Either the soil particles are holding the water too tightly for roots to extract it, or the tree doesn't have enough roots to gather enough water. As water concentrations become too low, roots are shut down and don't grow. Trees need water to make food and move essential materials. They also gain some heat dissipation from water evaporating from leaves. As water is allowed to evaporate from leaves, carbon dioxide for food-making moves into the leaves.
Water is the most valuable of raw materials trees collect.
But as water becomes less available, trees must close up leaves to save the water that remains. Stopping leaf functions means the tree can't make food and will have its internal temperature climb. As temperatures climb in and around the tree, many problems arise. For every 18-degree increase, water and food requirements double. As water becomes unavailable and leaves are closed, food continues to be needed in greater volume.