“I am on the alert for the first signs of spring, to hear the chance note of some arriving bird, or the stripped squirrel’s chirp,” wrote the naturalist Henry David Thoreau. Through his observations in Walden and other journals, Thoreau may well have been one of our nation’s first unwitting phenologists—precisely recording when the pond froze and thawed, as well as spring’s first blooms for several hundred plant species surrounding his rural retreat in Massachusetts.
You can think of phenology as nature’s calendar— observations on when saguaros blossom, when monarch butterflies migrate south, or when cicadas first appear.
“Phenology refers to the timing of seasonal events in plants and animals, such as when plants put on leaves or flowers open, when birds migrate, and when insects hatch,” said Theresa Crimmins, Assistant Director of the USA National Phenology Network.
165 years after Thoreau’s observations, phenologists like Crimmins are comparing historical records like his with contemporary data to identify long-term trends and develop a more complete picture of the regional effects of global climate change.
Putting Phenology to Work
Seasonal temperature has a lot to do with when phenological events happen. To better understand how springtime temperatures are changing and potentially affecting plants and animals, Theresa Crimmins, along with her husband Michael Crimmins, a professor and extension specialist in the University of Arizona Department of Environmental Science, recently evaluated three key springtime heat thresholds across the continuous United States.
Traditional methods used to evaluate changes in phenology focus on long-term trends in monthly or seasonal temperature, but the timing of many springtime phenological events, such as leaf-out and flowering, is better represented by accumulated warmth or Growing Degree Days (GDD).
GDDs are a measure of how much warmth has accumulated over a period of time. The specific quantity of warmth that must accumulate to trigger a phenological event is unique to individual species of plants and animals. In many species, the amount of accumulated heat needed to cue the event has been identified, and this amount is called a GDD threshold.
The Crimmins focused on three spring-season heat accumulation thresholds: 50GDD, 250GDD, and 450GDD. These thresholds encompass the initiation of spring-season activity including budburst, leaf out, flowering and egg hatch in the majority of plant and insect species in the U.S.
50GDD is associated with activity in species that start biological activity the earliest in the year, things like egg hatch in eastern tent caterpillar, first bloom in red maple, and first bloom in Corneliancherry dogwood.
Progressing further into the season, 250GDD is associated with first bloom in many species, including Ohio buckeye, Tatarian honeysuckle, common horsechestnut, and egg hatch in pine needle scale.
And after even more springtime warming has occurred, 450GDD is associated with first flowering in black locust and white fringetree, adult emergence of emerald ash borer—a devastating insect pest—and when strawberries start ripening.
“Our study is different than many other studies in that it evaluates trends in accumulated winter and spring warmth in different regions of the country,” Michael Crimmins said. “This is important for better understanding and anticipating species responses to changing climate conditions because we are evaluating how a biologically-relevant measure of spring warmth is changing.”
Tapping Historical Records
An important feature of their analysis was the ability to verify patterns by comparing them to long-term records of plant phenology collected at various locations across the county.
One great example of this was seen in Wisconsin, where Aldo Leopold—a man considered by many to be the father of wildlife ecology—and his family collected phenology observations of springtime events from 1936-1998. Leopold’s daughter, Nina Bradly, used these records to show phenological events occurring at the very beginning of the season, in February, had rapidly advanced over a 60-year period. In contrast, later in the season, this pattern dropped off.
Theresa Crimmins and Michael Crimmins saw a similar pattern—an advancement in the earliest season threshold at 50GDD, occurring in February, and no significant change in the later season 450GDD threshold, over the duration of their analysis of a similar timeframe.
An Advancing Spring
Their findings also demonstrate real change in the speed at which spring progresses across the U.S. While spring is sprouting earlier in parts of the Northeast, bringing it in line with the start of spring in the south, the opposite is occurring in the Western U.S.
“In the West, there is a longer lapse now than in previous decades between when spring starts in the Southwest and in the Northwest,” Theresa Crimmins said.
These patterns may have significant implications for a number of species. For example, migratory birds and insects looking to take advantage of peak springtime resources.
“Species migrating north in the spring in the East may need to speed up their migrations to keep pace with food, whereas species in the West may need to slow it down,” Theresa Crimmins said.
The strongest trend in the three thresholds the Crimmins identified directly impacts the Southwest, including Arizona and much of southern California, both large agricultural production areas.
“In the Southwest, we are seeing all three thresholds advancing dramatically, meaning pretty much all springtime phenological events have advanced, at nearly double the rate as in other regions,” Theresa Crimmins said.
“More broadly, these changes are consistent with shifts in the beginning of the growing season being observed across much of the U.S. impacting when crops are planted and mature, as well as impacts to natural ecosystems,” Michael Crimmins said.
The notably rapid advances in threshold timing in the Southwest may have less of an impact on the timing of phenological events in natural systems, where leaf-out and flowering are governed more directly by precipitation than temperature. However, a continued rise in temperature could result in a change of species ranges or even potential die-out.
“If conditions continue to get hotter, eventually these plants will be unable to tolerate them,” Theresa Crimmins said. “Where possible, these species will be forced to shift to higher latitudes or elevations, where temperatures will be more in line with their ideal growing conditions.”