Phenology

Phenology is the seasonal cycle of nature as well as a field of research that involves elements of ecology and meteorology. Phenologists study and record the timing of actions and growth of living things, such as plants growing and producing fruit, animals giving birth, birds migrating, and creatures going into dormant states at certain times of the year. The timing of phenological events influences and is affected by the lives and cycles of other creatures.

Researchers are examining the impact of global climate change on this cycle because temperature has the greatest influence on seasonal events in many parts of the world. Research shows that spring events are occurring earlier while fall events are happening later. However, changes in the schedule of life events do not occur at the same rate for all creatures. Many organisms’ life events are becoming out of sync with the elements they require. Research into phenological information is also of economic importance. For example, the agriculture industry must know when fruit trees will be in bloom so it can ensure bees are available to pollinate them and when leaf and flower buds will be at the right stage for spraying insecticides. In the medical field, flowering times of certain plants affect individuals with allergies, asthma, and other respiratory conditions.

Background

Early research and interest in phenology developed in the United Kingdom (UK) and is credited to Robert Marsham (1708–1797), owner of a sizeable property in Norfolk. He planted a forest of trees on his land and was keenly interested in wildlife and agriculture. In 1736 he began recording what he regarded as the signs of spring including the timing of trees leafing out and the return of migratory birds on the Marsham estate. He also recorded information about weather, temperature, and the growth of crops.

Marsham’s “Indications of Spring” records contain sixty-two years of data on twenty-seven natural events involving more than twenty animals and plants. He presented his findings to the Royal Society of London, the UK’s national academy of sciences, in 1789. His records were published in the Royal Society Journal, which prompted interest among researchers. After his death, many generations of his family continued to add to the phenological record on the estate, providing near-continuous data until 1958.

Phenology as an organized area of study developed during the nineteenth century. In 1875 the Royal Meteorological Society of London began collecting information about multiple species of birds, insects, and plants. This data was published annually until 1947.

Amateur phenologist Jean Combes began recording oak budburst dates in 1947 when she was twenty years old. Over the decades, her localized UK data drew the attention of researchers and government agencies and provided valuable data. Her records clearly indicate the gradual change of the timing of the opening of oak leaf buds. In 1950 budburst occurred on average in early May. By the late 2010s, the average date of this natural event was in early April. As Combes’s records became widely known, her work and interest in Marsham’s records prompted many other amateur phenologists to document natural events. A UK charity, the Woodland Trust, established Nature’s Calendar, a website that asks the public to share phenological data.

Phenology in the United States is traced to a book by Jacob Bigelow, a professor of botany at Harvard University. Facts Serving to Show the Comparative Forwardness of the Spring Season in Different Parts of the United States was published in 1817. During the 1850s, naturalist Henry David Thoreau recorded the flowering times of plant species near Walden Pond in Massachusetts. Their data is used in the twenty-first century to understand climate change and how species have responded. For example, Thoreau’s records prove plant flowering dates in Concord in modern times are up to three weeks earlier.

Overview

Phenology research has many uses. It can help predict events that can affect human health, such as pollen release and mosquito season. The agriculture industry can determine the best times to plant and fertilize crops and better predict harvest time and the need for more workers. Phenology can also help researchers understand how global climate change affects species and habitats.

Phenology compares events in one location from year to year, noting the sequence and timing of cyclical phenomena. Researchers also compare the timing of events in different places in the same year. Trends emerge when data over long periods can be compared; researchers can then use this information to make predictions of future change.

Although the length of day is one of the most obvious seasonal changes in all regions of Earth, it is not highly significant in affecting the timing of natural events. Day length variation is constant—it changes by location and time of year but is consistent in every place on the same date year after year. Other factors do change and affect the natural cycle. In temperate climates, the most significant weather influence on recurrent events is temperature. Other weather factors such as humidity or precipitation may be greater influences in tropical regions.

In temperate regions, spring is the season when the most resources are available for wildlife. In the tropics, resources are abundant during the rainy season. Plant and animal life cycles typically are in harmony with these periods. A common example is bird nesting and breeding, which often occurs in spring and for some species may repeat during the summer. The timing of breeding correlates to the period when food for chicks, such as caterpillars, is most abundant. This increases the chances of the survival of young and is least taxing on adults, which must expend energy on finding and delivering food to chicks. Phenological asynchrony can have a strong effect on the adults; when food is scarce, they must devote more energy to finding it. They may also have fewer and weaker offspring. Some birds, such as the American kestrel, are able to adapt their breeding schedule to changes in resource availability. If food is abundant and reliable, they can delay incubating until all eggs in the clutch are laid. All the chicks will then hatch at about the same time when resources are plentiful and more easily acquired. When resources are scarce or unreliable, the adults can begin incubating before the female has laid all the eggs. This allows staggered hatching of chicks, which reduces the drain on the parents because the chicks’ needs are at different stages. Researchers are trying to determine the long-term effects of this behavior, such as survival rates of the parents as a result of mismatched breeding.

The importance of phenology is apparent in the plant-pollinator relationship. Plants and insects respond differently to temperature shifts. In some cases, one or the other cannot adapt, and the species distribution changes so plant and insect no longer interact. In other cases, both adapt differently so their life cycles become mismatched. For example, a plant may bloom earlier, but the butterfly that pollinates it emerges later. The plant species may be affected because fewer flowers are pollinated, while the insect species could emerge when its food is not available. Researchers have looked at specific crops and their pollinators and predict that continued warming could have a dramatic impact on food supplies. For example, fig blossoms are pollinated by a specific species of wasp that has a lifespan of about two days. A tiny increase in the average temperature—about 5 degrees Fahrenheit (3 degrees Celsius)—in the equatorial tropics could drastically reduce the fig wasp population and have a significant effect on fig crops.

Studies have found that human health is impacted by changes in phenology. In Europe, cases of allergic conjunctivitis, allergic rhinitis, asthma, and eczema increased in the second half of the twentieth century. Research shows that the average length of the growing season increased by about ten days from the 1970s through the end of the century and pollen season began and peaked earlier. Asthma, allergies, and other respiratory issues also have a greater effect on many people in North America, where the pollen seasons have also become longer and pollen concentrations have become greater.

Some research suggests that the phenological shifts of some species can be overcome through a theory called biodiversity insurance. This involves preserving biodiversity so that other species may emerge as substitutes in relationships of phenological synchrony. Research suggests that although certain crops may be negatively affected by climate change, geographic shifts of species may prove beneficial to other crops that will have a greater number of potential pollinators.

Bibliography

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“The Impacts of Climate Change on Phenology: A Synthesis and Path Forward for Adaptive Management in the Pacific Northwest.” US Geological Survey, www.usgs.gov/centers/casc-sc/science/impacts-climate-change-phenology-a-synthesis-and-path-forward-adaptive?qt-science‗center‗objects=0#qt-science‗center‗objects. Accessed 18 June 2021.

“Important Phenologists.” Nature’s Calendar Woodland Trust, naturescalendar.woodlandtrust.org.uk/what-we-record-and-why/why-we-record/important-phenologists/#:~:text=Robert%20Marsham&text=Marsham%20continued%20to%20note%20down,with%20his%20death%20in%201797. Accessed 18 June 2021.

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