About Phenology

What is Phenology?

Phenology is the study of the timing of the biological events in plants and animals such as flowering, leafing, hibernation, reproduction, and migration. Scientists who study phenology are interested in the timing of such biological events in relation to changes in season and climate.

How does a bear know when it's time to hibernate? Why do April showers bring May flowers? Plants and animals don't have calendars or watches, but many of them take cues from the changing seasons. Changes in weather with the seasons, such as temperature and precipitation, signal many organisms to enter new phases of their lives. For example, buds form on plants as temperatures warm in the spring. As temperatures cool in the fall, deciduous trees and shrubs lose their leaves and become dormant. The study of the timing of these changes is called phenology.

Think about the changes where you live that tell you spring is almost here.

Phenology is literally "the science of appearance." The word phenology comes from the Greek words phaino (to show or appear) and logos (to study). Scientists who study phenology – phenologists -- are interested in the timing of specific biological events (such as flowering, migration, and reproduction) in relation to changes in season and climate. Seasonal and climatic changes are some of the non-living or abiotic components of the environment that impact the living or biotic components. Seasonal changes can include variations in day length, temperature, and rain or snowfall. In short, phenologists attempt to learn more about the abiotic factors to which plants and animals respond.

Examples of springtime phenological events that interest scientists include flowering, leaf unfolding, insect emergence, and bird, fish, and mammal migration. Think about the changes where you live that tell you spring is almost here. In the Washington, D.C. area, cherry blossoms are a sure sign that spring is on its way. In many parts of the country, people look forward to hearing the songs of the first robins of the season. California poppies are an indicator of spring to many along the Pacific shores. In the Midwest, the greening up of fields and pastures is a signal that winter is almost over.

Visitors to the National Aquarium observe plants with Budburst
Photo courtesy of the National Aquarium

Because spring temperatures are slower to warm at higher latitudes than they are at lower latitudes, the timing of life-cycle changes in living things differs from place to place. For example, a cherry tree might bloom in Atlanta two months before a similar tree blooms in Chicago because temperatures warm earlier in the year in Atlanta than they do in Chicago.

The arrival of spring gets a lot of attention in terms of phenological events, with flowers emerging from their winter slumber. However, equally important phenological events happen throughout the year. In the case of Budburst, don't let our name fool you: we want to know when you first notice the signs of all seasonal change in plants, like when the leaves change color in the fall and when summer wildflowers wither and finish their life cycle. You can tell us about these changes by making observations and sharing them with us through our website. By joining Budburst, you will be part of a community of thousands of people across the country that are furthering the understanding of plant phenology.

Plant Phenology and Climate Change

The timing of when a place warms up in the spring and when it cools down in the fall depends on Earth's climate. Regional differences in climate cause warm weather to arrive later in the spring at higher latitudes than at lower latitudes. But today global change in climate is affecting the timing of warming temperatures in the spring and cooling temperatures in the fall worldwide.

As Earth's temperature rises, it becomes warmer earlier in the spring and stays warmer later into the fall at any given location. The opposite is true as well: if Earth's temperature were to cool, warm weather would show up later in the spring and cool weather would arrive earlier in the fall. Today, the global climate is warming. According to NASA’s Goddard Institute for Space Studies (GISS), the average global temperature on Earth has increased by about 0.8° Celsius (1.4° Fahrenheit) since 1880.  This modest number averages all the seasonal variations—the cold winters and hot summers—as well as all the differences in latitude—the cold poles and warm tropics— which can make it difficult to imagine the impacts in any given location.

Image courtesy of the Intergovernmental Panel on Climate Change.
Fifth Assessment Report, Topic 1, page 41.

We know that the amount of warming is not the same around the world. The Arctic, for example, is warming more quickly than other areas of the planet. Understanding the impacts that this worldwide phenomenon has on specific places is an area of active research. Budburst is one of many studies investigating the impacts of global change in specific places. As the climate warms in the twenty-first century, places around the world will continue to be affected by changing conditions. Current models by the Intergovernmental Panel on Climate Change estimate average surface temperatures could rise between 2°C and 6°C by the end of the 21st century. (©2007 IPCC WG1 AR-4.)

