How Plants Are Responding to a Warming World — And Why It Matters
Winter and spring 2023 were very warm in much of the eastern United States, causing springtime biological activity to start much earlier than usual. The news media took note, with dozens of outlets trumpeting the earliest spring in decades, a premature beginning to the allergy season, and an early start to flowering in the iconic cherry trees of the mid-Atlantic region. Even “Saturday Night Live” gave it a nod, with an opening skit featuring early sightings of stereotypical characters who populate Central Park when the weather is nice.
It was mid-April and New York City had already reached 90 degrees Fahrenheit, three months ahead of schedule. Many of the clearest changes in phenology — a term that refers to the timing of seasonal activity — are occurring in spring. While we might appreciate the break from bulky winter clothing, changing climate conditions and progressively earlier springtime conditions create challenges for both plants and animals, with repercussions that ripple throughout ecosystems and extend to humans.
The biggest shifts in springtime activity are occurring in Asia, followed by Europe, though our ability to state this with confidence is hampered by limited information from the Southern Hemisphere. On each of these continents, the timing of leaf out in common overstory trees has shifted to between nine and 15 days earlier since the early 1980s. Changes in the timing of leaf out are less dramatic in the United States; since the early 1980s, spring leaf out has advanced by about a week.
A fantastic example of how overstory deciduous trees and understory herbaceous plants from temperate ecosystems, such as those in Europe and the United States, have adjusted the timing of spring activity since preindustrial times hails from New York. In the early 1800s, instructors at secondary schools across the state regularly documented when they observed meteorologic conditions as well as plant and animal activity. They continued this effort until the Civil War. These data languished, all but forgotten in dusty annals, until Dr. Conrad Vispo, cofounder of the Farmscape Ecology Program in Hawthorne Valley, discovered them in 2014. Vispo immediately began to digitize the old data, appreciating the gold mine these historical records offered. Yet he recognized that they had little scientific value without a modern context.
The trend toward earlier flowering in wind-pollinated plants is not good news for those of us suffering from seasonal allergies.
So he searched “phenology in New York” online and discovered the New York Phenology Project, a statewide phenology monitoring effort that had been initiated in 2012 by Dr. Kerissa Fuccillo Battle. The two scientists were astounded when they realized the similarity between the two efforts. They began a collaboration on the spot. When the two datasets were compared, the findings were eye-popping. Tulip trees now flower 27 days earlier than in the 1800s, and common milkweed, a plant that is critically important to endangered monarch butterflies, now flowers 13 days earlier. Overall, plants in the study leaf out 19 days sooner than they did in the early 1800s.
While the results of the New York study are impressive, they are not novel. Dozens of studies have shown impressive shifts in springtime plant and animal activity in the temperate forests of the United States. In West Virginia, two spring wildflowers, cutleaf toothwort and yellow trout lily, now flower approximately six days earlier than 100 years ago. In Mohonk, New York, hepatica, bloodroot, and trout lily now flower over a week earlier than in the 1930s. In Concord, Massachusetts, yellow wood sorrel now flowers 32 days earlier than 150 years ago. This pattern extends to Europe. In Switzerland, horse chestnut has shifted leaf out by 11 days since the early 1800s, and cherry trees have advanced their flowering time by 30 days over the past 130 years.
But not all plants are shifting their springtime activity earlier in the year. Some are exhibiting the opposite response, delaying their springtime activity. For example, in North Dakota, nannyberry, smooth sumac, and black walnut now flower approximately 10 days later than in the first half of the 1900s. And many other species seem not to care much about changing conditions and persist in undergoing springtime activity at the same time that they have in decades past. This is especially true in the Southern Hemisphere. Approximately 70 percent of the species evaluated in a comprehensive study in Australia and New Zealand showed no evidence of changes in phenology.
A handful of generalizations is starting to emerge from the hundreds of studies of plant phenology from around the globe. In general, plants active the earliest in the spring show the greatest changes in the timing of their activity. In addition, annual plants, which complete their entire life cycle — from germinating to producing mature fruits or seeds — within a single year, generally evidence larger advancements than their longer-lived counterparts.
Further, shifts in the timing of leaf out and flowering appear to be greater for wind-pollinated plants than those that are pollinated by insects. Wind-pollinated plants produce abundant pollen grains that are small and lightweight enough to be carried on air currents from one plant to another. These plants are the ones responsible for causing seasonal allergies, as the pollen grains produced by these plants are small enough that we breathe them into our lungs.
In contrast, the pollen grains produced by insect-pollinated plants are comparatively large and often sticky or spiky, enabling them to hitch a ride on the insects visiting the plants for nectar. The trend toward earlier flowering in wind-pollinated plants is not good news for those of us suffering from seasonal allergies; this means an earlier start to the allergy season! Shifts toward earlier activity are often greater for plants introduced to an environment than those exhibited by native species. And finally, plants at higher elevations and latitudes frequently show greater changes in their timing than their lower-elevation and latitude counterparts.
For many of us, autumn is a favorite season, bringing cool, crisp days, brilliant gold, crimson, and orange leaves, and pumpkin spice everything. Around the globe, however, the natural events that signal our autumn experience, like leaf color change and leaf fall, are changing as well.
In general, autumn events are drifting later in the year than in decades past, though patterns in autumn are not as clear or strong as those emerging for spring. Autumn phenophases — life cycle stages such as leaf color change and leaf drop — appear to be shaped not only by temperatures, beginning when temperatures start to cool in the late summer, but also by daylength and available moisture.
