FAIRBANKS — One hundred years ago, the growing season in Fairbanks was less than three months long. Last year, some local gardeners were still harvesting broccoli and cabbage in mid-September.

Fairbanks is 2.5 degrees Fahrenheit hotter and 11 percent drier than it was in the early 20th century, according to data gathered by the Alaska Climate Research Center. (The growing season is marked by the last frost in spring and the first frost in fall.) These changes have stretched the growing season from 85 to 123 days in the past century. And while warming might produce more potatoes and pumpkins in cold-climate regions, it could eradicate tree populations.

“Every change in climate will bring positive things and negative things,” said Gerd Wendler, director of the research center at the University of Alaska Fairbanks.

Agriculture is one beneficiary of a warmer climate in Fairbanks. But before you start planting olives, consider the discrepancy among growing seasons in the just past five years. In 2006, the last hard freeze struck June 6. Last spring, the final frost came May 15, according to data from the National Weather Service.

How long is it, really?

Agriculture depends not just on climate but on weather, a short-term measure of conditions, and seasonal anomalies like early frosts and heavy rain.

“The thing about agriculture is we can have really high temperatures or long seasons, but if you have a hard frost in the middle of August that wipes out everything, you can have another month that’s really good but you can’t take advantage of it because the crops are dead,” said Meriam Karlsson, professor of horticulture at UAF who has observed 20 years of growing seasons in Fairbanks.

Karlsson said the growing season has increased in the past couple of decades from 100 to about 105 days. Though summers typically contain more frost-free days, it’s still risky to plant before June.

Michelle Hebert, horticulture specialist at the Cooperative Extension Service, said last summer was the longest growing season in the 37 years she has gardened in the north.

“Within the last few years, especially last summer, I had, in my garden, 120 days,” she said. “We just had that Indian summer that went on forever.”

Longer seasons allow crops like potatoes to flourish and for second harvests of crops like broccoli and cabbage. They have also allowed the Fairbanks Experiment Farm to test certain varieties of corn, which need at least 100 days to grow, said Karllson.

Balance of warming

In the past 100 years, temperatures climbed the most in winter but rose in every season except fall. Yet every season has been touched by the consequences, as living conditions improve for insects, moisture content changes and freeze-thaw cycles are altered.

The single month with the biggest change was April, which warmed by more than 5 degrees Fahrenheit. That’s partly because April is when snow melts, which compounds warming, Wendler said.

“Snow reflects lots of sunlight so when the snow is gone, more solar radiation is absorbed at the surface. The higher surface temperature causes greater warming of the soil,” he said.

This helps soils thaw faster, a perk for gardening.

“With earlier snowmelt, the field’s going to dry up faster, the soil warms up faster and you can plant earlier,” Karlsson said.

Other effects

Another side effect of warming is drying, because warm air can carry more moisture than cold air. Drying has been less kind to agriculture and forestry, said Glenn Juday, professor of forest ecology at UAF.

“Warm air sucks more out of the plant,” he said, “so the supply of moisture available to trees is reduced.”

Precipitation has dropped 11 percent since 1916. Also, as spring arrives earlier and fall comes later, trees have less time to absorb moisture from the snowpack. As trees in the Interior run out of water, they become more susceptible to disease, he said.

Meanwhile, lower winter temperatures and fewer sustained freezes have allowed insects to over-winter here, causing outbreaks of aspen leaf-miners, spruce budworms and birch leaf scorch. In turn, dry, diseased trees are prime fuel for forest fires.

“Each species is being hit with something. There’s no winner in the boreal forest,” he said.

With agriculture, however, it’s easier to combat drying.

“The whole paradigm is different,” Juday said. “If there’s no water, we irrigate. If the soil’s not fertile, we fertilize it. If the soil’s too acidic, we lime it.”

The future

This season began with a warm, dry spring but evolved into a cool, wet July.

While the recent rain has helped this year’s crops, it’s not enough to save the forests, Juday said. If warming trends continue, he anticipates most native trees species in the Interior will be eliminated within the next few decades, replaced by more temperate imports from farther south.

“We’ve just gone through one of the two or three driest, hottest periods in the last century,” he said. “The cumulative drying we’ve had has just crisped things to the level that these higher precipitations are just not enough.”

While warming is provoking some negative changes in Alaska, it is not without positive impact. Agriculture could be one of these bright spots.

“People are already harvesting red tomatoes,” said Hebert. “The gardens overall across Fairbanks are unbelievable, healthy and thriving, and I think it’s all the moisture we’re having now.”

Contact staff writer Molly Rettig at 459-7590.