Pollination is extremely important to Fruit Tree Farmers, less so to Home Orchardists.
Pollination occurs within the trees flowers, causing the flower to set into fruit.
To keep it simple, there are three types of flowers, Complete Flowers (having both male and female parts), Incomplete Flowers (female parts only) and Incomplete Flowers (male parts only). Complete Flowers are often called Perfect Flowers, lncomplete Flowers are often called Imperfect Flowers.
Many fruit trees have Complete Flowers and require no cross pollination. Some fruit trees have both male and female flowers in comparable numbers (on the same tree) and require no cross pollination.
Cross Pollination is required for all trees with one type of incomplete flower only. Cross pollination allows a male Flowering Tree and a female Flowering Tree, who flower at comparable times, to set fruit.
Many Apples, Pears and Cherries require cross pollination, although some do not. Fuji Apple, d'Anjou Pear and Stella Cherries are self-fertile, so are many other common, very desirable varieties. Most Apricots, Nectarines, Peaches and Plums are self-Fertile. Persimmons, Quinces, Figs, and pomegranates are self-fertile. Walnuts are self-fertile, but most Almonds require cross pollination (All-in-One Almond is a great self-fertile variety).
Examples of Cross Pollination:
Ne Plus Almond requires Nonpareil Almond
Red Delicious Apple requires Golden Delicious Apple
Satsuma Plum requires Santa Rosa Plum
There are other pollinators for the previously listed varieties, these are just examples.
Do neighboring areas have fruit trees? Cross pollination can occur from quite a distance. Mild Winter and or cool Summer areas will sometimes cause a tree to produce some Complete Flowers, where otherwise they would produce only Incomplete Flowers.