Why do deciduous trees shed their leaves in autumn?

Deciduous trees in temperate forests shed their leaves in autumn primarily as a survival strategy to protect themselves from the harsh conditions of winter. Because these trees possess thin, delicate leaves, they are highly susceptible to freezing temperatures; if left on the tree, the water within the leaf cells would freeze and expand, causing the cells to rupture and rendering the leaves useless for photosynthesis.12

Beyond frost damage, shedding leaves prevents physical harm to the tree. The broad surface area of leaves would catch wind and collect heavy snow, which could cause significant breakage to the tree's branches. Furthermore, by the end of the growing season, many leaves are damaged or diseased, so dropping them allows the tree to enter dormancy and start fresh in the spring.1

This process is a highly controlled biological event triggered by shortening days and cooling temperatures, which activate the formation of an abscission layer that cleanly separates the leaf from the twig. Finally, the nutrients from the fallen, decaying leaves are recycled back into the soil, supporting the forest's ecosystem.1