Like other nut bearing trees, oaks produce a large amount of acorns; unlike other nut-bearing trees (pecan, walnut and hazelnut) the seed nuts cannot be consumed by humans or most animals because of the tannin in the seed. A huge quantity of acorns must be produced for an oak to propagate itself.
The oak tree is unisex, having both male and female flowers. The male flower makes pollen; the female flower makes the egg. According to The Nature Conservancy, the pollen is transferred to the female flower by wind. The seeds are the acorns that grow from the female flowers inside a brown woody cap.
Acorns are produced once a year during autumn. The number of acorns fluctuates from year to year; no two successive years will have strong production. The acorns fall before the leaves, which protect the acorns from frost and animals. Given proper humidity, temperature and rainfall, the oak tree can produce several thousand flowers. While full-grown acorns will show by late summer, the chance of an acorn becoming a full oak tree is 1 in 10,000.
Acorn production begins when the oak tree reaches 20 to 50 years of age. Production of acorns increases each year in proportion to the size of the oak tree's canopy. When oak trees reach near 100 years old, the acorn production slows to a yearly rate of 2,200. Even though oak trees have both male and female flowers, they still need neighboring oaks to pollinate. A study by the University of California found that the density of airborne pollen reaching a tree greatly diminishes as the distance between oak trees increases. The study also found that dry, warm conditions are also needed for optimum pollination (below 45 percent).
Factors which effect acorn production include sufficient nutrients, age of the tree, the diameter of the crown and height, branch density and the amount of sunlight, according to the Lehigh Earth Observatory.
Oak trees provide a food supply through the acorns to a variety of animals and birds. Acorn woodpeckers specifically use acorns as food to sustain a resident population. According to the Hastings Reserve, woodpeckers choose areas where more than one species of oak is found so that if one variety goes bad they will still have food. Insects, such as wasps, also use acorns for food.
Acorns have different rates of maturity, according to Purdue University. White oak trees have mature acorns within three months. Red oak trees have mature acorns in 15 months (or two growing seasons). Acorn production is unpredictable from year to year. Information from Purdue states that growth cycles occur between 2 to 4 year intervals for black, white and northern red oak.