oak mast year


It appears that our oak and Ohio buckeye tree (and all nut trees in the area) are producing a huge amount of nuts this year. Can you confirm this year as a mast year? Can you explain why this phenomenon occurs?


Thank-you for asking Toronto Master Gardeners your question. Based on the numbers of acorns falling and on the ground right now in Toronto, I think we can be confident that this is indeed a mast year!

The term ‘mast’ refers to the fruit/nut production of any forest tree or shrub.  A “mast year” is when the trees or shrubs produce far more fruit or nuts than usual. These years occur irregularly and vary by species, approximately every 3 to 5 years in most oak species. The full explanation of the phenomenon of mast years remains a mystery, but there are a number of interesting facts and theories regarding these irregular, but extraordinarily productive years.  A single oak can produce as many as 10,000 acorns in a mast year! The enormous amount of energy needed to produce these acorns means little regular growth in the tree for that year. Growth will resume its normal course in the following year when few or no acorns are produced.

Environmental factors alone do not account for a mast year. Weather can affect pollen production in a cold, wet spring, or acorn development in a hot, dry summer, but neither of these issues can account for the massively different numbers of acorns in a mast year versus a “normal” one. Acorns are a key food source for many forest animals. In the low-yield years, the population of predators such as squirrels, chipmunks and deer will dwindle due to lack of food. This means that there are fewer predators around to eat the acorns in the mast year. A fairly high proportion of seeds will survive and sprout. This phenomenon is called “predator satiation.” The year following the mast year, there will be a surge in animal populations, but this will be quickly controlled by the lack of acorns in that and subsequent years.

One of the most perplexing aspects of mast years is the question of how the trees of a particular species in a particular area coordinate their mast years. Predator satiation really only works if all of the oaks over-produce at once. The fact that oaks rely on wind-blown pollen suggests that a mast year is only possible in a year with warm, windy weather so that a mass fertilization is possible. Naturalists have postulated that there may also be chemical signals (either air-born or through soil/root/fungal connections) between the trees in an area, but this has not been proven scientifically. What is sure is that all of the trees of one of more species within a fairly large geographic area (i.e. Toronto) have achieved a “synchrony”, and a major mast year in 2023!

Check out this article in Botanical Journey, Oaks & acorns: the mystery of the mast, for a great overview of oak mast years. This answer to a similar question to yours was published in this useful New York Botanical Garden FAQ. A bit more commentary on “synchrony” amongst the trees in an are is available in this Charlotte News article.