Scientists have thought for some time that climate change is making flowers bloom earlier but weren’t quite sure if you could draw a connection between studies in the lab and historical records. Now, a new study has found that the relation between average spring temperature and flowering date holds up across time -- and the researchers are partially indebted to “Walden” author Henry David Thoreau.

The study, published on Wednesday in the journal PLoS ONE, also found that incredibly warm springs in 2010 and 2012 resulted in the earliest known flowering in 161 years at two sites in Massachusetts and Wisconsin.

Flowering plants often time their spring blooming based on temperature, waiting to sense warmth after a period of cold. But climate change and its warmer winters and springs are messing with the plant’s ability to gauge the seasons. April showers are falling on April flowers.

A team of scientists from Boston University, Harvard University, the University of Wisconsin and the Aldo Leopold Foundation wanted to see if the relationship between flowering times and spring temperatures shown in historical records would hold up in recent years.

“There has always been a critique of extrapolating beyond your data,” author and University of Wisconsin ecologist Stanley Temple said. “It’s always risky.”

Temple and his colleagues turned to some iconic naturalists for help: Thoreau and Aldo Leopold. Amid his transcendentalist writings, Thoreau found time to keep records of when flowers blossomed in Concord, Mass., from 1852 to 1858. About 80 years later, Leopold kept meticulous notes on flowering times in Dane County, Wis.

To try and draw a connection between past and present, Temple and his colleagues looked at flowering times for 32 native plants in Concord and 23 plant species in Wisconsin in contemporary times. In Thoreau’s time, the average spring temperature in the region was 5.5 degrees Celsius (42 degrees Fahrenheit), and flowers typically bloomed around May 15. But since 2004, mean spring temperatures have risen to 8.8 degrees Celsius (48 Fahrenheit), and mean flowering time has moved back to May 4.

2012 was the second-warmest spring on record for Concord, with an average temperature of about 10.7 degrees Celsius (51 Fahrenheit) and a mean flowering date of April 25.

“The change in response of the flowers can be shown to be directly related to the average spring temperature,” Temple said. “We can explain variation in flowering time with one variable.”

In Wisconsin, a similar effect registered: The mean flowering date in 2012 shifted more than three weeks earlier than what Leopold had observed in the 1930s.

“This demonstrates the utility of historic records, as well as the strong relationship between flowering and spring temperature,” co-author and Boston University researcher Elizabeth Ellwood said in an email Thursday.

Furthermore, the results show that plants still have the ability to adjust to changing climate conditions, though that may eventually change.

“There’s no evidence yet they’ve reached any physiological limit, but there are rumblings in the literature” that point to some breaking point in the future, Ellwood said.

Ellwood will soon be continuing her research in Japan, where there are records of cherry blossom blooming stretching back nearly 1,000 years.

As flowering plants bloom increasingly earlier, there may be rippling effects both in the wilderness and among agricultural plants.

“The flowering of plants is when the ecosystem kind of wakes up and the base of the food chain starts producing again,” Temple said. “If other species can’t stay in sync, you get these possibilities of ecological mismatches.”

The Wisconsin region where Leopold once lived is famous for its cherries but lost nearly its entire crop in 2012 after the trees bloomed too early, setting the flowers up to get dispatched by a late cold snap. Early flowering could also eventually put plants and their pollinators out of alignment if blooms open before key insects emerge. Really early bloomers could also risk awakening while the days are still short, leaving them starved of much-needed sunlight.

“It’s larger than just little wildflowers in the woods,” Ellwood said.

SOURCE: Ellwood et al. “Record-Breaking Early Flowering in the Eastern United States.” PLoS ONE 8: e53788, published online 16 January 2013.