Iconic authors help reveal record early flowering

"When I wrote the following pages, or rather the bulk of them, I lived alone, in the woods, a mile from any neighbor, in a house which I had built myself, on the shore of Walden Pond, in Concord, Massachusetts, and earned my living by the labor of my hands only." Those are the famous opening lines to Henry David Thoreau's Walden. Now, scientists have used Thoreau's notes on his surroundings to show how much earlier global warming has pushed plant flowering. Credit: Tim Hettler, via Flickr

“When I wrote the following pages, or rather the bulk of them, I lived alone, in the woods, a mile from any neighbor, in a house which I had built myself, on the shore of Walden Pond, in Concord, Massachusetts, and earned my living by the labor of my hands only.” Those are the famous opening lines to Henry David Thoreau’s Walden. Now, scientists have used Thoreau’s notes on his surroundings to show how much earlier global warming has pushed plant flowering. Credit: Tim Hettler, via Flickr

Notes from the last 150 years made by two environmental pioneers have helped show that the speed at which global warming is pushing spring events forward is not slowing. Boston University’s Libby Ellwood and her teammates compared flowering times recorded by Henry David Thoreau and Aldo Leopold to spring 2010 and 2012, the warmest and second warmest on record.  “Plants flowered earlier than ever before in these recent record warm years,” Libby told me. That advance is so closely linked to the warming our world’s experiencing, the researchers showed that they can predict flowering time from temperature. This knowledge could help predict climate change’s impact on crops.

“There will likely be winners and losers with climate change,” Libby said. “It is quite possible that some species will be able to use the warmer temperatures and longer growing season to their advantage. The risk for plants that begin growing as soon as the weather is warm though, is that the new spring growth and flowers are susceptible to late season frosts, and this can set back plant growth and reproduction.”

To understand what global warming is doing to other organisms, scientists have to find records about them from times when fossil fuel burning wasn’t as widespread as today. Thoreau and Leopold are best known as authors of books that lay the foundations of modern environmentalism. Both Thoreau’s Walden and Leopold’s A Sand County Almanac, published in 1854 and 1949 respectively, hold powerful ideas on the relationship between humans and nature. But both authors also studied phenology – the cycle of biological events such as plant flowering throughout the year.

Notebooks on nature

For several years, Richard Primack has been studying phenology - reading the “pulse of life” - at Walden Pond in Concord, Massachusetts, the same details Henry David Thoreau carefully recorded a century and a half ago. Credit: Boston University, via YouTube

In 2003, Boston University’s Richard Primack started looking for old phenological records in the area around Concord, Massachusetts, which Thoreau lived near when he wrote Walden. After asking everyone he could think of, Richard found several sets of information. The largest sets came from local botanist Alfred Hosmer covering, 1878 and 1888-1902 and Thoreau himself, covering 1852-1858. “His observations of plants and animals in Concord were well-known within the circle of Thoreau scholars,” Libby said. “These records are housed in various libraries in the northeast. Richard immediately recognized their importance in providing an historic baseline for his research and thus began a decade-long study of Concord ecology.” That study had already showed that by 2006 flowering times were already on average a week earlier than in Thoreau’s time.

Leopold’s records were better-known, as his work as an ecologist at the University of Wisconsin saw him collect data with his students in their local region between 1935 and 1945. From 1977 until she died in 2011 his daughter Nina Leopold Bradley and others at the Leopold Foundation resumed his work, and in 1999 showed climate change’s impact could be seen in the data. These measurements can today be viewed online, maintained by Stan Temple, who until his retirement held the University of Wisconsin chair occupied by Leopold and is now a senior fellow at the Foundation. And with spring temperatures hitting new heights, bringing the different sets of measurements together can help to understand how plants are responding, Libby explained. “The recent records gave us the opportunity to once again turn to Thoreau and Leopold to test just how remarkable these years were,” she said.

Flower power

In 2012, highbush blueberry flowers, shown here, appeared six weeks earlier than they did in Henry David Thoreau's records. Credit: Anita363 via Flickr

In 2012, highbush blueberry flowers, shown here, appeared six weeks earlier than they did in Henry David Thoreau’s records. Credit: Anita363 via Flickr

Together Libby, Stan, Nina, Richard and Harvard University’s Charles Davis looked at the links between flowering time and temperature in spring months. In a paper published in the scientific journal PLOS ONE last Wednesday, they describe how they reduced a list of hundreds of possible plants to 32 in Massachusetts and 23 in Wisconsin. “We narrowed the dataset to include only native species that were seen in most, if not all, years possible,” Libby said. “This eliminated issues such as the possibility that non-natives species may flower earlier than natives, or having missing data points.”

Some species have responded especially strongly to our warmer world. For example, in 2012 the highbush blueberry flowered on 1 April, six weeks earlier than seen by Thoreau. Similarly Leopold recorded serviceberry flowerings between 10 April and 9 May, when in 2012 it flowered on 25 March. On average across the Concord species, in both 2010 and 2012 plants flowered three weeks earlier than when Thoreau observed them. In Wisconsin in 2012, the average flowering was 13 April, 24 days earlier than Leopold saw. Libby and the other scientists found this early modern flowering is what they expected when projecting from the link between temperature and flowering dates seen in the past. That’s a little surprising, as other scientists think that shorter winters and fewer frosts could actually delay flowering.

“Plants have not reached a physiological threshold whereby they can no longer keep pace with warmer temperatures; flowering is ever earlier with warming spring temperatures,” Libby stressed. “We are able to predict flowering based on temperature using these very long historic datasets. This speaks to the value of historic datasets as well as to the strong relationship between spring temperature and flowering.”

The annual average first flowering dates of 29 years of data are shown from Massachusetts (a) and 47 years from Wisconsin (b). Blue diamonds are data from Henry David Thoreau in A and Aldo Leopold in B. Orange squares are data from Alfred Hosmer. Red triangles are data recorded by the teams of Richard Primack in A and Nina Leopold Bradley in B. Taken from paper cited below under Creative Commons license.

The annual average first flowering dates of 29 years of data are shown from Massachusetts (a) and 47 years from Wisconsin (b). Blue diamonds are data from Henry David Thoreau in A and Aldo Leopold in B. Orange squares are data from Alfred Hosmer. Red triangles are data recorded by the teams of Richard Primack in A and Nina Leopold Bradley in B. Taken from paper cited below under Creative Commons license.

Journal reference:

Ellwood, E., Temple, S., Primack, R., Bradley, N., & Davis, C. (2013). Record-Breaking Early Flowering in the Eastern United States PLoS ONE, 8 (1) DOI: 10.1371/journal.pone.0053788

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