Thoreau Waited Longer for Spring to Arrive

Besides authoring the nature classic, Walden, Henry David Thoreau kept records during the 1850s of the dates when various plants in Concord, Massachusetts began bursting into flower. Now scientists at Boston University have compared their own recent observations of plant flowering with those of Thoreau, as well as others that were made over the years.

The data track not only changes in spring's arrival since Thoreau's days, but reveal surprising variations in how plants have responded.

During the 155 years that these records span, Concord's mean annual temperature increased by 2.4 °C (4.3 °F). The rise is attributed to both global warming and the influence of expanding urbanization on local climate.

In response, the 43 common species that were most consistently observed from 1852 to 2006, now begin flowering one week earlier on average. Thoreau noticed these in the 1850s first flowering on May 14. By the 1890s, their average flower opening date had moved to May 10, and by 2006 flowers began blooming on May 7.

Some plants though, have leapt much further ahead. The native shrub, highbush blueberry (Vaccinium corymbosum) starts flowering three weeks earlier compared with 150 years ago. Yellow wood sorrel (Oxalis europaea), a native herb, now opens its flowers 32 days earlier.

When flowers first bloom each year correlates well with mean temperatures in January and the two months before flowering. This is true regardless of where a plant grows or whether it's a native or exotic species. For January, April and May, mean temperatures have climbed from 4.3 °C in Thoreau's time to 5.9 °C in the early 21st century.

Although flowering in well over 90% of the 300 plant species tracked is influenced by temperature, huge inconsistencies occur, even among closely related species. Black birch for instance, flowers nearly three days earlier for each degree Celsius rise in temperature for January, March and April, whereas gray birch, a member of the same genus that grows in the same habitats, is unaffected by temperature. Similarly, a 1 °C increase in January, May, and June temperatures prompts rough-stemmed goldenrod to move its flowering ahead by 11 days, while most other goldenrods don't respond to temperature changes.