In autumn, the earth receives less sunlight and warmth, which results in trees preparing themselves for the upcoming winter weather. The changes in weather trigger hormonal changes in trees. When this happens, leaves change color and abscission cells cut the leaves off of branches. This results in the falling of leaves in autumn.
The falling of the leaves helps trees survive in the cold, dry winter air. When the temperature is warmer, trees use their leaves to make food with the help of photosynthesis, a process that involves the absorption of water, sunlight and carbon dioxide through the leaves to produce glucose. In the winter, food production slows down, so there is little to no use for leaves using up energy to produce food. Trees also lose water through pores in their leaves, so in the cold, dry months when water retention is critical, leaves prove to be inefficient.
If leaves remained on trees, the water in the leaves would freeze. Frozen solid, the leaves would die. That means when spring rolled around again, the tree would have nothing but dead leaves, so it would die as well. Hence, the cycle of losing and growing back leaves prolongs a tree's lifespan.