When you report to Budburst about how and when the plants in your garden, park, town, or city are changing with the seasons, you are contributing scientific data that can help us understand how plants are responding to this year's seasons and long-term changes in climate. Scientists are using data about the timing of seasonal changes in species in computer models to predict how climate and ecosystems will change decades and even centuries into the future.

Changes in the timing of phases of the plant life cycle, known as phenophases, are directly affected by temperature, rainfall, and day length. While these factors change through the year in places where there are distinct seasons, the first two – temperature and rainfall – are also changing in many regions because of climate change. For example, if climate change causes warmer temperatures, warm weather may occur earlier in the spring and it may stay warm later into the fall than in years past. It will still get cold in the winter and warm in the summer, but the plant growing season will be longer, and that can have big impacts on living things.

When you report to Budburst about how and when the plants in your garden, park, town, or city are changing with the seasons, you are contributing scientific data that can help us understand how plants are responding to this year’s seasons and long-term changes in climate.

That's where plants come in. By monitoring plants and noting when the first buds appear, when the first flowers appear, when leaves drop in the fall, and other parts of plant life cycles, scientists can figure out how seasonal patterns are changing, and make predictions for the future.

Why is Phenology Important?

Phenological observations have been used for centuries by farmers to maximize crop production, by nature-lovers to anticipate optimal wildflower viewing conditions, and by many of us to prepare for seasonal allergies.

Cherry blossoms in Washington, D.C.
Photo courtesy of the United States Department of Agriculture (Wikipedia).

From a cultural viewpoint, we time festivals and events around specific phenological events. For example, the National Cherry Blossom Festival happens during the first two weeks in April in Washington, D.C., whether or not there are cherry blossoms. The festival traditionally ends with a parade through blooming trees, but over the past few decades the cherry trees have been blooming earlier and now the parade happens after the peak bloom. The flowers bloom in response to warming temperatures, so if climate change is causing it to be warmer earlier in the year, the flowers will bloom earlier as well.

Changes in phenological events can have a significant impact on how we humans live our lives and interact with our environment on a daily basis.

Having a parade for cherry blossoms while the blooms are fading is bad timing, but it is perhaps not quite as dire as some cases of bad timing that affect entire ecosystems. For example, in most ecosystems, there are insects and plants that need each other. Hungry insects searching for nectar from flowers inadvertently transport pollen from flower to flower. The pollen grains hitch a ride, often by sticking to an insect's legs. By distributing pollen, the insects, called pollinators, are fertilizing the flowers, allowing the plant to grow seeds and fruit.

Monarch caterpillar feeding on common milkweed.
Photo courtesy of H. Jean Bryan.

But it takes time for insects to develop from egg to larva to adult, and the timing of their growth can't be sped up just because the flowers are blooming earlier. As the climate warms, plants may become out of sync with the insects that pollinate them. If an insect is still a larva when the flowers blossom, for example, it will not be able to fly from flower to flower to transport pollen. Without pollination, the flowers will not be fertilized and will not produce fruit.

Mammals in the ecosystem can be affected, too. Consider mice: some mice eat insects and seeds. If plants bloom too early for insects to pollinate them, then the seeds won't grow. And if the insects are too late to gather food from the flowers, they will not survive, either. Without seeds or insects to eat, the mice may not survive. And animals that eat mice, like snakes and hawks, will also go hungry.

How plants react to seasonal change has a big impact on the natural environment.

Because plants are at the base of the food chain, anything that affects plants can impact other parts of the ecosystem. Phenology is important because it affects whether plants and animals thrive or survive in their environments. It is important because our food supply depends on the timing of phenological events. And, to scientists, changes in the timing of phenological events can be used an as indicator of changing climates.

Changes in phenological events can also have a significant impact on how we humans live our lives and interact with our environment on a daily basis. For example, the timing of when plants flower and fruit can affect our food supply and therefore our health. Pollen allergies can also be exacerbated by changes in growing conditions. People who are allergic to plant pollen will experience reactions to the changes in flowering times and the lengthening of the growing season.

