Deciduous trees drop their leaves before winter after they stop producing chlorophyll that covers up with green other colorful pigments remaining in the leaf. Just prior to leaves falling colorful displays catch the eye as the color green dissipates while other pigments remain visible. During their six to eight month life, leaves must work like hell to do their job. Leaves are the original solar panels turning sunlight into chlorophyll, tree energy. Trees have a broad range around our planet, from tiny( bonsai) to extremely huge( redwoods, sequoias) and they range as much in age as they range in habitat, some to living many 1000's of years.
Leaf dropping triggers a very dynamic stage in a tree's growth cycle : that is, the development of next year's flowers are wrapped up in a bud prior to the all important development of seeds after the flowers become pollinated. Through seed germination the tree species survives almost endlessly but with a couple million seeds dropped to the ground maybe only one or two will germinate and mature into a full growth tree.
The multitude of trees is so vast that almost every house built is built by using lumber from matured trees. In fact, lumbering tends to be a multigenerational business because once a forest is lumbered it will take over a generation or two for trees to reach an acceptable age for cutting them down. Remember the old tails of an era of sailing when sailors crossed this World looking for tall, straight, strong and somewhat flexible trees to turn them into the main masts of new sailing ships. The tree is so adaptable because it can be formed into doing multiple tasks and is easily worked by hand.
Leaves, the energy producers for trees, have another use when they dry up and fall. One of the most energy rich materials found and now understood by man is when chopped up leaves are piled up and left to compost through heating and bacterial action. The result is often termed "Black Gold" because in nature even without chopping leaves they decompose and provide much of the nutrients needed by maturing trees. Man always seems to need to accelerate a process that nature does in its own due time. If you rake leaves and don't compost them you defy the natural process, a process which has lasted billions( yes, billions ) of years. "Black Gold" can work for you, too.
Ronald C. Downie