Mater Gardener: Phenomenon of trees’ ‘Mast Year’

University of Minnesota Master Gardener, Brown County

Of all the horticulture related terms I have read over the past years, I had never heard of a ‘mast year’ or ‘masting.’ We even went through a mast year in 2015 and commented on this several times, but did not know it had a name.

So what are we referring to? Mast is the botanical name for the nuts, seeds, buds, or fruits of trees and shrubs that are eaten by wildlife. There are two main types of mast: Hard mast includes hard nuts and seeds such as acorns, hickory nuts, and walnuts. Soft mast includes berries and fruits such as crabapples, blueberries, and serviceberries. The definition of mast is sometimes expanded to include the winged seeds of trees such as maple and elm, as well as pine seeds and nuts and even buds, hips, and catkins such as rose hips. Both types of mast are important year-round food sources for wildlife.

All trees have boom crops of seed some years and generate a minimal crop other years. A ‘mast year’ is when trees go on a reproductive binge and produce a bumper crop that inundates the forest floor with nuts and seeds. Nut and seed-bearing trees like oaks, white spruce, sugar maples, beech and hickory trees all have mast years. Mast Years (also known as masting) happen anywhere from two to five years apart and when they do, all of the wind-pollinated, seed bearing trees join the fruit and nut bearing trees in a reproductive free-for-all that can cover hundreds of square miles.

So how do the trees synchronize so that all of the trees in a region will equally overproduce seeds or nuts? The ‘how’ and ‘why’ so many different kinds of trees get in synch for a mast year is a mystery. The University of California did an 11 year study of 10 separate sites over 434 miles and found that masting occurred in virtually every one of approximately 100 million trees. Temperature, rainfall, and predation may factor into the amount of crop, however the true reason for this phenomenon has baffled scientists for hundreds of years.

One theory suggests that since acorns are a major food source for many 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 take root so that the oak trees can propagate. During non-mast years acorn predators such as birds, mice, squirrels and chipmunks decline because the harvest is nominal. This reduces the number of predators storing acorns during the mast years. Supporters of this “predator satiation” theory point to long-term seed production records and note that in some tree species, seedling establishment is virtually confined to mast years. The predator satiation theory has many supporters, and over the years they’ve tested it and refined it, adding new elements to the case. Some scientists, for example, have suggested that the interval between mast years may also be necessary for trees to recover their energy and mineral reserves from the last high-output year and to accumulate enough energy for the next one.

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. The more trees that flower at once, the better the chance for pollination, and the greater the proportion of filled, viable seeds that result. It is an economy of scale, favoring large, occasional outputs of seed rather than frequent, small ones.

Another theory offers that masting behavior is caused by wide-ranging climate conditions, such as the periodic fluctuations of temperatures (perhaps caused by the cycle El Nino). This could explain masting over a large geographic area. However, scientists have proven that environmental factors are not the only reason for mast years.

Determining when a mast year occurs and what causes it is further complicated by the fact that most of the acorns or nuts are formed in a two-year cycle (depending on the species of oak). This suggests that if there is a weather connection, it could apply to the year before an actual bumper crop. 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.

In 2015 we noticed that our oak tree had hundreds, and hundreds, and hundreds of acorns. We commented to friends that we had never had so many acorns on the tree. In researching mast years, I found that 2015 was a mast year in our area. So, what triggers a mast year? Scientists do not know. So for now we should just enjoy the mystery and marvel at how ‘mother nature’ makes sure her trees will multiply.

Sources: Dr. Marc Abrams, a professor of forestry at Penn State; California Scientific Study and UC Berkeley study; MIT Study – William Needham, naturalist; University of Minnesota – My Minnesota Woods — Agroforestry


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