High 2012 temperatures led to earliest flowering ever in eastern U.S.

Record-high temperatures in 2012 led to massive droughts and wildfires, but tiny wildflowers like the dwarf dandelion and the shooting star were also affected by the especially warm spring. Their early blooming has the potential to disrupt the ecosystem, scientists say, but perhaps it could be a sign of plants' resilience.

Last year, scientists from Harvard University, Boston University and the University of Wisconsin documented the earliest flowering season on record in the eastern United States. Researchers believe climate change is hastening this sign of spring, according to an observational study published in the Public Library of Science yesterday.

Researchers used records dating back to 1852, when writer Henry David Thoreau wandered the land surrounding Walden Pond near Concord, Mass., taking meticulous notes on the wildflowers he found there.

Thoreau noted that the highbush blueberry's small white flowers appeared in mid-May, but in 2012, the highbush blueberry bloomed six weeks earlier, on April 1.

Using Thoreau's journals and records kept by another American icon, conservationist Aldo Leopold, researchers projected that for each rise of 1 degree Celsius in average spring temperatures, plants in temperate regions flower 3.2 days earlier, on average.

"I think sometimes it's hard for people to understand how big of a deal that 1 degree is," said Elizabeth Ellwood of Boston University, the lead author on the study. "But for plants that are going to be flowering four days earlier for every degree of warming, that really can play a large role in that plant's life cycle, and it will influence the rest of the ecosystem, as well."

How long can this go on?


Thanks to Thoreau and Leopold, long-term, reliable records of flowering times for 47 flower species were available at two locations: near Walden Pond and in Wisconsin's Sauk and Dane counties, where Leopold collected material for "A Sand County Almanac."

Later data sets were compiled by other researchers, including Leopold's daughter Nina Leopold Bradley, and compiled to predict flowering times in relation to spring temperatures. Because the data were collected at sites separated by more than 1,000 miles, the researchers believe their observations apply to regions throughout the United States.

The resulting graphs indicated a clear trend: "Spring-flowering plants at both locations ... largely responded to record-breaking warm temperatures as predicted by their historical responses to warming spring temperatures," the study said.

In Concord, the average spring temperature has risen by almost 9 C since Thoreau's time, a result of both global warming and the "heat island" effect of nearby Boston. The mean flowering date shifted from May 15 to May 4. During the spring of 2012, the second warmest on record at 10.7 C on average, plants flowered April 25.

According to Leopold's observations in Wisconsin during the 1930s and '40s, the mean flowering date at that time was May 7. Now, the average spring temperature has risen by almost 2 C, and the flowering date is seven days earlier. In response to 2012's record-high temperatures, plants flowered more than three weeks earlier.

Despite the fact that 2012's flowering time matched up with projected trends, the researchers did not necessarily expect flowers to continue blooming earlier as spring temperatures rose. To grow, plants in temperate regions need both a period of winter cold and sufficient daily exposure to sunlight.

"We were, at some level, not surprised at what we saw but, at other levels, completely stunned," said Charles Davis, a plant evolutionary biologist at Harvard who contributed to the study.

"What it suggests is that there is some built-in resiliency in the way that plant populations are responding to warming springs," he said. "What we still don't really know is whether this is a good or a bad thing."

But at some point, many plant species may no longer be able to adjust to warmer, earlier weather, resulting in a spring with no flowers at all.

"There's these rumblings in the literature that, at some point, this can't keep happening," Ellwood said. "We feel like there's only so much longer that they can keep pace."

What will be the ripple effects?

So far, the researchers haven't seen any fallout from the earlier flowering season.

"There aren't any indicators that this, so far, has caused any dramatic problems," said Stanley Temple of the University of Wisconsin's Department of Forest and Wildlife Ecology, who also contributed to the study.

But Temple and his colleagues warned that earlier flowering times have the potential to throw other plant and animal species out of sync. When plants flower, "it's essentially when the ecosystem wakes up in the spring," Temple said. "It really has ripple effects that are very complex."

Insect pollinators and migrating birds that feed on them could be negatively affected, the researchers speculated, and flowering crops like apples are more vulnerable to spring frosts. Plants' flowering times also affect the water cycle, Ellwood said, as well as how the environment stores carbon.

"It could be devastating, really, to different parts of the ecosystem," she said.

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