Phenology is a field of study that explains the seasonal changes and behaviors seen in plants and animals. Unfortunately, due to climate change, many phenological events are occurring earlier or later, which disrupts relationships between co-dependent plants and animals. Joining me on the podcast to explain what gardeners need to know about phenology is ecologist and phenology expert Theresa Crimmins.
Theresa grew up in Michigan and now lives in southern Arizona, where she is an associate professor at the University of Arizona in Tucson and the director of the USA National Phenology Network, a national monitoring and research initiative focused on collecting, organizing and delivering phenological data and forecasts to support natural resource management, advance the scientific field of phenology and promote understanding of phenology by a wide range of audiences. She is also the author of “Phenology,” a new book in the MIT Press Essential Knowledge series.
Theresa Crimmins is the director of the USA National Phenology Network, an ecologist and a University of Arizona associate professor.
I want to thank my friend Margaret Roach of A Way to Garden for connecting me with Theresa. (Margaret had her own podcast conversation with Theresa in March.)
We all need to be more aware of the importance of phenology, its influence on interactions between plants and animals — and why it matters. Gardeners can make better decisions for a successful growing season and can learn from these natural processes to make more informed plant choices and better timed gardening chores.
What Is Phenology?
Phenology, in simple terms, refers to when things happen seasonally in plants and animals.
While most people don’t recognize the term “phenology,” Theresa says we all know what it is. “If it isn’t a real explicit awareness, we feel it in our bones,” she says. “You can’t help but recognize the changes that are happening right outside your window, and our lives are very much shaped by when the leaves fall off the trees and pile up in our yard, or when the daffodils start poking up through the ground.”
When seedlings come up outdoors. When pine trees start to produce cones. When migratory birds reappear in the spring or head south in fall. These are all phenological cues that can give a hint about what to expect next.
“Most of those events are really largely shaped by local environmental conditions,” Theresa says. “A lot of times, temperature conditions have a lot of influence, but other factors like day length and precipitation and sometimes winter chill can also have an influence.”
She adds that, because in many parts of the world changes are happening rapidly, the timing of phenological events is changing.
It’s only in the last few decades that phenology has become prominent and paid attention to, she points out: “It’s one of the most visible ways that we can see how changing climate conditions are affecting individual species and ecosystems.”
Chilopsis linearis, desert willow, developing fruit. When trees develop fruit is influenced by temperature and day length.
Photo by Saran Schaffer / USA National Phenology Network
How Theresa Crimmins Joined USA-NPN
Theresa moved to Arizona for graduate school initially. She stayed because her husband found a job there quickly after graduation, and soon she joined the newly formed USA National Phenology Network, or USA-NPN.
“This was interesting to me because I was doing some research on looking at patterns in flowering in the mountains north of town, using some data that had been collected by a local individual,” Theresa recalls. “And interestingly, not only was this national-scale organization being established right here at my university, the headquarters were being set up right in the same building and just steps from my office. And the more I learned about the aims of the organization, which was to document when things were happening seasonally in plants and animals all across the country, the more I was intrigued.”
She became one of USA-NPN’s first employees back in 2006 and has been there ever since. She became the director in 2020.
The Origins of the USA National Phenology Network
The USA National Phenology Network was inspired by phenology networks in other countries, especially in Asia and Europe, where there has been a much longer history of tracking phenology formally, Theresa says.
Those long-term historical records have allowed researchers to see the fingerprint of how things are changing, and folks here in the United States recognized that we didn’t have similar data, she explains. “Other than a single network that tracked lilacs and honeysuckles for the last several decades, we largely haven’t had an intensive effort to track when things are happening. And because phenology is so important as this indicator of how things are changing, some folks felt like it was really necessary to formalize that.”
USA-NPN originally worked very closely with the U.S. Geological Survey, a federal agency that provided much of the network’s funding. That changed in 2020 when the USGS was facing funding cuts of its own.
“Since that time, we have continued to be funded by federal grants and different agencies, but it’s been more of a, I’ll say, cobbling together of funds from different sources,” Theresa says.
Because USA-NPN has been supported by federal dollars all along and has grant money in hand currently, the network can continue to sustain operations for a duration, she says, but she adds that she is anticipating that opportunities for federal support that USA-NPN has relied on in the past will no longer exist going forward due to federal cuts.
Philanthropy, commercialization and reducing the scope of what USA-NPN does are all being considered to sustain the network.
Theresa Crimmins delivers a TEDx Talk on phenology. Courtesy of Theresa Crimmins
‘Phenology’ – The Book
Theresa’s book “Phenology” is intended to introduce readers to the concept formally, give a little bit of the history, and share why people might care what it is.
Because of climate change, the historical timing of phenological events is changing, and that affects species in many locations nationally. Theresa says the consequences are ecological as well as economic. Food availability, the cost of agriculture and human health are all affected.
“We’re seeing longer allergy seasons, tourism’s being disrupted,” Theresa points out. “There’s an awful lot of ways in which this begins to intersect with our daily lives.”
