Acorns: Abundance, Absence, and the Race for Survival
Some years, oaks produce huge numbers of acorns. Such an event is known as a “mast year.” The word mast comes from the Old English word maest, which means the nuts of trees that drop to the ground. During a mast year, the nuts accumulate in unusually large quantities.
But mast years only occur every two to six years, and most individual trees do not produce large numbers of acorns in consecutive years. For example, last fall in southern Virginia, there were very few acorns. Why are there so many acorns one year, and so few the next?
Acorns may be very abundant one year and very scarce the next; their numbers are cyclical. Photo courtesy of www.flickr.com, Douglas Earl.
As with most natural phenomena, we aren’t exactly sure of the direct cause. Some biologists think that external factors such as environmental conditions are the main reason oaks periodically produce very few acorns. For example, a late freeze might kill most or all of the flowers on a particular oak species in a certain area.
Others think that high numbers of acorns followed by extremely low numbers can be explained by internal mechanisms. An oak tree spends a lot of energy to produce a large number of acorns. So it is energetically difficult to produce heavy seed crops year after year without taking a break.
Whatever the cause, it’s pretty well accepted that the mast cycle is an evolutionary adaptation of oak trees in response to seed predators. Birds such as jays, woodpeckers, and turkeys, along with mammals including mice, chipmunks, deer, bears, and squirrels feed heavily on acorns during the autumn. The population size of seed foragers often increases proportionately in response to an abundance of acorns. Some studies have shown that the number of individual animals increases as much as seven times following a mast year.
Blue jays Cyanocitta cristata feed heavily on acorns during the fall. Photo courtesy of www.flickr.com, shock264.
If oaks always produced large numbers of acorns, the populations of seed predators would continually expand over time. The trees would be in a never-ending race to produce more seeds than the predators could eat. Some of these animals (e.g., white-tailed deer) also eat oak seedlings. Therefore, high populations of deer result in increased predation on the young offspring of oaks as well as on their seeds.
In years when acorns are scarce, fewer animals survive the winter to reproduce. This causes a rapid decrease in the size of populations, and fewer animals are alive to eat acorns in subsequent mast years. As a result, more acorns escape predation and germinate, ensuring survival of the oaks.