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January 20, 2009

News Article: Richmond Times-Dispatch

Published: January 20, 2009

This fall, some oaks in parts of southern Virginia produced 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. Many city streets and lawns were covered with acorns.

But mast years only occur every two to six years, and most individual trees do not produce large numbers of acorns in consecutive years. Why are there so many acorns one year, and so few the next?

Scientists aren't sure of the cause. Some think 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 believe 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 difficult to produce heavy seed crops year after year without a break.

Whatever the cause, many scientists believe 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 autumn. The population size of seed foragers often increases in response to an abundance of acorns. Some studies have shown that the number of individual animals increases as much as seven times after a mast year.

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 -- white-tailed deer -- also eat oak seedlings. Therefore, high populations of deer would threaten the young offspring of oaks as well as their seeds.

In years when acorns are scarce, fewer animals survive the winter to reproduce. This causes a decrease in the populations, and fewer animals are alive to eat acorns in later mast years. As a result, more acorns become seedlings, ensuring survival of the oaks.

Virginia science Standards of Learning: K.6; K.9; 1.4; 1.7; 1.8; 2.5; 2.7; 2.8; 3.5; 3.6; 4.5; 4.8; LS.8; LS.9; BIO.9.


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