Trees that lose their leaves in autumn are collectively described as deciduous trees.
The word deciduous means “falling off at maturity.” In temperate climates, deciduous trees usually shed their leaves as daylight shortens and temperatures fall. This seasonal leaf drop helps the plant reduce water loss and avoid maintaining foliage during a period when cold conditions limit photosynthesis.
The process is called abscission. A specialized layer of cells forms where the leaf stalk joins the stem, gradually cutting the leaf off while sealing the break. As chlorophyll breaks down, yellow, orange, and sometimes red pigments become visible, creating autumn colour.
Deciduous does not always mean a tree loses leaves in autumn. In tropical and subtropical regions, many deciduous plants shed foliage during a dry season instead. The opposite term is evergreen, although semi-deciduous and semi-evergreen plants occupy intermediate categories. Oaks and beeches may also show marcescence, retaining dead leaves into winter because the abscission layer develops incompletely.