Many species of fruit and nut trees tend to "alternate" bearing. A more precise term is "synchronized" bearing because the years of heavy bearing are not always on a two year cycles.
Not all individuals in a give species show synchronized bearing traits. That tells us that there must be some significant selection advantage to synchronized bearing in those species where most individuals do exhibit sychronized bearing.
Environmental factors that contribute to synchronized bearing
Specimens that have fully exploited the nutrients in their root zone tend to be more strongly synchronized. The specimen will set an enormous crop. The maturing crop sucks an enormous amount of both carbohydrates and nitrogen (protein) out of the tree. The summer buds "decide" to take the vegetative form based on the dearth of resources needed to ripen next years crop. Depending on the site and climate it may take several years before the tree "triggers" another mammoth crop.
Commercial fruit growers have bills to pay every year. They overcome "alternate" bearing tendencies by managing the crop load. They also manage fertility and soil moisture to ensure moderate annual shoot extension and a high-but-balanced level of Nitrogen and carbohydrates.
The cycle is often synchronized by a synchronizing event like a late freeze or a serious drought.
But WHY?
What advantage might a species gain by having most individuals having one (or several) years of no seed production followed by a bumper crop? Specimens of synchronized bearing and annual bearing tendencies both exist within a given species. Under conditions that strongly favor annual bearing one would expect synchronized bearing individuals to be rare. There must be compelling reasons that favor individuals with synchronize bearing characteristics.
The leading theory for synchronized bearing is predator avoidance. Trees cannot flee weevils. They are rooted to the ground. The effect of several years of not bearing is to starve out those insects that would otherwise come to an equlibrium with the food base if the trees were to bear annually. The greatly diminished population of pests cannot find and infest all of the nuts, or fruit, of those infrequent, massive crops.
Some pests can infest many different species. The different species sharing a give habitat often follow their own synchronized schedule. So how does the "predator avoidance" theory mesh with the messiness of multiple species?
They mesh rather elegantly, actually. Each plant species typically has a different time window of maximum vulnerability. A peak crop of a plant species with a late window will drag the timing of (i.e., genetically shift) the pest to peak later in the season. Consequently, the peak pest pressure will miss the window of maximum vulnerability for earlier species. In a similar way a peak crop of a very early species will shift the peak pest pressure to earlier in the season thereby clearing the way for later species.
Is there any evidence?
There is anecdotal evidence that some of the selections most notable for annual bearing (McDaniel's burenglish oak) also appear to be one of the most susceptible to weevils.
