Henry Thoreau and Phenology

Henry Thoreau walked in Concord for several hours every day looking carefully at the plant life around him. He took notes on the different species of grasses, wildflowers, shrubs, and trees he encountered—when their leaves emerged, when they flowered, and when their leaves turned color. This science is called phenology.

Thoreau wrote these observations (among other thoughts) into his journal each night. In this way he assembled years of observation on hundreds of species. In 1860, he began to extract these observations from his journal and to compile them into charts. The charts compiled the observations of many individual species over a number of years, allowing for comparison of flowering times and other phenomena from year to year.

In 2003, Boston University biologist Richard Primack learned about Thoreau’s data and recognized immediately that it was exactly what he was seeking for his research: an extensive, site-specific, accurate set of historical observations that he could compare with present-day observations at the same location. This comparison, building on the observations of Thoreau from the 1850s, shows that the climate does not just change from season to season; it changes in measurable ways over decades and centuries.