Thoreau's Plants

Thoreau's Plants Photo Credit: Cbaile19 (Own work) [CC0], via Wikimedia Commons

Global warming has altered the composition of plants on the shores of Walden since the time of Henry David Thoreau.


Thoreau’s botanical legacy. I'm Bob Hirshon and this is Science Update.

The famous 19th-century philosopher, Henry David Thoreau, lived on the shores of Walden Pond in Massachusetts for two years, taking meticulous notes on the plant life he observed. Now, 150 years later, plant biologist Charles Davis of Harvard University and his colleagues at Boston University are studying Thoreau’s journals to learn about climate change. Davis says that while some plants have adapted to warming temperatures by flowering up to three weeks earlier, others haven’t made the transition.


Groups of related species are unable to respond to climate change, and it’s these groups that are being hit hardest.

He says some of the Northeast’s most familiar plants have declined in abundance, including orchids, roses, and dogwoods. He suspects they’ve become out of synch with their insect pollinators. I’m Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

More than a century and a half after its publication, Henry David Thoreau's Walden has become an American classic, and required reading in many high-school and college courses. Thoreau's long retreat to the woods was a rebellion against the increasingly technological, urban lifestyle of the Industrial Revolution. While Thoreau worried that the technology boom would rob humanity of the pleasures of simple living, he probably didn't suspect that it would someday change his beloved Walden Pond by altering the planet's climate.

Because of its historical significance, Thoreau's woods in Concord, Massachusetts have been mostly spared from direct assaults like deforestation, construction, and pollution. That makes them an excellent case study for the effects of climate change. Thoreau's detailed notes on the types of plants he encountered, and the dates on which they bloomed, provide a rare and valuable record that compares well with scientific surveys today.

Since Walden was published in 1854, the average annual temperature in Concord has risen by 2.4 degrees Celsius, or 4.3 degrees Fahrenheit. In the past, many scientists had assumed that similar species, like flowering plants, would respond to local climate change in similar ways. As you heard, that isn't the case. Instead, some plants have shifted their flowering time to keep up with the earlier thaws, while others have stuck to the blooming schedule of the 1850's.

The “late” bloomers don't just miss out on a few extra weeks of warm weather. They also may bloom after bees and other pollinating insects have finished their work and moved on. Insects tend to be more adaptable to climate change than most creatures, so it stands to reason that they started arriving earlier when flowers started blooming earlier.

This theory is consistent with the fact that the early bloomers are generally thriving better than their stubborn counterparts. If he were to come back from the dead, Thoreau would be startled to find that more than a quarter of the species he once wrote about have vanished from Walden, and another third have dwindled to near-extinct levels. Unfortunately, the losers in this adaptive competition include some of the most colorful, picturesque species, like lilies, orchids, buttercups, violets, roses, dogwoods, and mints. Climate change has been much kinder to weeds and non-native plants: a pattern that also can be seen in other ecosystems around the world.

It's not yet clear why some plants have an easier time adjusting their flowering schedule than others. What is clear, however, is that climate change may dramatically alter the world's biodiversity, with only a small fraction of today's species coming out on top.

Now try and answer these questions:

  1. Why study the plants at Walden Pond? What's scientifically useful about this location?
  2. What was the key finding of this study? Why is that significant?
  3. Suppose all the plants had adjusted their flowering schedule by the same amount of time. What would this say about climate change? Would it mean that climate change is not a cause for concern? Why or why not?
  4. Why do you think some plants, particularly weeds, have more adaptable blooming times? What would be the evolutionary advantage of this trait?
  5. Is there an advantage, from an evolutionary standpoint, for a plant to bloom on a consistent schedule, regardless of temperature changes? Why or why not?


Going Further

For Educators

In the National Geographic News article South African Desert Becomes Global-Warming Lab, read about a South African desert called the Succulent Karoo, where researchers are studying flora for impacts of global warming.

The ReadWriteThink resource, Henry David Thoreau, focuses on the life and work of this great American naturalist.

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