In general, when temperatures are warmer, leaf senescence — when the chlorophyll that is responsible for giving leaves their green color breaks down — is delayed. Moreover, the reveal of the brilliant yellows and oranges that were there all along but masked by green as well as the generation of the anthocyanins responsible for dazzling red in some leaves are both delayed. Yet midsummer drought can cause leaf color change to occur earlier in the season or even cause leaves to drop with no coloration at all. The opposite is true for fruit ripening, though. Following suit with leaf out and flowering, fruit ripening is generally occurring earlier in the year now than in the past in response to warmer temperatures.
Why Should You Care?
Changes at the Grocery Store
Agricultural crop losses are one of the most direct ways we experience changing phenology. Crop success is dependent on many factors, including daily temperatures, rainfall, extreme weather events, and the presence of pollinators, pests, and disease, all of which farmers track and use to shape immediate decisions.
As climate conditions have changed rapidly in recent decades, farmers have had to adapt. In many parts of the world, farmers are planting earlier in the year. As well, in many places they are harvesting earlier in the year as plants reach maturity more rapidly. For example, some varieties of wine grapes in the north of France are now harvested four weeks earlier than only 40 years ago. Sap flow in sugar maples — the key ingredient in maple syrup — has also inched notably earlier in recent years. With continued warming, sap flow is anticipated to shift to an entire month earlier by the end of the century, dramatically affecting the timing of the harvest as well as where sugar maples successfully grow.
Likewise in India, the flowering period for saffron has been substantially shortened in recent years. Saffron “threads,” highly valued for the flavor and rich color they bring to dishes and drinks, are actually the female reproductive parts of the saffron crocus flower. In parts of India, saffron flowers now open when temperatures are too warm for their development. This leads to a high rate of flower death, and with no flowers, there is no saffron. Between 2013 and 2017, saffron production in Kashmir declined by 90 percent. Consequently, many saffron farmers are shifting their plantings to higher elevations with cooler temperatures.
The northeastern United States is a major fruit production region, as are southern states. As temperatures in these regions have warmed, leaf out and flower bud development as well as the last spring frost have shifted earlier in the season. In many locations, however, the date of the last frost has not shifted earlier to the same degree as plant activity. Consequently, tender plant tissues are at greater risk of exposure to damaging frosts. Many of the plants that produce fruits we enjoy, including blueberries, apples, and cherries, open their flower buds early in the season, sometimes even before they break leaf buds. Once flower buds begin to open, they become sensitive to cold temperatures.
We can expect to experience early warm springs followed by damaging freeze events in nearly one out of every three years by the mid-21st century.
As with saffron, if flower buds are killed by frost, there are no fruits. So advancing phenology is expected to worsen the risk of frost damage in the coming decades. The start of springtime biological activity in the United States is projected to advance by up to three weeks by the end of the century. One set of predictions indicates that we can expect to experience early warm springs followed by damaging freeze events in nearly one out of every three years by the mid-21st century. The same is predicted for Europe and Asia, with up to a third of Europe and Asia’s forests predicted to be threatened by frost damage in future decades.
Longer Growing Seasons and Your Health
Achoo! Sniffle. Does it seem like your allergies have gotten worse lately? It sure feels like it to me. If you said yes, you’re not wrong. Since 1990, the pollen season has lengthened by over 20 days in the United States and the amount of pollen generated by wind-pollinated plants has increased by about 20 percent. Why is this happening? The higher atmospheric carbon dioxide concentrations and warmer global temperatures we are now experiencing essentially fertilize plants, enabling them to grow larger and produce more pollen than in the past. The consequences for those of us who suffer from seasonal allergies are real.
Dr. Stanley Fineman, an allergist who has practiced in Atlanta, Georgia, for over 40 years, recently shared that in years past, he would instruct his patients to begin using their allergy medications on Saint Patrick’s Day (March 17) to be prepared for upcoming surges in airborne pollen. Yet because the growing season — and consequent pollen production — starts so much earlier now, he has changed his advice to initiate medication on Valentine’s Day (February 14), a full month earlier.
Changing climate conditions also benefit many insects that carry disease. Milder winters and a longer growing season encourage earlier emergence and later fall activity in ticks, which carry Lyme disease and other pathogens, and mosquitoes, which carry dengue, West Nile virus, malaria, and multiple forms of encephalitis. The longer seasons of insect activity increase the period during which humans can be exposed to these disease vectors. The same is true for waterborne illnesses: warmer fresh and marine waters promote growth in harmful algae, bacteria, viruses, and parasites, and likewise increase the potential for exposure through a longer period of activity.
In mid-December 2023, my friend and colleague Dr. Jorge Santiago-Blay sent me photos of cherry trees on the Penn State York campus in Pennsylvania, United States, bearing many open blooms. While he expressed pleasure over the lovely sight, the phenomenon sparked confusion and disorientation because typically, eastern Pennsylvania is wracked with frigid temperatures and snow by this time of the year — hardly favorable conditions for delicate cherry blossoms.
With increasing variability and change in the earth’s climate, we are poised to experience oddball phenological events such as this with increasing regularity, with impacts to not only to the species exhibiting the unseasonal activity but also our health, cultures, diets, and mental states. Changing phenology, while seemingly innocuous, impacts our lives in many clear and tangible ways.
Theresa Crimmins is Associate Professor in the School of Natural Resources and the Environment at the University of Arizona and Director of the USA National Phenology Network. She is the author of “Phenology,” from which this article is adapted.
