Kellen Calinger-Yoak couldn’t believe her luck. In 2009, The Ohio State University forest ecologist found a 1915 Monthly Weather Review journal with nearly three decades of late-1800s growing season records. She read row after row of details from Ohio farmer Thomas Mikesell, who tracked weather patterns, seasonal shifts in trees and plants, and crop yields around Wauseon, Ohio. The farmer had penned painstaking information on the same tract of land where he was born in the state’s northwestern corner, and the journal had published it.
Inspired to visit Mikesell’s farm, Calinger-Yoak mapped the small town and realized it was just over two hours northwest of her office. Shortly after visiting, she decided to replicate some of Mikesell’s data, so she could compare modern growing seasons to past ones.
Mikesell was self-taught, devouring science and history books during the decades he spent meticulously recording weather patterns, bird migrations and the seasonal changes of trees and plants. His first obsession was weather patterns, which he began tracking in 1865.
Mikesell initially kept records for himself but began to recognize his accomplishment later in life, and hoped future generations might use his data. In 1882, he was among the first observers for the Ohio Meteorological Bureau and, later, briefly served as a regional observer for the U.S. Weather Bureau. “Quietly, carefully, conscientiously, this man has merely kept his eyes open to see and systematically recorded the movements of nature about him year after year,” wrote meteorologist J. Warren Smith, who compiled Mikesell’s notes into that 1915 journal. “He has done what thousands of other men might have done, but which no other one has done. … Science owes a great debt to such a man.”
Smith also noted Mikesell’s perseverance: “There is scarcely an observation lost during all these 45 years.”
Mikesell’s data formed a robust baseline for Calinger-Yoak’s research. Property records led her to his former farm, and a friendly approach accomplished the rest. “I basically knocked on doors and asked people if they owned the woodlots, explained to them what I was doing and asked for permission to stomp around on their land for the next five years,” she says. “They were really into it.”
She spent a half-decade trekking up to Wauseon weekly to create a modern record of observations that could be compared with Mikesell’s to see how growing seasons had changed. Between spring and fall, she noted the first buds of spring, peak fall colors and the last leaves to drop before winter. She tracked the same details as Mikesell for American elm, black oak, black walnut, eastern cottonwood, staghorn sumac, sassafras and white oak trees, species common across the eastern United States.
Calinger-Yoak discovered that, with temperatures rising significantly since Mikesell’s lifetime, hardwood growing seasons are now about 20 days longer on average, findings she published in March in PLOS One. Growing seasons for certain species were up to 26 days longer, she found, but rainfall was about the same as in Mikesell’s lifetime. While forest ecologists often track climate change impacts over time, it is rare to have detailed data from the 19th century.
A 2004 Geophysical Research Letters report that analyzed weather station data collected from 1895 to 2000 showed that the continental U.S.’s average growing season was about two weeks longer by the end of the 20th century than it was at the century’s start, with a marked increase in the rate of change starting in the 1970s. Because Mikesell’s data was so detailed, Calinger-Yoak was able to create a similar study, tracking and comparing the same events in the same species at the same location more than 100 years later.
While some studies have shown that a longer growing season can be helpful, allowing plants to store more carbon dioxide, the increase can also disrupt the natural balance. “It can really throw an ecology or a system into disarray in the sense that the pests don’t have the control and mechanisms that kind of keep them at bay at the right time,” says Christopher Evans, a forest ecologist at the University of Illinois at Urbana-Champaign. “The birds that arrived don’t have the food sources at the right time, or the pollinators that may pollinate these plants aren’t out yet.”
Midway through Calinger-Yoak’s 2010 to 2014 data collection period, Wauseon clocked its warmest March in more than 100 years, in March 2012. Two years later, as she took her final measurements, the town saw the fourth-coldest March ever recorded.
Both Calinger-Yoak and outside experts say her observations encompass a modern-day pattern of unpredictable weather and seasonal shifts driven by climate change. “To me, it just highlights how variable the climate is,” Evans says. He notes that the study’s five-year snapshot is still relatively short. As climate change accelerates, Evans says scientists are having a harder time predicting growing seasons, driving a need for longer studies and more data-gathering.
The longer growing seasons Calinger-Yoak noted stretched late into the fall, rather than starting earlier in the spring, which she tracked by marking when trees lost their leaves. “Most studies only focus on the start of the growing season and often leave the end of the growing season under-studied,” Calinger-Yoak says. By ignoring when the growing season ends, or assuming it’s not as variable, “we’re losing a huge amount of really important information.”
For trees, these impacts can be outsized. Longer growing seasons can mean an increase in invasive species, an uptick in pests and drier soil as snowpacks shrink.
“Farmers can adjust to longer growing seasons, but you can’t cut down a forest and have it suddenly come back,” says University of Maine forest scientist Aaron Weiskittel. “The trees that are there have to adapt to these conditions.” Growing seasons and environmental conditions are only part of forest health, and trees also sometimes have to adapt to overcrowding, shifts in species diversity and human impacts like logging.
While much has changed since Mikesell’s lifetime, the importance of citizen scientists has persisted. Individuals who help track natural phenomena over time, now using apps like iNaturalist, add a rich layer of observations and measurements to academic efforts. Mikesell, who only missed a measurement when his crops and workload elsewhere got in the way, might have appreciated today’s explosion of apps and programs designed to involve laypeople in data tracking. And he may have reveled in the fact that researchers are now relying on citizen scientists as collaborators.
“The scientific community is saying, ‘Hey, we see you out there, we see you interested, we see you engaged,’” Calinger-Yoak says. “You can be scientists, too, and help us gather far more data than we ever possibly could have on our own.”