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What is a "mast year" and why do I have a bumper crop of acorns?

by Jim Binnings, Collin County Master Gardener

Question of the month: Every few years my neighbors and I ask each other the same question. Where are all of the acorns coming from? Some years, my yard is covered in acorns. Can you help us understand what is going on?

What you and your neighbors are experiencing every few years is a “mast year.” The term “mast year” comes from the botanical term, mast, for tree fruit—seeds, nuts, acorns, etc. Nuts and acorns are considered “hard mast,” while berries and apples are “soft mast.” A mast year occurs when trees produce an overabundance of fruit.

I started reading and researching “mast year” and while there are many theories, at the end of the day all the experts have are theories. The trees just aren’t talking. They are keeping their secrets to themselves. I did find three theories that kept coming up in my research: 1.the weather, 2.the resources of the tree, and 3. predation. Let’s start with what we do know. How is an oak tree acorn formed?

Jim Finley, of Forest Resources Management at Penn State says it this way: Both red and white oaks produce female and male flowers on the same tree. The process of producing an acorn starts late in the growing year when the male flowers form as the tree’s growth slows toward the end of summer. That is the end of the first year (year 1) in the process. Then, in the second year (year 2), as the tree comes out of dormancy, female flowers form in the axil of the leaf stem and the twig and remain dormant. As the spring leaves begin to unfold, the male flowers emerge and are very apparent as rather-long, drooping, greenish-yellow catkins. These appear about two weeks before the much smaller female flowers emerge. Once the male and female flowers are present, pollination should get started. In ideal conditions, an acorn will form in two years.

And now the theories. Most experts agree the weather seems to have a large impact on the successful production of acorns. If there is a drought during the summer in year one of the process, the acorn formation process will suffer. And if there is above average spring rainfall and late spring freezes in year two of the process, pollination rates and acorn formation will fall.

Experts also agree that it takes a lot of a trees resources to produce a large number of acorns. If the tree uses its resources in one area, it will not have enough resources and energy for other processes the tree needs to live. So, it makes sense that it would be in the trees best interest to produce a large number of acorns every few years when it has the resources and energy available.

And now the interesting theory of predation. Predation is defined as the killing of one organism for food for another organism. In the case of acorns, predation by seed-eaters like squirrels, deer, turkey and even weevil larvae can greatly reduce the number of viable acorns. It may take a very large acorn crop to have many acorns escape from the numerous species that depend on acorns for food. This irregular cycle of large crops can be beneficial for the oaks by overwhelming the seed eaters. Populations of wildlife that depend on acorns may eat most of the seed during normal seed crops, but may not be able to utilize all the seed produced during a mast year. This surplus seed is available to produce the next generation of oak seedlings (Purdue.Edu, Forestry and Natural Resources Dept, 10-11-21).

How do the trees know a mast year is coming? Something is going on. Cycles emerge in mast years occurring around 2-5 years for trees like oak. One theory is that trees are using chemicals to communicate with each other. Another is pollen coupling, where trees synchronize through pollen exchange. Again, the trees aren’t talking so the experts can only theorize.

While we can enjoy thinking about all of the possible explanations for masting, there is one undeniable truth. Masting helps ensure the survival of many plant species. And as Jeremiah Sandler, an arborist for Royal Oaks Michigan said, there is another benefit of masting. It gets people outside looking and wondering about their trees.   

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