runoff period could happen earlier in the year, the annual snowpack could decrease and there could be more drought periods, the plan says.

In the Colorado River Basin, higher temperatures caused by climate change stole an estimated 10 trillion gallons of water out of the basin from 2000 to 2021, according to a study from UCLA hydrologists

The river basin had a swell of water this year, but overall, the amount of water entering Lake Powell has dropped over recent decades, said Cody Moser, senior hydrologist at the Colorado Basin River Forecast Center.

This year, water flowing into one of the basin’s largest reservoirs, Lake Powell, between April and July was 166% of the 30-year average from 1991 to 2020.

That 30-year average, 6.39 million acre-feet, is less than the average volume of water running into Lake Powell between April and July from 1981 to 2010: 7.16 million acre-feet. 

One acre-foot provides enough water for about two typical urban households for a year.

Forecasters can only really gauge conditions with certainty a few weeks into the future, Moser said.

“It’s fair to say that, as temperatures do increase, that’s obviously going to impact precipitation type, so rain versus snow. And it’s going to increase the snow line, so more rain at the lower elevations,” he said.

For West, variability seems to be the only constant. He said state forest and resource managers should focus on longer-term planning when thinking about tree conditions and water resources.

“We know that precipitation is so variable now. The years of the mid- to late-’80s, where we saw above-average precipitation for eight years straight — those years are gone I think,” West said. “I don’t think that in present day, that’s the norm. Really what’s the norm is variability.”

Shannon Mullane writes about the Colorado River Basin and Western water issues for The Colorado Sun. She frequently covers water news related to Western tribes, Western Slope and Colorado with an eye on issues related to resource management,...