Leaves are food generation unit for the trees.During summer, tree supplies adequate water to the leaves and leaves are busy in making and storing food.
During summer days, leaves make more glucose(food) than the plant needs for energy and growth. The excess is turned into starch and stored until needed.
In winter sun light is very limited also trees itself will not get more water.So in order to sustain, trees stop sending water to the leaves also it shutdown the food production.Trees will be survived with excess food generated in the summer.
Since the water supply is stopped, leaves cant survive anymore.Due to this leaves start dying ,change its colors and fall during winter.Depends of the condition some of the trees will not have any leaves during winter.
Leaves grow again in trees once spring starts.