Aspen leafing out early? Don’t trust your memory, but yes, this year is a peek into the future

Aspen stands this year had leafed out by May 5 on the north-facing slopes above Eagle-Vail, the subdivision of duplexes where I spent the 1990s. They were lovely, light and green. And seemed weeks early.

In my memory, the leaves turned out by Memorial Day, but not much before.

“You’re definitely on the money in your observation,” said Theresa Crimmins, assistant director of the National Phenology Network, when I called her about my aspen observation.

The network, founded about a decade ago in Tucson, has been documenting the impact of a generally warming climate on plant and insect life. By its estimates, spring arrived 11 days earlier in the Vail area while some parts of the nation were 30 days ahead of schedule this year.

Those estimates are based in part on the direct observations over time of thousands of people nationwide. The network has 856 registered observers in Colorado in the program called Nature’s Notebook.

Phenology is not strictly the province of professional scientists. It can be—and increasingly is—done in an organized way by people looking out their kitchen windows, noticing changes underway literally in their own backyards. People have been observing nature’s calendar—such as when lilacs bloom, hummingbirds return from the tropics, and crickets start getting noisy—for decades.

It is a discipline that depends not on memory, but upon written records, carefully and conscientiously curated.

The network also has a website here with maps created by models that draw upon data from the National Oceanic and Atmospheric Administration, among others. Two maps speak to this year’s unusual warming through April. One, the spring index map, uses a model that tracks the heat accumulated since the start of the year. The map predicts the day of the year that this index, or threshold, “that would cause plant and animals to start doing their thing,” said Crimmins.

A second map, the spring index anomaly, compares when the date for reaching any given heat threshold, or cumulative degrees-days, as compared to the average from 1981 through 2010. Meteorologists use a rolling 30-year period, updated every decade, to define what is average. Red on this map indicates early spring, and blue shows late spring.

Donald Trump would delight in how much red there was this year. On May 10, the maps showed red from the Rocky Mountains eastward, some areas as crimson as ripened cherries or dried chili peppers. Only the Pacific Northwest showed strong blue. (Portions of mountainous Colorado are green, a reflection only of insufficient data.)

“This is a general picture, and yes indeed, spring has been much warmer than usual, and it does seem that many of the plants are responding accordingly,” said Crimmins.

Does this include aspen? Not necessarily.

“If we wanted to predict when certain alpine species are changing, we should calibrate a different model with what we know about them,” she said. “A lot of it has to do with snowmelt.”

Heavy snow in Colorado this winter during December and January was followed by exceptional warmth. Temperatures in Eastern Colorado from February through April were five degrees above the 20th century average, a record. It was the second warmest three-month period in Western Colorado in records beginning in 1895. It lagged only 1934, a year of drought—and elsewhere, the Dust Bowl.

Some nights in March temperatures never fell below freezing even above 10,000 feet in elevation, says Jeff Lukas, the research integration specialist for the Western Water Assessment at the University of Colorado-Boulder.  “That’s pretty unusual,” he said.

If there is always natural variability—a hot and dry 2012 followed by a coolish, wettish 2013—does this year’s weather also portend the future?

“This year is the future norm,” said Lukas.

And how much of this warming spring weather is due to atmospheric pollution, the greenhouse gases produced by modern civilization? “Some fraction,” Lukas says. That has been the overall trend over the last 30 years, he adds.

Might the Aspen Center for Environmental Studies have my answers about aspens? Jim Kravitz, the director of naturalist programs, has been tracking the aspen during the last six years. He has posted the photos of the aspen clones changing into their green garb of spring and summer and posted the comparison photos on the center’s Facebook page. The comparisons have gotten thousands and thousands of views, he reports.

This year, he reports, the aspen started leafing out four weeks earlier than compared to the last six years before May’s snow and cold put spring on pause.

Kravitz and other naturalists emphasize the value of direct observation in understanding the changing climate. “I know six years of 10 years, or even 10 years, doesn’t constitute science, but we can see things that are being projected (by climate models),” he said.

In Colorado’s alpine areas, that shift has already been documented by the Denver Botanic Gardens. Rebecca Hufft, the associate director of applied conservation, recently completed a three-year project that tapped thousands of those observations from 30-plus museums.

Filtering out observations of less than 10 years, Hufft came up with 385 species with records of seasonal changes since 1950 for alpine areas, defined here as 10,400 feet or above. That’s just a little lower than Echo Lake, near Mt. Evans. On average, these species shifted toward earlier blooming and other spring activities by an average eight days during the last 60 years.

Species did not march to an earlier climate drummer in unison, however. The study found that 45 species—including rosy pussytoes, the thickleaf ragworth, and the giant red Indian paintbush—are coming out an average of 32 days earlier.

But 340 others species—including Englemann spruce, kinnikinnick, and Colorado blue columbine— didn’t change from 1950 to 2011, the study period. No species showed a trend toward later blooming in spring.

Hufft, a plant ecologist, plans similar studies of four other ecosystem types in Colorado, such as the piñon-and-juniper and the short-grass steppes. The latter cover nearly all the eastern half of the state.

As for aspen gaining leaves, Hufft’s study did not include the species, likely because there are relatively few aspen above 10,400 feet. That doesn’t mean my memory was wrong. But neither am I sure that I’m right.

What we do have is clear evidence of warming. But if climate change is a polarizing phrase, Hufft sees plant phenology as a valuable but relatively simple tool for talking about the same thing.

“It takes the discussion away from the controversy,” she says. The evidence is there, outside local windows.

“I think when people can see that it changes, it makes a better connection to their own lives. It’s no longer abstraction, like something happening at the equator or the sea level rising. It’s concrete. There are actually things changing in their own backyards.”

As for those aspen changing in Eagle-Vail—I’d bet a beer that I’m right. But not the house.


Photo:  An aspen tree along Buck Creek, in Avon, was wearing all of its summer leaves by May 5. Photo Credit: Allen Best