Fruits, the ripened ovaries surrounding seeds, are the result of a tree first initiating the production of flower buds based on environmental or genetic cues. Flowers contain male pollen and/or the female ovary which ultimately becomes the fruit or nut. Pollination by wind, animal or insect ensures fruit formation around the fertilized, developing seeds.
Some trees, but not all, require an environmental stimulus or threshold to be met in order to ensure a production of flowers. In fruit trees like apples, peaches and cherries, a prolonged exposure to winter cold is needed for the plant to initiate any flowering the following spring. This process is called vernalization and is found in many tree species native to the world's temperate climate regions. Tropical trees do not have a winter cold requirement, but certain species may require a dry dormancy broken by rainfall, or a specific duration of day or night length called a photoperiod in order for flowers to be produced.
Flowering is the means by which a plant reproduces, an attempt to bring the genetic material from male and female flower parts together to create a fertile seed. Fruits are the swollen, ripened female ovaries of flowers, with or without viable seeds encased within. If a tree fails to flower, fruits cannot subsequently develop.
Flowering does not necessarily occur each year. Environmental factors like soil moisture or seasonal temperatures or plant genetics can cause some tree species to flower in alternating years. Some crab apples and litchi trees often bear flowers more heavily every second year, or when preceded only by an optimal dormancy and favorable start to the growing season.
Pollination, the process of male pollen fertilizing the female ovary of the flower, is typically necessary for fruiting. More accurately, however, pollination results in formation of seeds. Some plant species will still produce fruits after flowering without producing seeds, such as the banana.
Trees are pollinated by any variety of vectors, from wind or falling raindrops to more specialized means through animals and insects. Typically insects are the most efficient and numerous pollinators of fruit-producing trees, including flies, butterflies, wasps and especially bees. Some trees may only have one specialized pollinator, such as a bat or tiny midge that can navigate the uniquely shaped flower.
Moreover, trees may be "self-fertile", meaning they will be pollinated effectively from flowers across its own canopy. Pollination can be improved in most tree species if more than one individual plant is present. In some plant types, male and female flowers occur on separate plants, therefore multiple plants are a necessity to ensure both genders are present for pollination.