Why trees shed their leaves in winter?
Shedding of leaves is part of process called senescence (ageing) this happens in normal plants as well when leaves becomes old, infected or when they are not getting enough sunlight. But in case of trees that shed all of their leaves at same moment before winter do so in order to transfer all the energy storage chemical compounds and such material from leaves to trunk. This is because trunk can with stand the winter. This gives such trees advantage over those that do not shed their leaves. Those who keep their leaves are unable to utilized them most of the time during the winter because they are covered with snow or there is not enough sunlight. And most important benefit is that when winter is over and spring comes trees who shedded their leaves have backup nutrients in stock and have head start compared to those who did not do so.
