When fruit trees are just starting out, they can grow fairly close together without any problems. Their roots are shallow, their trunks are short and their branches do not meet. Some backyard gardeners and others with a limited space actually prefer to grow their fruit trees about half as far apart as commercial grows. These gardeners select dwarf trees, prune them carefully and keep them from growing into their neighbors. This allows them to squeeze a large number of trees into a small space.
If the fruit trees do grow tall, however, close planting can cause problems. The trees create too much shade, preventing light from penetrating to the lower branches. The trees can form a canopy which stifles are circulation under the leaves. This can encourage the growth of fungii and reduce the production of fruit, potentially damaging or ruining the fruit harvest. It may also be impossible to get vehicles between closely planted fruit trees, making it difficult and time-consuming to harvest the fruits.
Sometimes, fruit trees too close together can actually become grafted naturally. The wind causes their branches to rub together, causing a particular spot to rub bare on each tree. The trees ooze out sap from the wound and press together even tighter as they both continue to grow. Eventually, two different fruit trees can actually heal together, become one tree with two trunks.