An amazing mast

A mast year for nut-bearing hardwood trees is upon us, meaning there is a glut of acorns, black walnuts and other tree fruits. With that come the squirrels and chipmunks en masse.

“Bam! Crack! Bam-bam-bam! Clank! Bam! Thud!” You’d think I was watching a war movie or maybe visiting a construction site, but I was just walking my dog in my neighborhood. The acorns were dropping by the thousands from 60 feet, maybe even higher. The worst was when they hit some of the neighbors’ metal roofs and cars. Well, maybe not the worst; I shuddered at what it would feel like if one caught me on the noggin, especially if it didn’t hit a lot of leaves and branches on the way down.

This is what botanists and foresters call a “mast year.” “Mast” is the botanical term for the fruits of woody trees, shrubs and vines. Sometimes mast is soft, such as raspberries, and sometimes mast is hard, like beech nuts and acorns. And for many woody species, the individuals periodically synchronize their seed/fruit production, with some years very light productions and some years with tremendous production.

How often is periodically?

For oak trees, the timing of mast years is generally every two to five years. Why the variation? One reason is that there are around 600 different species of oak around the world, including about 200 here in North America. But, that variety of species doesn’t account for the variation in timing even within a species. Truth is, scientists don’t fully know how mast years are triggered. Some hypothesize that weather conditions or the availability of resources such as water and nutrients might be a contributing factor.

Why do the trees synchronize their acorn production?

Another hypothesis for mast years is that if the trees keep seed production low for a number of years, the populations of acorn eaters such as squirrels will be kept under control. Then, in a mast year of very high production, the squirrels will be overwhelmed by the high numbers of acorns and many acorns will go uneaten and turn into young oaks. The key to this strategy is for all the oaks in an area to produce low numbers of acorns together and high numbers of acorns together in those periodic mast years.

How do the oak trees synchronize with each other?

This is another not fully understood aspect of mast years. Perhaps there are cues from the environment to turn “off” and “on” the genes responsible for acorn production, but why would all the oaks have their genes turn off or on at the same time even if the local environments varied in the resources need for reproduction? Perhaps the oaks have some way to communicate with each other that this year is the right year to produce an almost unbelievable number of acorns? Don’t scoff at this idea! Scientists have been discovering in recent years that trees, in association with fungal threads called mycorrhizae, are able to communicate chemically in forest soils. The networks they form are remarkable in how trees of the same species and even of different species are able to “talk” with each other. It may not be the same as our self-conscious language, but there is communication going on.

Count your lucky stars

This year, my neighbors were moving their cars out of their driveways to keep the car roofs from getting dented. They were keeping their windows closed even on mild fall nights to lessen the 24/7 noise. For me, I was thanking my lucky stars that my dog escaped this fall uninjured and that I wasn’t seeing stars from getting conked on the head. And now, I know I can breathe easy for one to four more autumns before the “bams” and “cracks” come again.

Fred Kosnitsky has been teaching biology, ecology and environmental issues at Community College of Vermont over the past 35 years. He is a member of the inaugural class of the South Burlington Master Naturalist Program. You can contact him at with suggestions for future columns.

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