Acorns and Mast Years: Masses of Acorns and Why

If you've been clocked on the head more than once this autumn by what can only be described as a missile-launched tree-nut, you may be wondering "what's up with all of the acorns this year?"

Only oak trees produce acorns. But all trees have boom crops of seedlings some years and generate minimal crop other years. Temperature, rainfall, and predation do factor into the amount of crop, but they are not the central reason for "mast years", or years when an immense amount of tree-nuts are produced. The true reason for this phenomenon has baffled scientists for hundreds of years.

Mast trees synchronize as well, so not just one tree has a mast year, but all of the oak trees in a region will equally overproduce acorns. One theory proffered suggests that since acorns are a major food source for a multitude of animals mast years make up for the intense predation--with so many acorns falling, it is impossible for all to be consumed. Therefore, the leftover acorns are able to embed into the soil and take up roots so that the oak trees can propagate. On the flip side, during non-mast years acorn predators such as birds, mice, squirrels and chipmunks decline because the harvest is nominal. This "tactic" reduces the number of predators squirrelling away acorns during the mast years.

A second hypothesis is that masting trees are trying to maximize pollination efficiency. New evidence suggests that deciduous trees flower and release pollen at the same time in order to increase their chances of reproduction - and huge amounts of pollen is correlative with larger amounts of germination, and ultimately more acorn production.

Acorns begin formation one to two years (depending on the species of oak) before they fall, so it is the temperature and amount of rainfall at the time of formation that helps determine some variation in acorn production from year to year. However, scientists have proven that environmental factors are not what prompt the the prodigious crop of mast years.

The good news for your head is that mast-years occur every two to five years in oak trees, so you don't have to worry about being clonked on the head again for several years. But you might want to wear a hat for the rest of the harvest season.

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