Plant Phases: Senescence
You wouldn’t go out into a blizzard without a parka, boots, and gloves. A bear doesn’t hibernate without putting on a fat layer that will allow it to survive the winter. Plants, too, prepare for winter through a process called senescence, or biological aging. Except near the Equator, which has its own wet and dry seasons, plants need to prepare for winter every year.
Plants have different strategies for dealing with winter. For some plants, the seasonal changes are minor and not always obvious. For example, while an evergreen like a pine tree does go through some changes in preparation for winter, the changes are not nearly as apparent as they are on an oak or maple, which loses all its leaves. Many perennial plants and herbs die off entirely above ground, hiding away as root or bulb until it’s time to send out new stems and leaves. True annuals don’t even try to keep a root alive. Instead, like the mayfly, they put all of their adult energy into mating and reproduction, often with colorful blossoms to attract pollinators. Once the flowers are fertilized with pollen, they begin to produce seeds, creating new generations whenever conditions are right. Each of these strategies is different, but plants must prepare for winter, just as we do!
As fall comes, the days get shorter and the nights get longer and colder. Seasonal changes trigger plants to move nutrients, especially energy-containing sugars, from expendable parts like leaves to the trunk or roots, where they will be stored until spring. Every wonder why maples turn such bright colors in the fall? Maple leaves contain high levels of carotenoids and anthocyanin sugars, which are brightly colored. In the spring and summer, the leaves are rich in green chlorophyll, but as the sap returns to the roots, the leaf dies, and the remaining traces of these compounds show what kind of a year it has been for the maple. Bright colors in the fall mean lots of healthy sugars have been produced—which promises a healthy tree, and a good run of sap for maple syrup in the spring!
Some plants bear fruits or berries. This strategy can get seeds deposited, with a healthy organic fertilizer dose, a great distance from where they were eaten initially. Other plants disperse their seeds directly on the wind, like the Utah cottonwood tree or the dandelion. Still others, like burdock or fox-tail, can hitch a ride without being eaten. The coconut can float from one island to another (or, according to Monty Python, be carried on a string by a migrating swallow). Plants are exceptional in their variety and ingenuity!
In nature, nothing goes to waste. Leaves and stems that fall to earth become mulch, which first protects soil and roots from hard freezes, and then decomposes, feeding soil bacteria and fungus, which provide food to roots in the spring. For plants, as for us, fall is a
season of gathering, preparation, and thanksgiving.