From historical records and observations, we know that phenological events can vary from year to year. Ecosystems can recover from variation between years, but when these changes happen consistently over many years, the timing of events such as flowering, leafing, insect emergence, and allergies can impact how plants, animals, and humans are able to thrive in their environments.

History of Phenology

Phenology is one of the oldest branches of environmental science, dating back thousands of years. Observations of phenological events have provided indications of the progress of the natural calendar—when seasons begin and change—since pre-agricultural times.

Photo courtesy of the Aldo Leopold Foundation (www.aldoleopold.org).

Many cultures have traditional proverbs and sayings that attempt to forecast future weather and climate using phenological observations: “If oak’s before ash, you’re in for a splash. If ash before oak, you’re in for a soak.” But the indications can be pretty unreliable, as an alternative version of the rhyme shows: “If the oak is out before the ash, ‘twill be a summer of wet and splash; if the ash is out before the oak, ’twill be a summer of fire and smoke.” While phenological observations may not let you predict the weather from one season to the next, they can be used to identify climate trends over decades and centuries.

The Chinese are thought to have kept the first written records of phenological observations dating back to around 974 B.C.E. And for the past 1,200 years, the Japanese have recorded observations of the timing of peak cherry blossoms.

By participating in Budburst, you are contributing to this long-established history of phenologists. You also join a legion of citizen scientists across the world and through the ages who are helping to understand changes in plants over time.

In Europe, the Swedish botanist Carolus Linnaeus (1707–78) systematically recorded flowering times for 18 locations in Sweden over many years. His meticulous notes also recorded the exact climatic conditions when flowering occurred. Linnaeus and a British landowner, Robert Marsham, share the honor of being considered the “fathers” of modern plant phenology.

Photograph of Henry David Thoreau. Image courtesy of Wikipedia Commons.

Marsham could be considered one of the first citizen scientists in modern times. He was a wealthy landowner who kept systematic records of “indications of spring” on his estate in England. His observations were in the form of dates of the first occurrence of events such as flowering, bud burst, and emergence or flight of an insect. For generations, Marsham’s family maintained records of phenological events over exceptionally long periods of time, eventually ending with the death of Mary Marsham in 1958. The records of the Marsham family showed trends that were observed and related to long-term climate records.

Phenology is one of the oldest areas of environmental science, dating back thousands of years.

Phenological observations have provided indications of the progress of the natural calendar—when seasons begin and change—since pre-agricultural times. The Chinese are thought to have kept the first written records dating back to around 974 B.C.E. And for the past 1,200 years, observations of the timing of peak cherry blossoms in Japan have been recorded. Read more about this history and how you can be part of it through Budburst.

Aldo Leopold is another prominent figure in early plant phenology and is considered to be a founder of the wildlife management field. In 1949, he penned his best-selling book, A Sand County Almanac, a series of essays about wildlife, conservation, land ethics, and phenology taken from his experiences living and working throughout the United States. Leopold felt strongly that record keeping was important to understanding the ecosystems, plants, and animals he encountered. He wrote, “Keeping records enhances the pleasure of the search and the chance of finding order and meaning in these events.” After Aldo died, his daughter, Nina, picked up where her father left off and began keeping phenological records once again. In 1999, Nina and others published a paper in the Proceedings of the National Academy of Sciences titled “Phenological Changes Reflect Climate Change in Wisconsin,” based on the phenological observations she and her father had collected all those years.

The detailed journals of naturalist and writer Henry David Thoreau provide a compelling example of the great contributions that volunteers can make to science. They also provide a unique link between current Budburst data and historic observations, which in turn can be used to make important scientific discoveries. Thoreau kept a daily journal of natural history observations from 1851 to 1858. This journal included first flowering date observations for close to 500 plant species around Walden Pond. Several naturalists continued to make observations in the same general area over several other time periods up until 1993.

“Keeping records enhances the pleasure of the search and the chance of finding order and meaning in these events.” —Aldo Leopold, A Sand County Almanac

In 2003 phenology scientists Richard Primack, Abraham Miller-Rushing, and their collaborators started collecting the same kind of data that was collected in the past, primarily dates of first flowers and dates of when trees and shrubs leaf out (equivalent to the Budburst first leaf phenophase). Of particular interest, these studies show that plant species vary widely in their ability to change the dates of their phenophase events as weather and climatic conditions change. The researchers found that plants in some families have not changed the dates of phenology as much as others, and that these plants tend to be less common now than they were during Thoreau’s time. This suggests that with Budburst data it will be important to see which species are changing their phenology most quickly, and to identify those that are flowering or leafing out on the same dates, regardless of changes in weather or climate.

