161-Year Study Shows Warmer World Causes Earlier Spring Flowering
January 17, 2013

Study Shows Warmer World Causes Earlier Spring Flowering

Lawrence LeBlond for redOrbit.com - Your Universe Online

Scientists from three universities have found that record temperatures in 2010 and 2012 resulted in the earliest spring flowering season in the eastern United States in more than 160 years. Using the phenological records of two iconic American naturalists, Henry David Thoreau and Aldo Leopold, scientists have shown that some springtime bloomers have appeared as much as a month earlier in response to the warming climate.

This study is of great significance as it gives scientists a good look at ecological change in response to global warming. The work also helps predict effects on important agricultural crops, which depend on flowering to produce fruit.

The study, conducted by researchers from the Universities of Harvard, Boston and Wisconsin, was published January 16 in the open access journal PLoS ONE.

The authors said that many plants need a long winter´s nap to undergo physiological changes that make them bloom again in the springtime. Previous studies have shown that this blooming is now occurring earlier due to warmer springs, which could have dire implications for plant productivity, their pollinators, and the ecosystem in which they thrive.

"We're seeing plants that are now flowering on average over three weeks earlier than when they were first observed — and some species are flowering as much as six weeks earlier," said senior author Charles Davis, a Harvard Professor of Organismic and Evolutionary Biology. "Spring is arriving much earlier today than it has in the past."

To explain the early arrival of the spring bloom, Davis and his colleagues, Boston´s biology Professor Richard Primack, Boston´s postdoctoral researcher Elizabeth Ellwood and Wisconsin´s Professor Stanley Temple, point to temperature increases due to global climate change. Using data collected in Massachusetts from the mid-1800s to the present day, the team found that the two warmest years on record–2010 and 2012–also produced the earliest spring flowering.

The team compared their data with that of Thoreau, who took flowering measurements near Walden Pond, Mass beginning in 1852, and with Leopold, who began taking measurements in Wisconsin in 1935. According to the analysis, many plants flower up to 4.1 days earlier for every degree Celsius (1.8 F) rise in mean spring temperatures.

However, the relationship is linear from Thoreau´s time to the present day. Meaning long-term observations could be used to predict plant response to weather extremes outside of the historical range. While spring rising temps are causing record earlier flowering, temps have likely not yet reached a point where plants are not able to respond in terms of flowering times, the team explained.

"It appears that many spring plants keep pushing things earlier and earlier,” said Davis.

What is most striking “is that we see the same pattern in Wisconsin as we see in Massachusetts,” Davis added. “It's amazing that these areas are so far apart and yet we're seeing the same things—it speaks to a larger phenomenon taking place in the eastern United States.”

“Thoreau and Leopold are icons of the American environmental movement and it is astonishing that the records both kept decades ago can be used today to demonstrate the impacts of climate change on plant flowering times,” Primack noted.

“We were amazed that wildflowers in Concord flowered almost a month earlier in 2012 than they did in Thoreau's time or any other recent year, and it turns out the same phenomenon was happening in Wisconsin where Aldo Leopold was recording flowering times,” added Ellwood.

Compared to the timing of spring flowering in Thoreau´s day, the team found that native Massachusetts plants such as serviceberry and nodding trillium are blooming 11 days earlier, on average. The change was even more striking for native Wisconsin plants, with plants appearing to bloom as much as a month earlier than they did 67 years ago when Leopold made his last entry.

"We used relationships revealed in historical records to predict how 47 species of native plants would respond to unprecedented spring temperatures, but that has only been possible because naturalists, past and present, kept good records of what they observed in nature," Temple explained.

One comparison, that of the Massachusetts black cherry, was particularly notable. In 1942, when the spring temperature in southern Wisconsin was an average 48 degrees F, the plant bloomed on May 31. In 2012, with average spring temperatures of 54 degrees F, the black cherry bloomed as early as May 6.

"Leopold and Thoreau had no idea their observations would help us understand responses to human-caused climate change," said Temple. "But Leopold knew his records might be useful in retrospect when he wrote: 'Keeping records enhances the pleasure of the search, and the chance of finding order and meaning in these events.'"

While continued monitoring of flowering times is needed, Davis expressed hope that the study provides a substantial example of the potential consequences of climate change and global warming.

"The problem of climate change is so massive, the temptation is for people to tune out," Davis said. "But I think being aware that this is indeed happening is one step in the right direction of good stewardship of our planet."

"When we talk about future climate change, it can be difficult to grasp. Humans may weather these changes reasonably well in the short-term, but many organisms in the tree of life will not fare nearly as well," he concluded.