One of the challenges for scientists attempting to understand and document climate change is finding reliable records from the past in order to compare data from the present.

Flowers are losing direct sunlight as trees put out leaves earlier.

A recent study by a team including a University of Tennessee professor makes use of a particularly rich trove of real-time information collected more than 160 years ago: the detailed observations of natural life around Concord, Mass., recorded during the 1850s by naturalist and philosopher Henry David Thoreau.

“Humans are really affecting the landscape on many levels,” said Susan Kalisz, who is the department head of ecology and evolutionary biology at UT Knoxville. “We’re fragmenting habitat, we’re introducing invasive species, we’re changing global climate and we’re using resources at a phenomenal rate.”

One way to see that is to chart the changes in plants and flowers from Thoreau’s time to ours. What Kalisz and her colleagues found was that relative to the 1850s, both shade-grown wildflowers and the trees that tower above them are budding and flowering earlier than they used to -- but in an unequal way.

Wildflowers bloom earlier than trees put out their leaves. For flowers that cluster around trees, the head start gives them time to absorb as much sunlight as possible before the leaves above them emerge to cut off the solar supply.

But the potential problem for the wildflowers is that while they are blooming a week earlier than they did in Thoreau’s day, the trees are budding two weeks earlier -- meaning the flowers are losing up to a week of unfettered sunshine.

“The ramifications are that over time, because these plants really have very short windows for photosynthetic carbon gain, that they would become carbon starved in some ways,” Kalisz said. “They wouldn’t grow very much each year.”

That could mean a long-term loss of wildflowers.

Kalisz specializes in the development and conservation of flowering plants and their ecosystems. In previous studies, she and colleagues had developed a model for measuring wildflower assimilation of carbon via photosynthesis during different times of year and different levels of light exposure. It showed that the short period of direct sunlight before the emergence of the leaf canopy was critical to the flowers’ health.

Looking for a way to extend that model over time to measure the impact of a warming climate, Kalisz was introduced to the work of Boston University biologist Richard Primack. About 15 years ago, Primack was himself seeking historical climate data for the Eastern United States when he learned of Thoreau’s detailed record-keeping.

Beginning about four years after his famous sojourn at Walden Pond, Thoreau had been a consummate chronicler of the natural world around him in Concord. Day by day and season by season, he documented with great specificity the flora and fauna he encountered on long daily walks.

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"It’s really about reining in the rate at which climate is changing. “The forecasts are very dire and they’re real. It’s not people being alarmist.” – Susan Kalisz, head of the department of Ecology and Evolutionary Biology, University of Tennessee

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He even compiled his observations in convenient charts, the originals of which are held by the Morgan Library & Museum in New York. In a phone interview from Boston, Primack said it was receiving photocopies of those charts that made him realize how valuable the work was.

“As soon as we saw those records, we knew they were kind of a gold mine,” Primack said.

Since 2004, Primack and many other researchers have mined Thoreau’s precise reports for comparative data. Kalisz’s study, which includes Primack as a co-author along with three other researchers, is the latest result.

“We probably know more about climate change and the biology of Concord than anywhere in the United States,” Primack said. That’s not only because of Thoreau -- partly inspired by his work, generations of naturalists have flocked to the area and left abundant records.

Kalisz said there is a fairly straightforward explanation for why trees are responding more quickly to a warmer climate than the flowers below them.

“The air heats up faster than the soil,” she said. “Below ground where these plants are, it’s cooler than the air temperature where the buds on the tips of the stems on the trees are.”

That means warmer temperatures register with the trees relatively faster than with the flowers, cutting into the flowers’ time in the sun.

Compared to some of the other potential effects of a changing climate, like severe storms and rising sea levels, reduced carbon budgets for forest wildflowers might seem like a small issue. But, Kalisz said, it’s all connected.

“It’s really about reining in the rate at which climate is changing,” Kalisz said. “The forecasts are very dire and they’re real. It’s not people being alarmist.”

Kalisz and Primack said the study has opened the way for further research into the effects of increased shade on wildflowers, among other wide-ranging effects of climate change.

Thoreau, a philosophical founder of the conservation movement, might be gratified by the 21st-century utility of his data. In a letter to a friend in 1860, he wrote, “What is the use of a house if you haven’t got a tolerable planet to put it on?”