Early Phenological Tracking
Aldo Leopold, the renowned environmentalist active in the first half of the 20th century, wrote about phenological events in his book “A Sand County Almanac,” based on his nature observations around his Wisconsin home.
“Some of the best insights that we have into how things have changed are because folks took the time to write things down in the past, and they didn’t have the same motivation that we do now,” Theresa says. “Now, our major motivation for documenting phenology is because we want to understand how things are changing. We know that they’re changing. We know there are consequences for those changes. But there’s a lot, a whole lot, that we don’t know back in the past.”
Aldo Leopold died in 1948 but his children continued his annual observations and even published a study in the 1990s demonstrating that some phenological events had shifted by as much as a week in a few decades.
In Japan, cherry blossom timing has been tracked since about the year 800. There was little to no variation for hundreds of years, but after the age of industrialization, around 1850, when carbon dioxide and more started to be pumped into the atmosphere, blossom time has trended earlier.
Historical records of plant and animal seasonal timing informs phenology studies today.
Photo Credit: Brian Forbes Powell, USA-NPN
The Impact of Temperature, Day Length and Vernalization on Phenological Events
Temperature accounts for the most drastic changes in the timing of phenological events.
In a temperate arboreal system, when the temperature dips, plants go dormant.
“As global average temperatures have increased, we are now experiencing more and more time above those limits, enabling those plants to basically get going earlier in the year and stay active longer, later into the growing season, or extend the growing season at the tail end,” Theresa says.
Species that depend upon one another aren’t necessarily keeping pace with one another in these periods of change, she points out.
“The reason for that is that while temperature does play the greatest role in shaping when species undergo their seasonal events, it is more complicated than that,” she says. “For example, if we talk just about plants for a moment, some plants in the spring just respond to warmth, meaning as soon as it starts getting warm, they start waking up and preparing to put on their leaves and their flowers.
When we have an extra warm spring, those plants wake up earlier. “We see the crocuses coming up earlier. We see the lilacs starting to break bud earlier. We see the snowdrops coming up earlier, and that’s because they’re like, ‘Hey, it’s warm. Great!’” Theresa says.
But there are other plants that respond to day length — the minutes of daylight each day — more than they respond to warmth. “Those ones, they aren’t necessarily as responsive to a very early spring because if that extra early warmth is coming before the days are long enough, then they’re not going to respond. They’re going to wait until the days are long enough, and then they’ll get going.”
A third category of plants is dependent on a period of vernalization.
“They must be exposed to a certain amount of chill in the winter before they will start paying attention to the warmth,”Theresa explains. “And if it’s another one of those really early springs where the warmth comes very early and they haven’t yet achieved their sufficient chill, they actually won’t respond to that warmth for a good long time because they’re still waiting for their chill. They’re thinking it’s not yet spring — this is a funky, warm spell in the middle of winter.”
Plants that follow that vernalization strategy are delaying when they put on their leaves or flowers in spring because they are waiting so long for that chill that never comes.
“Some of those plants normally would be undergoing their leaf-out and flowering at the same time as these other guys that advanced way ahead, but now they’re lagging,” Theresa says. “And so things are all discordant in those years.”
Back in 2017, here where I live in Georgia, the peach orchards experienced 80% crop loss because the chill hours hadn’t been sufficient.
Crocuses emerging is a sign of spring.
Photo by Ellen G Denny / USA National Phenology Network
How Climate Change Is Making Pollen Season Longer and More Severe
Another example is pollen season — which is now 20 days longer than it was in 1990, with 20 percent more pollen in the air.
Pollen monitoring stations, mostly privately operated in the United States, sample the air continuously. The pollen is collected by trained professionals then examined, identified and counted under microscopes.
“That’s how we know how much and what kind of pollen is in the air,” Theresa says. “It’s still very, very manual, but we have some stations in the country that have been in operation for many decades. And so we can use those data to construct a picture of how that timing and intensity of the pollen season has changed.”
Ragweed (Ambrosia) is one plant that is spreading its range, growing bigger, producing more pollen, and seemingly more allergenic than ever.
The monitoring stations record pollen that is carried and spread by the wind. The pollen being monitored is not coming from plants that make pretty, sweet smelling flowers to attract pollinating insects, birds and bats.
A warming climate results in a longer pollen season, which is bad news for people with allergies.
Photo by Patty Guertin / USA National Phenology Network
Invasive Species and Phenology
While a graduate student at Penn State University, Erynn Maynard-Bean published a 2020 study on how invasive shrubs in the deciduous forests of eastern North America leaf out earlier and hold on to their leaves for longer than native shrubs and tree canopies. While the invasive shrubs benefit from more photosynthesis, the native flora and fauna suffer from the new understory shade.
Erynn leveraged the power of USA-NPN volunteers to accumulate data for the study.
