Similarly, at The Shack, as average spring temperatures rose about 3 degrees Fahrenheit (1.7 degrees Celsius) over the last eight decades, first flowering came a week early for the 23 species they studied. During the hottest years in the United States (2010 and 2012), flowering came 24 days earlier than in Leopold's time.
The research may have tracked just two sites, but has broad implications, said Elizabeth Wolkovich, a climate change ecologist at the University of British Columbia who was not involved in the study.
"One is deep within the country and one is on the coast," Wolkovich said.
That means the findings probably apply to temperate climates throughout a large swath of the United States, she told LiveScience.
Though Thoreau and Leopold's works have highlighted how much climate change alters ecosystems, in some ways, the findings are good news.
At some point, the climate will get too hot for plants to survive without evolving, but the fact that the plant flowering time is still changing in step with the temperature means they haven't hit that point yet, said David Inouye, a University of Maryland biologist who was not involved in the study.