Predicting Acorn Production

The unpredictable nature of flowering in oaks results in the irregular occurrence of acorn crops and thus new seedlings (Cecich 1991). On the average, most species produce a good acorn crop once every 3 or 4 years (Olson 1974). Numerous biotic and abiotic factors influence acorn viability, germination, initial seedling establishment, and survival. For example, dry weather, droughty soils, and freezing temperatures can reduce acorn viability and germination (Korstian 1927). Acorn crops also are frequently destroyed by unpredictable but frequent infestations of acorn weevils (Christisen and Kearby 1984). Most of the remaining acorns may be consumed by rodents, deer, birds, and other animals (Marquis and others 1976, Sork and others 1983). So, even after a bumper acorn crop, few acorns may be available for seedling production. Among the few remaining viable acorns, many fall into microsites unsuitable for germination and seedling establishment (Johnson 1993).

Significant numbers of new oak seedlings thus occur as unpredictable population waves associated with bumper acorn crops and a spatial distribution resulting from patchy germination suitability and seedling survival. The relatively infrequent occurrence of large seedling populations originating from one acorn crop (cohort) usually coincides with a bumper acorn crop combined with other fortuitous events, such as weather, that favor the preservation of acorn viability through fall and spring germination periods and low populations of acorn consumers. (Johnson 1993)

The acorn producing capacity of a stand, and thus the rate of seedling input into oak forests, changes with time. Although variation in stand structure and age can account for some of the variation in both the temporal and spatial variation in oak seedling establishment, inherent variation in acorn production among trees introduces an essentially random element into predicting seed and seedling inputs into oak forests (Johnson 1993). Some of the variability noted:

1. Large trees usually produce more acorns than smaller trees because, other factors being equal, acorn production increases with crown area. In turn, crown area is correlated with bole diameter (Goodrum and others 1971).
2. However, in some species there is a threshold diameter above and below which acorn production decreases (Downs 1944). Large, senescent oaks are poor acorn producers (Huntley 1983).
3. The production of acorns per unit of crown area is also greater in open-grown trees than in forest-grown trees of the same size (Gysel 1956, Sharp 1958).
4. Some trees are better acorn producers than others even when tree size and environmental factors are the same (Sharp and Sprague 1967). For example, in mature white oaks in Pennsylvania, only 30 percent produced any acorns even in good seed years (Sharp 1958), and an even smaller percent produced a good crop in those years (Sharp and Sprague 1967).