Daily and Seasonal Rhythms

Every fall, many monarch butterflies fly south through Virginia toward forests in the mountains of central Mexico. Every spring, the cherry trees around the Tidal Basin in Washington, D.C., just across the Potomac River from Virginia, burst into bloom.

People have written down the date of peak bloom for these trees every year since 1921. A butterfly has no calendar, and a tree has no eyes. So how do they know when to leave, or when to bloom? And can that timing change?

Explore: turn the year dial

The simulation shows a year dial with a cherry tree, a groundhog and a monarch. As you slide the date through the year, day length and average temperature change. A second slider makes spring up to 3 °C warmer.

Start here: drag the date slider from January to December and watch the three living things.

Date Jan 1 Warmer spring +0 °C

Go to bloom day Reset

bloom day fall cue day today. Ring color: blue is colder, orange is warmer.

Bloom day in this sketch

93

With no extra warmth

93

Change

0 days

Dormant buds

Awake

In the forest in Mexico

In this sketch, drag the date slider to start the year.

Assumptions made up for this sketch: one warmth line starts bloom, and with no extra warmth it falls on day 93 (about April 3, near the lesson’s decade averages). The same warmth line starts the groundhog’s warm-up and the monarch’s trip north. The size of the shift is not matched to the Tidal Basin record. One day-length line is the only fall cue. Every year is the same, with no weather swings. The delays, the spans of days and the short groundhog warm-ups every 20 days are invented. Bars are relative units, not hours or degrees.

Rule to keep: cues such as day length and temperature start these seasonal responses, and a tree whose buds respond to warmth may bloom earlier in a warmer spring.

Try this.

1. Predict the month when the monarch leaves. Then slide the date and see what changed just before.
2. Predict when the groundhog’s body temperature bar will fall and when it will rise. Then test it.
3. Predict whether a warmer spring moves the bloom day earlier or later. Set the warmer spring slider to 3 °C and compare the bloom day number. In this sketch the slider moves only the cherry tree.

Explain: daily cycles and growing toward light and water

Think about your own day. You get sleepy at night and hungry around mealtimes. Earth spins once about every 24 hours, so most places have day and night.

A circadian rhythm is a pattern in a living thing that repeats about every 24 hours. Light and darkness have the biggest influence in keeping these rhythms in step with the day. In many animals, the rhythm keeps going for a while even in steady darkness. That shows part of the timing comes from inside the body.

Different animals are active at different times. Many owls and many bats hunt at night and rest in the day. Tree squirrels and most songbirds do the opposite.

Plants respond too, but they do it by growing. A tropism is growth of a plant toward or away from something in its surroundings.

• Toward light. Cells on the shaded side of a stem grow longer than cells on the lit side. The longer side pushes the stem into a bend toward the light.
• Toward water. In lab tests, young roots bent toward the wetter side of their dish.

The bend is made of new growth, so it builds up slowly, from about half an hour to days.

Explain: getting through the seasons

In a Virginia winter, days are short, nights are cold, and many foods are hard to find.

• Migration is the seasonal movement of animals from one region to another and back. Monarchs fly south in fall. In spring, monarchs move north again over several generations.
• Dormancy is a resting state in which a living thing slows or stops growing but stays alive until conditions improve. Seeds and tree buds wait out winter this way.
• Hibernation is a kind of dormancy in animals. The body temperature drops and the heart and breathing slow, so the animal uses less of its stored fat. Groundhogs hibernate. It is not one long sleep: groundhogs and their relatives the ground squirrels warm up for short times, then cool down again.

Virginia’s deciduous trees, such as oaks and maples, drop their leaves each fall. As days get shorter, a layer of cells forms where each leaf stalk joins the twig. This layer weakens the joint, and the leaf falls. Without leaves, the tree loses less water in winter, when frozen soil makes water hard to get.

Season	Common cues (they differ by species)	Some responses
Fall	Shorter days, cooler air	Leaves drop, buds go dormant, monarchs fly south, groundhogs hibernate
Winter	A long time of cold	Dormant buds become ready to open
Spring	Warmer days	Buds open, seeds sprout, groundhogs come out

Buds of many trees need cold before warmth. In a 2018 experiment, young aspen trees got 10 weeks of short, 8-hour days, and their buds went dormant. Then the trees spent 5 weeks at 4 °C. Back in warm, long days, the buds swelled and green leaves came out.

Are these behaviors learned? Mostly not. An instinct is a behavior an animal inherits and can do without being taught. A monarch flying south has never made the trip before. In a lab study of ground squirrels, most were young animals hibernating for the first time, and genes explained much of when they started. Some animals do learn: young whooping cranes flew straighter routes when older cranes were in the group.

Explain: when the long-term pattern shifts

Some changes last for years. They can be natural, such as wildfires, floods and climate shifts. They can be caused by people, such as pollution, habitat destruction and overharvesting. Long-term changes can change how many organisms live in a place, where each species lives, and how a whole ecosystem fits together. As the climate has warmed, many land species now live farther toward the poles or higher up mountains than before. You will study these changes in Populations Rise and Fall and Big Changes (a later lesson).

Timing can shift too. If winters and early springs warm, a tree whose buds respond to warmth may bloom earlier.

Elaborate: a century of cherry blossoms

Peak bloom is the day when about 70 percent of the Yoshino cherry blossoms around the Tidal Basin are open. Dates are given as a day of the year: January 1 is day 1, and day 91 is April 1 in a year that is not a leap year.

Years	Number of years	Mean peak-bloom day (day of year)	About the same as
1921–1929	9	92.7	April 3
1930–1939	10	96.8	April 7
1940–1949	10	93.4	April 3
1950–1959	10	96.8	April 7
1960–1969	10	97.4	April 7
1970–1979	10	95.5	April 6
1980–1989	10	93.3	April 3
1990–1999	10	90.9	April 1
2000–2009	10	90.7	April 1
2010–2016	7	91.7	April 2

Source: peak-bloom dates for the Yoshino cherries at the Tidal Basin, Washington, D.C., 1921 to 2016, from the National Park Service and the National Cherry Blossom Festival, as published by the U.S. Environmental Protection Agency in 2016. Each mean was calculated from the yearly dates in that file.

1. Which group of years had the latest mean bloom day? Which had the earliest?
2. How many days earlier was the 2000–2009 mean than the 1930–1939 mean?
3. Did the mean get earlier in every decade? Compare 1940–1949 with 1950–1959.

Design problem

Does the direction of light change which way a bean seedling grows? Plan an investigation.

One way to do it

• Question: Do bean seedlings bend toward light that comes from one side?
• Independent variable: the direction of light. Some boxes have a hole on one side. Others have the hole on top.
• Dependent variable: the angle of each stem from straight up, in degrees, measured with a protractor after 5 days.
• Constants: same kind of bean, soil, pot, daily water, room and amount of light.
• Control: seedlings in boxes lit from the top. Their stems should grow straight up.
• Repeated trials: five seedlings for each kind of box. Find the mean angle.
• Safety: use a window or a cool LED lamp, not a heat lamp. Wash your hands after handling soil, and do not eat the seeds.

If stems lit from the side bend toward the hole and the control stems grow straight, the data support the idea that shoots grow toward light.

Evaluate: check your thinking

Monarchs and cherry trees respond to cues such as day length and temperature, and many of their responses are inherited. Because a cherry bud opens after it warms, a long-term change in climate can move the bloom day, and the Tidal Basin record shows the mean moving earlier over the decades.

Science practices you used: testing predictions in a model, reading a data table, comparing means, naming the limits of data, and planning a fair test with a control and repeated trials.