The initiation of fruit buds in fruit trees is a complicated matter. At present date there is still much not agreed upon in the scientific community with regard to the exact mechanisms which lead to fruiting. Despite the complexity, enough is known to give general guidance and understanding.
For specifics on each species, please look to the Apple, Tree, and Bush sections of the WHNO site. Following however is a basic introduction to the phenomenon.
During the growing season, varying by species from June to November, fruit/flower buds are differentiating. These will be ready to erupt into blooms the following growing season.
The precocity (tendency of early flowering as regards age of plant) of the variety or species is dependent on a many factors. The exact mechanism is still somewhat unclear, but many individual factors have been shown to affect bearing. Hormones (both specifically and balances of) play a large part in initiation. Rootstock choice, soil conditions, pruning, tree age, tree health, injury, and chemical applications all prove influential.
GA (giberellic acid), for instance, is instrumental in the inhibition of fruiting. Found to occur in seeds (especially immature) and in leaves of the plant in question. In its absence, studies have shown initiation of fruit buds is more likely.
The interruption of certain hormones is likely to be the cause of the production of fruit buds, in our estimation, and here is why: Two non-natural processes, grafting, and bark scoring both often cause earlier fruiting in a given cultivar. Both result in an interuption in the flow of materials in the transport system of the plant. In some cases, substances may not be able to cross the gap of a union, and build up at before the site. Furthermore, with interstem grafts the translocation can more complicated. Particlar rootstocks can be less efficient at producing or transporting some substances and more efficient as regards others.
It has long been held that the stress produced by heavy cropping was the cause of the lack of fruit production the following season, as with biennial bearers. It stood to reason that the nutrient drain was to blame. Some success with early thinning of fruit seemed to substantiate the claim. More recent studies, however, indicate that chemical production in the fruit seed (ie-GA) results in fruit bud inhibition. Although the heavy drain of nutrients can indeed play a part, it is now generally believed to be hormones in the fruit are the reason that thinning can be helpful. Incidentally, it is necessary for the thinning to be done in the three week period following petal fall.