Climate change is devastating the flowers of Walden Pond, picking off those species that cannot react to rising temperatures. Comparing data meticulously gathered by Henry David Thoreau more than a century and a half ago with more recent observations, Harvard biologists report in the Proceedings of the National Academy of Sciences that more than a quarter of Walden's plant species have already been lost. And an additional 36 percent are in imminent danger, including lilacs, roses and buttercups.
"It had been thought that climate change would result in uniform shifts across plant species, but our work shows that plant species do not respond to climate change uniformly or randomly," said co-author Charles Davis, a biologist at Harvard, in a release.
The Walden study shows that even small changes in temperature can have outsized impacts on plants that are evolutionarily adapted to fulfill ecological niches. Together with changes seen in other locations, like the unprecedented pine beetle damage in the West, the new work suggests that finely tuned biological systems are having a difficult time keeping up with the rapid pace of human-induced climate change.
The average temperature around Walden has risen by more than four degrees over the last century as increasing greenhouse gas concentrations from burning fossil fuels changed the earth's climate.
But the warming is not just mowing the forest down, it's shaping it as some plant species thrive under the new global conditions.
The earth is not a mere fragment of dead history, stratum upon stratum like the leaves of a book, to be studied by geologists and antiquaries chiefly, but living poetry like the leaves of a tree, which precede flowers and fruit — not a fossil earth, but a living earth.
Henry David Thoreau -- Walden
"Most strikingly, species with the ability to track short-term seasonal temperature variation have fared significantly better under recent warming trends," the authors write.
Although the design of the Walden study is simple, it depends on the value of Thoreau's rare pre-industrial data.
"Whenever you have an opportunity to get a dataset where someone who has made very careful efforts to observe things in a systematic way, it gives you a snapshot of a particular time period and lets you make comparisons," said Mark Schwartz, a world expert in phenology, the field of seasonal changes in living things, at the University of Wisconsin, Milwaukee.
Unfortunately, very few ecosystems have been recorded in such excruciating detail.
"We don't have a large number of datasets of this sort," Schwartz said. "Most of them are concentrated in Europe and in Asia. There are very few in North America."
For example, Isabelle Chuine at France's Center for Evolutionary and Functional Ecology, published a paper in Nature using detailed grape harvest records in Burgundy dating from as far back as 1370. Schwartz also noted that many European weather services record phenological data along with their weather measurements, while American weather stations do not. As a result, Americans know less about when our plants bloom than many other countries.
But Schwartz is trying to change that by empowering Americans to contribute their own Thoreau-style data. He's the chair of the National Phenology Network, a new organization attempting to incorporate data from ecological stations, citizen scientists and other types of fieldwork.
Already, one of the NPN's efforts — Project BudBurst — has marshaled several thousand people to track the timing of plant flowerings in their backyards as they shift due to climate change.
Their data could not only benefit scientists of the present and future, but could aid in providing Americans with direct evidence of climate change, helping to create the political will necessary to address the nation's greenhouse gas emissions.
"When someone asks me about climate change, I say, 'You can go observe it in your own backyard,'" Schwartz said. "If you want to see what's happening, start taking records and see for yourself."
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