Thoreau’s Phenology: Past Plant Wisdom for a Changing Climate

by Melissa Harding


Serviceberries are one of the first trees that flowers in the spring. With their tiny, white flowers, these small trees show some of the first stirrings of life in the winter forest. The plant gets its name from the early settlers who used it as an indicator of spring; when the tree was flowering, they knew that the ground was thawed enough to bury those who had died over the winter. The delicate blossoms served as a beautiful memorial to the dead and marked the time for funeral services. For these settlers, early spring started in April or even May. Winter was bitter and long, lasting many months. Fast forward to 2012 and the serviceberry plant is starting to bloom as early as March. This large change in bloom times is measured by the practice of phenology, or the study of plant and animal life cycles.

Many famous authors and statesmen were phenologists; from 1766-1824, Thomas Jefferson famously wrote volumes on the plants and animals at Monticello; from 1852-1858, Henry David Thoreau passionately recorded the coming of spring flowers around Walden pond in Concord, Massachusetts; from 1935-1945, Aldo Leopold chronicled the coming and going of life in Dane County, Wisconsin. These men spent every day walking acres of land and taking note of what they saw happening around them. They developed an intimate knowledge of the land they walked, noticing every minor occurence. Every spring, Thoreau recorded in his journals when hundreds of different flowers first opened.

Today’s scientists are using the records left by these early naturalists to predict the changes that climate instability will bring. Scientists in both Wisconsin and Massachusetts, working collaboratively on separate but parallel studies, published the findings of research comparing data from both Thoreau and Leopold to current phenological data. Their research shows that the record high temperatures of last spring resulted in earlier bloom times. In Massachusetts, the pattern they found was that for each degree Celsius rise in mean spring temperature, plants bloomed 3.2 days earlier. In Wisconsin, it was 4.1 days earlier for each degree rise. In an average year, spring plants will bloom around 11 days earlier than in the time of Thoreau.

Is this good for plants or bad? A little bit of both. For example, plants might benefit from longer growing seasons, but they could suffer if their pollinators don’t adapt as quickly. Before this study, scientists didn’t know if plants would be able to adjust their bloom times to the changing climate; there was concern that flowering, leaf out, and growth might be delayed for plants that have not experienced long spring photoperiods or that need a long cold period to hibernate. This peek into the future by way of the past allows scientists to predict bloom times for current temperatures, though they caution that as temperatures increase further, plants may not be able to keep up.

To learn more, check out NPR’s Science page or the peruse entire study at the journal PLOS One.

The above image is courtesy of May Dream Garden.

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