“There is this prevailing understanding that many invasive plants are successful in part because typically they will initiate growth earlier in the season than natives, and that gives them an advantage because they get early access to nutrients and to light,” Theresa says. “And that especially matters for these understory canopy plants because everybody’s competing to get their growth done before the overstory leafs out and basically shades them out.”
Out of Sync
A consequence of climate change that is heavy on the minds of everyone who works in the field of phenology is what happens when plants and the animals that rely on them become out of sync.
“Mismatches occur when two species that have evolved to interact and be active at the same time are actually driven by different cues,” Theresa said. “And when those cues then change, that can result in their timing no longer being in sync.”
One of the clearest examples of that are long-distance migratory birds and the food sources that they rely upon once they arrive at their breeding ground.
“We have some strong examples of that that have been documented both in Europe and Africa as well as on this side of the globe as well,” Theresa says.
Long-distance migratory birds and butterflies are cued to initiate their migration by sun angle, which is consistent. But the plants that attract the insects those birds meet are influenced by temperature, which is increasingly inconsistent from year to year.
“It’s a function of the earth revolving around the sun, and so on any given calendar day, the sun angle should be the same from one year to the next,” Theresa says. “And so therefore day length would be the same too.”
After a long migration, often over long stretches of ocean, migratory birds are in need of nourishment when they arrive at their destinations. They could really use some caterpillars to eat.
“If they happen to arrive and it’s been a really early spring and everything got going earlier than normal, what can happen is by the time the birds arrive, the caterpillars have already pretty much moved on,” she says.
Bird populations are suffering as a result. They have to keep moving north in search of food, and their breeding schedule is affected negatively.
Acer rubrum, red maple, native to the eastern and central United States. If invasive species leaf out before native plants, the invasives get the first shot at taking up nutrients and also shade out native plants.
Photo by Gina Loyd / USA National Phenology Network
Nature’s Notebook
“The best thing we can do is get our heads in the right spaces first and foremost,” Theresa says. “ … The way that I have found to do that is to get outside and be in the presence of other living things. That helps me recover, especially when I feel like I’m being bombarded with bad news. And then if we do want to go a step further, we can also write down what we are seeing. And that can take any number of forms. It can be just jotting things down in your journal, or a gardening notebook.”
The USA National Phenology Network offers a mobile app, Nature’s Notebook, where citizen scientists can record their observations and contribute to phenology studies.
“How it works is that you make regular visits to individual plants on your property, or document signs of animals on your property,” Theresa says. “I definitely focus on plants more because I’m a plant person. And how that works is that every time that you go out to visit a particular organism that you’re tracking, you’re answering a series of questions that are basically yes or no. And so you’re documenting, do you see breaking leaf buds? Do you see fully expanded leaves? Do you see flowers or flower buds? Do you see open flowers? Do you see fruits?”
That data is fed into a database maintained at the National Phenology Network.
“We now have well over 40 million records, and those data are used very widely by different decision makers and scientists to help put together this puzzle of, how are things changing?” Theresa says. “And again, are different species changing in different ways? Are things changing in different parts of the country at different rates? And all of that is in service to understanding what the consequences of those changes might be, and how we collectively can position ourselves to adjust and try to get out in front of really devastating consequences or conserve and adapt as much as we can.”
You can watch a video recorded at Audubon Star Ranch in California that demonstrates how the app works on the USA-NPN YouTube Channel.
“The intention really is to be able to tell a comprehensive story about what an organism is doing over the entirety of the growing season, and really the whole year,” Theresa says. “… That’s where the real true benefit begins to come to, because it starts to become personal. You start to really establish a relationship with this organism that you’re observing. You really start to appreciate what it’s doing and anticipate what’s coming next.”
She adds: “I keep hearing from observers that not only do they learn so much about the organisms that they’re observing, they learn how to see differently, and that extends to many other parts of their lives.”
The Nature’s Notebook app allows anyone to become a citizen scientist.
Photo Credit: Brian Forbes Powell, USA-NPN
I hope you enjoyed my conversation with Theresa Crimmins on phenology. If you haven’t listened yet, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
Have you noticed phenological events happening earlier or later? Let us know in the comments below.
Links & Resources
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Episode 373: The Land Ethic: Aldo Leopold’s Conservation Philosophy
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“Phenology” by Theresa M. Crimmins on Amazon
“Phenology” by Theresa M. Crimmins on PenguinRandomHouse.com — Use code READMIT20 for 20% off orders with a U.S. mailing address.
USA National Phenology Network
Observing the Outer World to Change Our Inner One | Theresa Crimmins TEDx Talk
“Nature’s Notebook: Help Our Planet in a Changing Climate” |
“Citizen scientists record novel leaf phenology of invasive shrubs in eastern US forests” by Erynn Maynard-Bean, Margot Kaye, Tyler Wagner, Eric P. Burkhart
“Invasive shrubs in Northeast forests grow leaves earlier and keep them longer” By Jeff Mulhollem | Penn State University
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Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you make a purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast was based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Milorganite, Soil3, Territorial Seed Company, Proven Winners ColorChoice, and Dramm. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.
