What is phenology?
Phenology is the study of the timing of natural events: first flower, leaves opening or falling, fruit ripening, birds arriving or leaving, insects hatching. These events follow the seasons, so their dates tell us how the climate is behaving.
Records can be old: Japanese records of cherry-blossom dates in Kyoto go back over 1000 years.
What decides the date of flowering?
Plants use clues from the environment:
- Temperature: warm days in spring speed up the opening of buds.
- Day length: the number of daylight hours tells a plant what time of year it is.
- Winter chill: many trees need a cold spell first, like an alarm clock being wound up.
Because temperature is one of these clues, a warming climate shifts the dates.
Data: record, graph, trend
To see a change you need many years of data. Choose a rule (for example "the first day when 3 flowers are open"), record the day of the year every year, and plot year on the x-axis and day on the y-axis. Draw a straight line that follows the dots (a trend line). Its slope tells you how many days per year or per decade the date is moving. A negative slope means flowering is getting earlier.
Day of the year: 1 January is day 1; 10 April is day 100 in an ordinary year.
When timing gets out of step
Not every species shifts by the same amount. If a plant flowers earlier because of warmth but its pollinator (bee, butterfly, bird) comes out on a different clue such as day length, they may miss each other. This is called a mismatch. Fewer flowers get pollinated and the animals find less food.
Typical shifts in many northern places are 2 to 5 days earlier per decade, though it varies a lot with the plant and the place.
Try it: your own phenology calendar
Choose one tree or plant near your home. Every week note whether it has buds, flowers, fruit or no leaves. Write the date of the first flower. Ask a grandparent when it used to flower. Next year compare. Then use the 3D slider to see how a warmer spring moves the bloom date.
Key formulas and definitions
- Day of the year: Jan = 0 days before; Feb starts at 31; Mar at 59; Apr at 90; May at 120 (ordinary year)
- Trend (days per decade) = (day in later year − day in earlier year) ÷ years × 10
- Mismatch gap = date pollinator is active − date flowers open
- Negative trend = earlier flowering
Worked examples
1. First flower opens on 10 April in an ordinary year. What is the day of the year?
January 31 + February 28 + March 31 = 90 days. 90 + 10 = day 100.
2. First-flower dates: year 2000 = day 100, year 2020 = day 90. How many days earlier per decade?
Change = 90 − 100 = −10 days over 20 years. Per decade = −10 ÷ 2 = −5 days. So 5 days earlier per decade.
3. A plant flowers 5 days earlier for each 1 °C of spring warming. Spring warms by 2 °C. The flower date was day 100. What is the new date?
5 × 2 = 10 days earlier. 100 − 10 = day 90 (1 April).
4. The bee comes out on day 100. The flower now opens on day 90. How long is the gap, and who has to wait?
Gap = 100 − 90 = 10 days. The flowers open 10 days before the bees are out, so the first flowers may not be pollinated.
Common mistakes
- Judging a trend from only 2 or 3 years. Weather varies each year, so you need many years of data.
- Thinking all species shift by the same number of days. They respond to different cues.
- Saying an earlier flower date is always good for the plant. It can miss its pollinators or get hit by late frost.
- Mixing up the axes. Put the year on the x-axis and the day of the year on the y-axis.