Does lack of water cause leaves fall off plant?

Generally the answer is "Yes, lack of water causes leaf fall", but an equally important reason is the approach of Winter. The leaves of some trees are able to withstand frozen conditions because their leaves are tough and shiny or needle-like. Such trees are called "evergreens" for that reason. But "deciduous trees usually have leaves with a greater surface area, and are more fragile. Such trees conserve water and food by removing it from the leaves, which causes them to change colour and fall off. We call that time "Autumn" but the Americans call it "Fall" when they appreciate the many rich colours of the tree leaves just before they are due to "fall".