Deciduous is the word used for the trees and shrubs that shed their leaves at the end of each growing season. The word deciduous is from the Latin word decidere, meaning “to fall off.”
Plants respond to their environment in a variety of ways and rely on chemical messengers (hormones) that affect all aspects of plant life. The interaction of these hormones regulates different stages of plant growth. Plant hormones control the process of going into dormancy, which starts before the leaves begin to change colors.
Most plants are green, the result of chlorophyll, a green pigment, and responsible for the absorption of light to provide energy for photosynthesis. When autumn arrives, chlorophyll levels drop, a signal that triggers the release of a series of hormones. Auxin is the primary plant growth hormone, produced in the roots and shoots of a plant. As the days shorten and temperatures fall, signals for the plant to stop using energy to grow lead to the reduced production of auxins. As auxin levels decrease, the hormone ethylene is activated and begins the process of recycling the leaf - senescence. The process of leaves changing colors in the fall is called "senescence." It's a natural process where the green chlorophyll pigments in the leaves break down and reveal other pigments, which give the leaves their autumn hues. These pigments have been present within the leaf’s structure all along but masked by the dominant green of chlorophyll. Most deciduous trees and shrubs begin to show their “true” colors in autumn. The four primary pigments that produce color within a leaf are: chlorophyll (green), xanthophylls (yellow), carotenoids (orange), and anthocyanins (reds and purples).
In deciduous trees and shrubs entering dormancy, the leaf separates from the plant at the “abscission layer” located at the base of the leaf stem (petiole). The hormonal changes described above lead to changes in cell walls throughout the leaf stem that eventually lead to leaf drop.
Leaves that tend to hang on to their twigs are known as “marcescent leaves”. In marcescence, the abscission zone does not develop until the leaf bud breaks in the spring. Trees that exhibit marcesence include oak, beech, and hornbeam trees.
Do fallen leaves have a benefit?
Words of wisdom from Leave Leaves Alone, located in Bedford Hills, NY - “The Many Ways Leaves Can Be Used in the Landscape: Natural mulch, protect roots, suppress weeds, amend soil, support biodiversity, fertilize naturally, encourage mycorrhizae (beneficial fungal growth in the soil).”
The greatest way to benefit your garden and the environment is to leave the leaves.
As the leaves decay, they add organic matter back into the soil, which lessens the need for fertilizer. The leaves also provide winter habitat for wildlife including birds, turtles, frogs, and insects (butterflies, bees and other pollinators) that overwinter in the fallen leaves. If the leaves are removed, these creatures will not survive.