Like people, the leaves of trees also age, but do so much more rapidly. Each spring the young leaves develop from buds and then rapidly mature to full size to carry on photosynthesis (the process that provides food needed for the tree to grow and maintain itself). By autumn, the leaves of many hardwood trees reach old age and do not function well. At this time a tree must also prepare for the harsher weather conditions of winter. Signals to the tree indicating it is time to prepare for winter include shorter days, less intense sunlight and cooler temperatures. Prior to the leaves falling, nutrients like nitrogen and phosphorus move from the leaf into the adjacent stem to be reused during the next growing season. The following spring a new crop of leaves are produced and the cycle is repeated.
To sum up, in early autumn, in response to the shortening days and declining intensity of sunlight, leaves begin the processes leading up to their fall. The veins that carry fluids into and out of the leaf gradually close off, as a layer of cells forms at the base of each leaf. Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall.
Thus while trees can live for decades and even centuries, some parts of the tree like the leaves, live for only a few months or years (in certain varieties of trees).