The Rocky Mountain wildflower season has lengthened by over a month since the 1970s, according to a study published Monday that found climate change is altering the flowering patterns of more species than previously thought.

Flowers used to bloom from mid-May to early September, but the season now lasts 35 days longer, from April to mid-September, according to researchers who collected 39 years of data at the Rocky Mountain Biological Laboratory near Crested Butte, Colo.

Earlier spring snowmelt and other climate shifts have changed the timing of blooms for more than two-thirds of 60 species of native wildflowers in mountain meadows, stands of Aspen trees and conifer forest that were surveyed from 1974 to 2012, according to the study published in the Proceedings of the National Academy of Sciences.

The scientific paper is the latest to document one of the strongest signs that global warming is shaking up the natural world. Scientists studying phenology — the timing of seasonal events in nature — are observing rapid shifts in when flowers bloom, trees leaf out and bees, birds and butterflies appear in the spring.


Scientists have documented the trend using historical records from writers and naturalists, including Henry David Thoreau, who in the 1850s began recording in his journal the first blooms of the season around Concord, Mass.

Previous studies largely have focused on the first appearance of flowers in the spring, but that probably underestimates the true extent of the changes they are going through, the paper says.

To go beyond that, researchers analyzed wildflower species throughout the season. They found that half of them flowered earlier, more than a third reached their peak blooms sooner and 30% flowered later into the year due to a warming climate.

“We don’t know if it’s good or bad for these plant species at this point,” said Amy Iler, postdoctoral biology researcher at University of Maryland and co-author of the study.


The findings nonetheless raise many questions about how disruptive the changing bloom times might be to bees, birds and pollinators and other plants that are adapted to flowers appearing at very specific times, she said.

“Climate change is reshuffling flowering plants over a short time period,” Iler said. “So it might be changing things that were set in place by natural selection over a long time frame.”

Wildflowers at the Rocky Mountain Biological Laboratory, which sits at about 9,500 feet in elevation, bloom almost immediately after the spring snowmelt and stick around until the first hard frost in the fall. But as temperatures rise, snow is melting earlier and the first hard frosts are occurring later.

The study is the product of decades of work by David Inouye, a biology professor at the University of Maryland, who since 1974 has amassed an exhaustive record by systematically counting dozens of species of wildflowers at the research station in the mountains of Colorado.


“It is probably the most detailed, long-term data set on flowering times that exists in the United States and perhaps even the world,” said Richard Primack, a biology professor at Boston University who has used Thoreau’s records to study the effects of climate change on plants and animals.

Primack, who was not involved in the study, praised the paper as an “extremely innovative type of analysis” that would stimulate a flurry of new research.

“As soon as I read this paper I thought, my God, why didn’t we analyze our data that way?” Primack said. “This study shows us that if you don’t just focus on the first flowering date but also on the peak flowering date and the final flowering date, there’s a much greater impact of climate change than we previously suspected.”

tony.barboza@latimes.com


Twitter: @tonybarboza