The sky is falling!

by Karen Schik

August is a great time of year to be a squirrel, because that’s when the great abundance of tree nuts falls from the sky – acorns, butternuts, walnuts etc. These nutritious fruits, also called mast, are an important food source for many animals, including deer, bears, turkeys, squirrels and mice, helping them to survive the winter. While many tree species produce mast – either hard or soft fruits - masting or fruit-masting is a specific reproductive strategy employed by certain species, most notably oaks, whereby an entire population of trees over a large geographic area has synchronous fruit production one year, followed by one to several years of little or no nut production. According to a Minnesota Department of Natural Resources article: “Bur oak seems to be on a two-year cycle, with good acorn crops statewide in 1994, 1996, 1998, 2000, and 2002, and poor crops in the intervening years. White oak and northern red oak appear to be on a three-year cycle and had good crops statewide in 1994, 1997, and 2000. The cycles of all oak species seemed to converge in 2000 with an extraordinary combined crop of acorns throughout Minnesota and much of the Midwest. Predictably, 2001 was a poor year for acorn production, and no species produced an abundant crop.” 2009 should be a year for red oak masting, but from our observations it appears to be a year of poor acorn production for both bur and red oak. (What are your observations? Send an email to ).

The reason for the masting phenomenon is not known, but studies indicate that it provides survival benefits. In years of large mast production, there is such an overabundance of food that seed predators cannot eat it all and there is a better chance that some acorns will survive and germinate. In years of low mast production, predators such as deer and squirrels may decline, further benefiting the trees. Another benefit may be at the other end of fruit production -pollination. Flowers of wind-pollinated species may have better chance of pollination if the “market is flooded,” so to speak, with vast quantities of pollen.

One of the real mysteries of this phenomenon, however, is the how part. How do trees “know” to all produce at the same time, what is the mechanism of coordination? The answer, of course, is not known! Theories include chemical or pheromone communication, physical connection, and climate factors.

While masting may seem like merely an interesting phenomenon to observe, there may be direct effects on humans. One, of course, would be the affect on game animal populations. But there may be another affect with more direct health implications. High mast production may promote rapid population increases in white-footed mouse and deer populations. The mice are hosts for the bacteria that causes Lyme disease, which matures on and is dispersed by deer. So masting years may consequently result in increased incidence of Lyme disease.

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