Much could be learned by doing this kind of analysis with Budburst data, since it covers the entire country (not just Walden Pond or Wisconsin) and also includes a broader range of phenophases than what was originally recorded by phenologists of the past. This will allow scientists to identify how different regions of the country and different species are responding to climate change, and also to determine which are the most important species to watch.

Phenology Today

Today, the well-established science of phenology is seeing a resurgence of interest as researchers seek observations of changes in plant phenology to help them better understand changes in our environment. The Lilac Network (1956-1986) an excellent example of a plant phenology effort dependent on the observations of large numbers of volunteer observers. The Lilac Network made significant contributions to the science of phenology.

The success of The Lilac Network in engaging citizen scientists formed the basis of today's USA National Phenology Network, established in 2004 to monitor the influence of climate on the phenology of plants, animals, and landscapes. This national network and its growing database, Nature’s Notebook, has great potential for promoting scientific interest in plant phenology in the U.S., as well as providing data and models that help scientists monitor and predict drought, wildfire risk, biological invasions, and the spread of diseases more accurately from region to region than is possible now. Budburst is a partner in the USA–NPN.

The urgency of environmental and climate change considerations is generating more interest in plant phenology.

While not exactly a household world, phenology is a rapidly growing field of science that lends itself to engaging citizen scientists in making simple observations of changes in their environment. Be part of the growing of interest in plant phenology in the United States by joining Budburst.

In Europe, the European Phenology Network has active monitoring, research, and educational programs. Nature's Calendar in both the Netherlands and the United Kingdom has active websites and events. Canada has PlantWatch, India has SeasonWatch, and many other countries, including China and Australia, also have phenological programs.

Publications in Phenology and Budburst

View the Budburst publications page to find links to journal articles and other publications that feature Budburst and/or make use of Budburst data.

People Behind Phenology

The dedication of many men and women has shaped the rich study of phenology. In this section, you can learn more about the scientists who have dedicated their careers to plant phenology. Some of the people you will be reading about are the very ones who may be using the data you contribute through Budburst!

Jennifer Ison

Jennifer Ison

Jennifer Ison, Ph.D., collaborates with Stuart Wagenius, Ph.D., of the Chicago Botanic Garden to monitor the daily flowering phenology of purple coneflower. She's learning more about how flowering time affects pollen movement between plants. Read Jennifer's story.

Julio Betancourt

Julio Betancourt

Julio Betancourt, Ph.D., was 10 years old when he and his family fled Cuba and Fidel Castro for the United States. Their first stop was the nation's capital, where his most vivid memory is of raking autumn leaves in an uncle's backyard. Read Julio's story.

Caitlin McDonough McKenzie

Caitlin McDonough McKenzie

Even before she knew how to define phenology, Caitlin McDonough MacKenzie was monitoring it. Her research aims to track the ecological effects of climate change on plant communities through time. Read Caitlin's story.

David Inouye

David Inouye

David Inouye, Ph.D., has spent more than 37 summers making careful observations of over 100 wildflower species as part of an ongoing scientific study at the Rocky Mountain Biological Laboratory (RMBL) in Gothic, Colorado. Read David's story.

Kay Havens

Kay Havens

Kayri Havens, Ph.D., was born and raised in the Chicago region, and she has always eagerly anticipated the first signs of spring. It was a happy coincidence that she was able to collaborate on the development of Budburst because it brought together many of her interests…the effect of climate on plants, citizen science, and her love of spring! Read Kay's story.

Mark Schwartz

Mark Schwartz

During a fateful day in 1983, Mark Schwartz, (now) Ph.D., was discussing possible topics of research when his advisor mentioned that phenology might be an interesting area that offered many future options. "What's that?" he recalls asking, little realizing at the time that this was to become his life's work. Read Mark's story.