This is the second essay I am posting about Henry David Thoreau, recycled from my encyclopedias. In this essay, I concentrate on Thoreau’s oft-forgotten contributions as a scientist.
Thoreau was also a scientist, though without formal training. His observations of nature suggested hypotheses to him, which he (however imperfectly) investigated. He was passionate about making observations (for example, the colors of Walden Pond ice and the stages by which it thawed) and measurement (the depths of Walden Pond). Scholars puzzle that his last writings were all mere observations of seed dispersal and spring budburst dates of plants. But his observations were the basis upon which important aspects of ecological science was later based, such as the following:
- Seed dispersal. In his long essays “The Dispersion of Seeds” and “The Succession of Forest Trees,” Thoreau presented many detailed observations to prove that trees grew only where their seeds had been planted by wind, water, or animals. As unbelievable as it may sound today, some intellectuals (none of them scientists) believed that trees just sprang up from the ground. Thoreau demonstrated that wild cherry trees grew where birds had carried their seeds, and that cherry seeds would not grow unless they had passed through a bird’s digestive system. He tested a hypothesis that mice dispersed hazelnut seeds by microscopically comparing tooth marks on a hazel nut with the teeth of a mouse skull.
- Forest succession. He also systematically surveyed tree stumps, counting the rings to reconstruct forest history by determining when each tree had begun to grow. He applied his observations to an understanding of the successional processes by which forests changed over time: oaks grew up underneath pines, not underneath oaks, not only because oak forests cast too much shade upon the seedlings and oak leaf litter contained toxins, but also because animals such as squirrels and jays carried acorns preferentially into pine forests. Thoreau also knew about natural disturbances, as well as human disturbances (he noted that Native Americans deliberately set fire to forests).
- Stochastic processes. Thoreau recognized what scientists today call stochastic processes. Ecological succession is not a deterministic process, always occurring in the same way, but by stochastic chance depended upon which species of plants as a seed source might happen to be near the disturbance.
- Spatial patterns of plants. Thoreau also studied spatial patterns in plants, noting that species whose seeds were dispersed by wind tended to grow in clumps, while animal-dispersed plants were more evenly spread on the landscape.
- Seasonal patterns. Thoreau also kept copious notes about the dates on which trees burst their buds in the spring, on which birds migrated, on which the ice thawed, and on which wild fruits matured. Modern scientists, such as botanist A. J. Miller-Rushing, have used Thoreau’s data to document that springtime comes earlier, and winter comes later, in New England today than in Thoreau’s day. Thus Thoreau started the science of phenology, which is the study of seasonal adaptations of the life cycles of plants and animals.