Our two fawns have grown tall and fat this summer, and their spots are just starting to fade. Their diet of browse and mother’s milk augmented by many pounds of black oil sunflower seeds and shelled corn from my bird feeders has successfully fueled their growth and development. Now, though, a new resource is occupying their attention: an abundant crop of apples that is falling nearly continuously from my old apple trees. The sound of the apples hitting the ground and the sight of fawns chewing the often bitter, still green apples are signs of the end of summer, indeed.
The dog star, Sirius, the brightest star in our sky, started to rise with the morning sun in late summer and signaled the beginning of the “dog days.” These dog days are also the time for the annual cicadas (called the “dog day cicadas”) to rise up into the trees and begin to sing.
The various species of dog day cicadas have life cycles that range from two to five years in length and almost any area will have cohorts that reach their adult stages in the late summer of any year. So, as we go through a given August, we will be greeted by the nearly continuous day-time singing of these “annual” cicadas. The developmental stages of these cicadas are found in the soil typically feeding on the sap of tree roots (especially oaks!). In the late summer they crawl up out of the soil and climb back up the trunks of surrounding trees where they carry out their molt into adults. The dry exoskeletons of their pre-adult stage can often be found empty but still clinging to the rough surface of the tree bark (a visual sign of the coming fall!).
Male cicadas then climb further up the tree and begin to sing. They have thin, exoskeleton membranes (called “tymbals”) on the sides of their abdomens that they can pull inwardly and then release to make a loud “click.” The males’ bodies are also quite hollow and act as amplifying, resonance chambers for the generated sounds. The purpose of the song is, of course, to attract females for mating. The mated females will then lay their cluster of eggs under the bark of a twig or branch of the tree and start the life cycle all over again. Interestingly, the females have very solid, “meaty” bodies. They require more metabolic energy and more elaborate internal organs for the production of their eggs. One consequence of these morphological gender differences is that females are the preferred food for most cicada predators (including birds, squirrels, raccoons, and even people (many cultures include annual cicadas as a popular, seasonal food!).
It is thought that the cicadas reduce their overall losses to predation by concentrating their adult emergence into a very narrow time window. Their numbers overwhelm potential predators and then they suddenly disappear. This transient existence also keeps predators from specializing on the cicada adults.
The mornings are increasingly foggy as summer begins to fade. These ground clouds are called “radiational fogs,” and they are caused by the extensive radiational cooling of the night air due to the longer, clearer nights of late summer. When this cool air comes into contact with soil and surface water that still retain their daylight heat, moisture condenses into the fog. The warming sun in the morning quickly reheats the air and dissipates the fog. This morning as I walked my dog Izzy out in my cool, foggy field, a small flock of five Canada geese flew over honking loudly and quickly disappeared into a dense fog bank. So many sounds and signs of Fall!
A more mundane sign of the end of summer is the appearance of crab grass. Although many of my friends and neighbors positively hate the species and do not want to hear anything good or amazing about it, in my rich mix of plants that I call my lawn the rise of the crab grass is an exciting, late summer event.
Crab grass thrives in the heat and dryness of the late summer because of elegant adaptations in its cellular structure and molecular physiology. In most plants, the cells that carry out the two parts of photosynthesis (the “light reaction” that captures the energy in sunlight and makes both energy molecules and also the “waste” molecules of oxygen, and the “dark reaction” that uses carbon dioxide and the light reaction’s energy molecules to make sugars) are intermixed together. This allows effective contacts and transfers and lets photosynthesis run quite efficiently. When a plant becomes stressed by a lack of water, though, it closes its leaf pores (the “stomatae”). This stops the potentially fatal water loss but also shuts off the delivery of carbon dioxide to the “dark reaction” cells. There is, then, an imbalance between the “light” and “dark” reactions, and one of the consequences is that oxygen begins to build up inside the plant. This oxygen starts breaking down the functional molecules of the dark reaction (a process called “photorespiration”) and the photosynthetic rate (and the growth rate) of the plant plummets.
Crab grass avoids photorespiration by anatomically separating and isolating the cells that carry out the light and dark reactions. The dark reaction cells are sealed away from the air spaces inside the leaf, and the oxygen from the light reactions, then, cannot break down the dark reaction cell’s molecules. A system (involving the synthesis of a four carbon molecule (hence, a C-4 plant!)) that transfers carbon dioxide from the air spaces to the dark reaction cells then occurs to keep the carbon chemistry of the dark reaction cell operating.
At temperatures above 30 degrees C (about 86 degrees F) a C-4 plant is 200% more efficient than the typically photosynthesizing plant (which is called a C-3 plant, by the way!). So, when you see the spiky heads of crab grass rising about the dead and dying grass plants around them, think about the biological elegance that allowed that event to occur!
The birds around our house have been subtly changing here at the end of summer. We have only been seeing one of our three “high- summer” hummingbirds of late, and the rate of seed consumption has declined from the nestling/fledgling gorging cycles of the summer. Also, the sound of evening robin songs has become rare so that when the gathering or passing robins do night-roost in our spruce trees and grace us with their rolling, “nightly news” songs, we feel not only blessed by their presence but also strongly sense the end of the summer season.