As light intensity and temperature decline in the winter the amount of glucose needed for respiration to maintain the leaves and produce chlorophyll is more than the amount produced by photosynthesis. The leaf is also more likely to be damaged by gales and storms in the winter. Deciduous trees in temperate climates lose all their leaves in winter and remain dormant until the days lengthen and temperatures rise in spring.
The leaves produce ethene in response to reduced concentrations of auxin. The abscission zone is two layers of cells sensitive to ethene. Genes are switched on resulting in the production of enzymes which digest cell walls in the separation layer of the abscission zone. Vascular bundles are sealed off and fatty material is deposited on the stem side of the separation layer. Cells retain water and swell putting strain on the cells and the leaf is then blown off by the wind.