Asked by: Barry Cull, by email
Deciduous trees shed their leaves as an active process that evolved to conserve resources and protect the tree from being blown over in the windier winter months. The process is controlled by the plant hormone auxin.
As light levels and temperatures drop, the flow of auxin to the leaves slows and levels of another hormone, ethene, rise. This signals the cells at the base of the leaf to weaken their cell walls, at the same time as other cells expand to break the connections between the weakened cells. The result is like tearing perforated paper, and the leaf drops to the ground.