With all of the cookouts and crab feasts, it’s hard to imagine that summer is waning. Autumn doesn’t officially start until Sept. 21, but nature is already preparing for the metamorphosis.
Trees and other plants are beginning to alter their physiology. Some trees have begun to change color, hinting at the falling of leaves.
Actually, this leaf-shedding process, known as abscission, has already started. For several weeks now, the cells where the leaf stem is attached to the tree have been toughening and starting to form a protective waterproof scar. The cells in the leaf stem itself swell, weaken and degenerate.
This interferes with the flow of moisture and nutrients to the leaf, reducing the production of a pigment, known as chlorophyll, which gives leaves their green color. Leaves contain other pigments, but these colors are hidden most of the year by the abundance of chlorophyll.
Chlorophyll uses the sun’s energy to convert carbon dioxide and water into sugar, food for the tree. As the days shorten, there is less sunlight to manufacture food. Nutrients and minerals are withdrawn from leaves and transported to the permanent parts of trees such as the trunk, stems and roots.
When the chlorophyll breaks down, leaves reveal their autumn colors as other pigments are unmasked. Xanthophyll produces the color yellow and carotene, like that in carrots, produces yellow-orange. Sunny days and cool nights can produce a sugar-related pigment, anthocyanin, which produces fiery reds. Other chemicals and breakdown products produce bronze, purple, and crimson.
Trees aren’t the only things making preparations. The monarch butterfly, easily recognized by its dark orange and black wings, is one of few butterfly species to migrate, and is on the move.
As the days grow shorter, millions of monarchs make their way south. Monarchs in the West migrate to southern California, while those in the central and eastern parts of North America overwinter in the Gulf states and remote mountain valleys of south-central Mexico.
From the first week of September to the third week of October, the monarch butterfly makes its way south through the Chesapeake watershed.
The Bay watershed also lies within a major migration path for birds known as the Atlantic Flyway. Mountain chains to the west and coastal shorelines to the east serve as geographic boundaries that channel millions of migrating birds through the Bay region.
Among these are the raptors, a group of birds that includes eagles, falcons and hawks. Raptors begin their annual southward migration just before the fall foliage color change.
By early September, many raptors in Canada and northern United States have begun to fly south. The earliest of migrants may go unnoticed. But as the the first autumn cold front passes through the area, the skies fill with more and more raptors. As they approach the Chesapeake Bay, some are funneled along the coast, while the others are steered along the mountains.
At the same time, songbirds and shorebirds are preparing for long flights from northern breeding grounds to tropical wintering areas. The shortening days and crisp frosts of early autumn signal waterfowl to begin moving south along the Atlantic Flyway. Instead of just passing by, though, the Chesapeake serves as the wintering ground for swans, geese and ducks.
One of the most familiar waterfowl is the Canada goose. Migrating flocks are noted for their distinctive "V" flying pattern. Some believe that this flying pattern reduces wind drag and lessens collisions between birds. Loud honking signals their arrival to southern wintering grounds.
Blue crabs mate between May and October, with mating peaking in late summer. Ordinarily, females mate only once after their last molt, while they are still in the soft shell phase. The male crawls on top and cradles the female between his legs until she molts. Cradling protects the female while her shell is still soft. After mating, the female’s shell hardens and the two separate. Males remain in fresher portions of the Bay and rivers and females migrate to spawning areas near the mouth of the Bay.
Juvenile American shad spawned in freshwater this past spring prepare to leave their natal homes. By autumn, the young shad gather in schools and swim toward the ocean, where they will remain for the next three to six years, until they are sexually mature. Then, they will return to freshwater to complete their life cycle.
Other juvenile fish that were born in freshwater but mature in the brackish or saltwater are beginning their migration toward the sea. These species include alewife, blueback herring, hickory shad and Atlantic menhaden.
So even though the trees may still be green and the days warm and sunny, look a little closer and you’ll notice subtle changes around you. A reddening sumac tree, a broad-winged hawk flying south, crab doublers swimming in synchronization, monarch butterflies flitting through your yard … all are signs that autumn is just around the corner.