We're hungry for reports for our Phenology Talkbacks segment. Please don’t hesitate to reach out with your observations, nature tales and insights! Get in touch with me (smitchell@kaxe.org), John Latimer (jlatimer@kaxe.org), or text "phenology" to 218-326-1234.
Right now, there are enough “undeniable, inescapable, and sobering” signs of fall that John Latimer grudgingly admits summer is beginning to wane.
The changing of leaves has particularly caught his attention, recalling this essay he wrote in 2016:
Why do leaves fall?
“We all know that fall marks the end of the road for leaves on the deciduous trees. In some cases, trees go out in a blaze of glory. Others may not change a thing, but simply drop from the tree as green as the day they emerged. Some change a leaf at a time, and some go from green to red faster than a stoplight. What accounts for the change and why do they drop their leaves at all?
“Trees make a significant investment in leaves. It takes a lot of energy to make a leaf. If the tree fails to make a positive return on its leaves, then the tree will die. In effect, each leaf needs to produce more carbon than it took to make it. So, why do they drop their leaves?
“In our cold climate one reason is that the winds and freezing temperatures would raise hell with the delicate machinery inside the leaf. Perhaps more importantly, the tree needs to balance the carbon received from the leaf against the cost of maintaining the leaf.
“The leaf starts in carbon debt. It takes more carbon to make the leaf than it will produce immediately. Over time, the leaf will repay that debt and begin to record a profit, to stay with the economic nature of our conjecture. Age will begin to limit the amount of carbon the leaf can sequester. At some point, the amount of carbon produced will start to approach stasis with regard to the maintenance costs. At this point the tree will begin a process called senescence.
How do trees shed their leaves?
“Senescence is the word given to the overall process of leaf fall including the color changes. Abscission is the term that refers to the actual separation of the leaf from the tree. It begins with a layer of cells developing at the junction of the leaf petiole and the stem. On the stem side, the cells will be corky and will seal the stem against entry of foreign bodies. The resulting scar is singular to the type of tree. So in the winter you can identify the tree by the nature of the leaf scar. On the leaf side the cells become less strongly attached to one another until the leaf falls.
“Day length and cooler temperatures initiate senescence, but the actual genetic processes that cause the cells to begin to grow are not yet known. What is known is that once the process begins, the tree resorbs much of the nitrogen and phosphorous from the leaf back into the twig. In the spring, this cache of chemicals aids in the production of new leaves. This allows the tree an opportunity to “jump the gun” so to speak. A few weeks before the soils are thawed and the roots can begin to transport nutrients from the soil to the branches, the tree can start putting out new leaves.
Why are fall leaves colorful?
“Once the tree begins resorption, the green photosynthetic parts of the leaf start to fade. As their numbers dwindle the xanthophylls and carotenoids begin to show through. These yellow- and orange-producing chemicals have been in the leaf all summer. They were simply overwhelmed by the greens of the chloroplasts.
“What about the reds, the anthocyanins? These brilliant reds are produced by the leaf in the fall. Sun striking the leaves causes the sugars in the leaf sap to convert to anthocyanins once the phosphorous has been removed. Bright sunny days and cool clear nights can enhance the production of these red chemicals.
“The brightest fall colors come at the end of a good growing season. Good soil moisture throughout the summer will create lots of sugars that can be converted to anthocyanins in the fall. Cool, clear skies and dry weather will ensure the best colors. Rain tends to wash out the colors as the leaf cell walls degrade. Frost can further damage cell walls and hasten the separation of the leaf from the twig.”
Once the leaves fall, they are still not done contributing to the health of the tree. The tree has reabsorbed some, but not all of the chemicals it invested in the leaf. Once the leaf hits the ground an army of microbes attack and quickly break down the last of the carbon bonds within the leaf. This action frees up the last of the chemicals and allows some of them to be re-captured by the tree’s root system: These will be stored and put back into new leaves the following year.
Plant progression
- Turning color: Woodbine, maple, ash, pin cherries, pagoda dogwoods, round-leaf dogwood and nannyberry.
- Flowering: Red clover, alfalfa, birdsfoot trefoil, tall sunflowers, smooth aster, Northern heart-leafed aster, flat-topped aster, and many-flowered aster.
Not yet flowering: New England aster.
Insect inspection
Ants, dragonflies and mayflies are also preparing for fall. John said the ants responsible for the “tiny sand volcanoes” in your yard will be opening up their anthills to allow the winged ants to exit. Two types of ants have wings: queen females and males.
Meadowhawk dragonflies are out: males are generally red-bodied, while females tend toward the yellow side of the spectrum. These meadowhawks will be the dominant type of dragonfly throughout the fall: John has seen them as late as November.
Some green darners, a larger species of dragonfly, are getting ready to migrate. Part of the population migrates south for the winter, while another portion overwinters as adults. They can migrate as far south as Panama!
While most people are unlikely to bother identifying a mayfly species, John can attest that there are many species of mayfly native to Minnesota. Some of these species emerge in late summer: keep an eye out for a fresh swarm of mayflies as they emerge. For a few days, they’ll carpet every surface near the water as they mate and lay eggs. Shortly after, they’ll die out. Each adult mayfly only lives a day or two, so the cycle is very short!
Yellow jackets are leaving their nests, their summer work completed. While next year’s queens go out, mate and settle in for hibernation, the rest of the hive disperses and survives while they can, feeding on nectar and sugar until the cold sets in.
A hive of paper wasps gave John a scare: he was looking at a black cherry tree, looked down, and saw a large paper wasp nest bumping against his calf.
“For all of those of you who have wondered if it’s possible for a man my age to run, the answer is yes, it is,” he declared. “It all depends on the incentive.” A hive of stinging insects is plenty persuasive, apparently! Luckily, he escaped without getting stung.
Other woodland insects John prefers to keep at arm’s length are the wood and deer ticks. Luckily, they have been leaving him alone during the summer heat.
“If you’re a wood tick, your surface area is quite large, but your volume is quite small because you’re shaped like a penny,” John explained. “You’re very round and very flat, and it doesn’t take long in the sun to just boil the water right out of a wood tick.”
As you can imagine, wood ticks prefer not to be boiled alive and make themselves scarce during the summer months. However, they will be reappearing as the temperatures drop.
John expects to see monarch butterflies through October, though he suspects the late-departing individuals are less successful at making the long migration to Mexico.
Avian emigration
Nighthawks are soaring through the sky, scooping up the mayflies with their broad beaks. Look for them flying over rivers, fields and city streetlights as they hunt for aerial insects. On average, John sees the migrating nighthawks around Aug. 21. His friend Sam called on Aug. 18 to report a sighting.
Rose-breasted Grosbeaks are becoming scarcer as they migrate south to Central and South America. John has not seen an adult male for over a week!
While male Ruby-throated Hummingbirds are still jealously hovering over John’s feeders, he is expecting them to depart soon. He increases the amount of sugar in his nectar from 1 part sugar in 4 parts water to 1 part sugar to 3 parts water. The increased concentration helps the birds pack on more energy as they prepare for their long journey south.
That does it for this week! For more phenology, subscribe to our Season Watch Newsletter or visit the Season Watch Facebook page.
Funding for this project was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR).
