Any two plants growing near each other are in a competition for food, light, soil and water. Especially when they belong to the same species, their fight becomes fiercest because their needs are quite the same. If you are study of trees and have keenly observed, you may find that a chemical warfare can happen between two plants. That too when the region is drier, more is this warfare.
One good example is that of an apple tree and a walnut tree. Apple trees grow poorly near a black walnut tree while tomatoes never grow near the walnut tree at all. This is because the roots of the walnut tree give off a poison. This poison inhibits the growth of the nearby plants. If you have also visited a eucalyptus grove, you will not find a blade of grass under the tree. This is because the poison is produced in the leaves of this plant. When the leaves drop off, rain washes the poison into the ground. Beech leaves also release a chemical one that prevents the sprouting of its own seeds.
In deserts of southwest America are the bushes that also prevent any other plant becoming a part of their community. There grows sagebrush. They form thickets in which no other plants or even their seedlings grow. This is prevented to ensure there is not any competition for the moisture. Around the thicket grow only the sagebrush seedlings.