New Hampshire

Fact-checking Thoreau’s observations at Walden Pond shows how old diaries and specimens can inform modern research

flower blooms

Credit: Pixabay/CC0 Public Domain

Henry David Thoreau, the environmental philosopher and author of “Walden,” was a keen observer of seasonal change. For example, in 1862 he wrote in the Atlantic Monthly:

“October is the month of painted leaves. Their rich luster now flashes round the world. As fruit and leaves and the day itself take on a bright tinge just before falling, so the year approaches its end. October is its sunset sky; November the late dawn.”

Over the past 20 years, researchers have used Thoreau’s observations of plant flowering, tree and shrub leaf budding, bird migration, and springtime ice melt on Walden Pond to study how these events have changed since the 1850s, primarily in response to the climate change.

Ecologists have also drawn data for modern research from museum specimens, hunting guide journals, and reports from bird and butterfly clubs. Comparisons with historical records have provided insight into changes in the natural world caused by climate change and other human influences. Examples include coral decline in American Samoa, amphibian losses in Mexico, and bird range shifts in California’s Sierra Nevada.

But how do scientists know that these historical dates are appropriate? How can they distinguish good data from bad? And how can you know if any records you may have, such as B. the diaries of an ancestor or a shell collection that could be useful for science?

We recently published an article in the journal Bioscience that outlines a three-step approach to assessing the quality of historical observations. With this approach, we believe scientists can safely use historical resources to inform studies going back to times and places where formal scientific data is not available.

A three part test

Shortly after Thoreau died, critics questioned the accuracy of his natural history observations. In 1919, John Burroughs, a leading nature essayist of the time, offered perhaps the harshest criticism.

Burroughs asserted that Thoreau’s “observations are frequently flawed or dead wrong”. He asked if Thoreau knew basic facts such as that hickory trees grew in Concord, Massachusetts and that pine trees had seeds.






Researchers are using a collection of photographs of British landscapes taken between 1910 and 1935 to analyze the current effects of climate change on those locations.

To determine whether Burroughs and other critics were right, we propose a simple three-step process.

  • Is the information collected using rigorous methods that are well documented and clearly described? Modern researchers should be able to repeat them – for example, locating places where earlier naturalists worked, making observations on the same number of days per week, and following other important parts of their methods.
  • How accurate are the observations, e.g. B. Species identification? Do they have any prejudices? Can researchers or naturalists replicate aspects of the observations that are expected to be consistent over time?
  • Do the data have the accuracy, frequency and rigor that scientists need now? No data is suitable for all purposes. Modern researchers must decide whether the information can answer the question they are studying.

Was Thoreau a good naturalist?

When we judged the rigor, accuracy, and usefulness of Thoreau’s naturalistic observations, we found that he was indeed a good naturalist.

Thoreau thoroughly documented the dates, locations, and descriptions of observations he made while walking around Walden Pond and Greater Concord. We can read in his diaries how often and for how long he made these notes.

We compared Thoreau’s notes to modern observations and found that his observations of seasonal events such as leaf bud break, flowering, fruiting, and bird arrival correlated strongly with modern knowledge. This told us that Thoreau was picking up similar patterns.

For example, we can see that the order in which flowers bloom around Concord in spring is almost the same in Thoreau’s journals as it is in modern observations. In both datasets, certain species bloom early while other species bloom late in the season.

Thoreau’s historical observations have enormous utility in research. We and other researchers used them to learn more about the effects of climate change on plants and birds in Concord. Using Thoreau’s results as a basis, we found that spring leaf bud break and flowering occur earlier, but the timing of bird arrival does not change significantly.

Beyond Thoreau and Walden






In this 2011 video, Boston University biologist Richard Primack explains how he and his research team used Henry David Thoreau’s observations of nature from the 1850s to measure the effects of climate change in New England.

Researchers can use this approach to evaluate other historical observations. For example, American field biologist Joseph Grinnell and his colleagues recorded observations of species in California between 1904 and 1969. Her team carefully described most of their methods and collected samples and photos to document their work.

However, their sampling methods have sometimes been inconsistent, and researchers are unable to locate some of their sampling routes. These uncertainties make the Grinnell team’s observations unsuitable for answering questions about changes in the abundance of some species. But their observations are excellent for answering questions about how climate change is altering the ranges of many species, including birds and small mammals like mice, voles and chipmunks, which Grinnell’s team has observed there in the past and are still found there.

Museum exhibits such as dried plants, bird nests, and animal skins are another source of historical information. The specimens themselves remove uncertainties in species identification and preserve many physical properties that interest researchers.

However, the people who collected the samples are sometimes unable to record precise location information. And some collectors target specific species, locations, or seasons, which can affect what they find.

For example, if a collector is targeting early flowering plants, their collection may be missing plants that flower later in the year. We urge researchers to be mindful of these biases when using historical data.

It is not uncommon to find historical records with little or no documentation as to when, where and how the data was collected – for example, observations from one’s daily walks, collections of photographs, or reports from a birder to an ornithological association. Even in these cases, it may be possible to determine how rigorous and accurate the data is.

For example, the frequency of photographs or observations can indicate how often someone made observations. And even poorly documented data can be useful in answering some ecological questions or in proposing new hypotheses that deserve further investigation.

Scientists are looking for more historical data. After careful evaluation, we may be able to use this information to learn more about the impacts of climate change, land use practices, and other environmental issues. Individuals who have records that may be scientifically valuable should consider contacting ecologists, research stations, natural history clubs, and the US National Phenology Network, which collects, stores, and provides data on the timing of seasonal events such as bird migrations across the United States passes on


Scientists use Thoreau to measure the impact of climate change on wildflowers


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Citation: Fact-checking Thoreau’s observations at Walden Pond shows how ancient journals and samples can inform modern research (2022 October 27) retrieved October 27, 2022 from https://phys.org/news/2022-10- fact-checking-thoreau-walden-pond-diaries.html

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