Have you noticed a bumper crop of acorns this year? You’re not alone – trees are having what’s known as a mast year. The bountiful harvest is a clever strategy to ensure the future generation of oak trees and its welcome food for wildlife too. But how and why do trees produce a mast year?

Every species of tree and shrub has a distinctive way of reproducing.

Every few years, some species of trees and shrubs produce a bumper crop of their fruits or nuts. The collective term for these fruits and nuts is ‘mast’, so we call this a mast year.

Two of our most recognizable trees, oak, and beech, fluctuate massively year on year in the number of acorns and beech nuts they produce. Some years seem to have very little while in others, the nuts create a thick carpet beneath the trees. 

One of the main theories for this behavior is ‘predator satiation’. Take oak and beech as an example again. Animals like squirrels, jays, mice and badgers feed on the acorns and beech nuts. When the trees produce smaller crops for a few consecutive years, they are in effect keeping the populations of these animals in check. But during a mast year, the trees produce more food than the animals can possibly eat.

This abundance causes a boom in populations of small mammals like mice. More importantly, it guarantees some will be left over to survive and grow into new trees. Mast years have a major evolutionary advantage for the tree. Producing nuts is costly work and slightly stunts the tree’s growth, but as it tends to happen every 5-10 years, it’s worth the payoff for some of the crop to germinate into new saplings. 

Mast years are not just one off events for individual trees. The vast majority of trees in a particular species will have a fantastic crop in the same year. How the trees co-ordinate this when they’re so far apart is one of nature’s many mysteries.

We don’t yet know exactly how trees communicate with each other and we can’t predict when any one species will have a mast year, but we do understand better what causes it. Weather certainly has a part to play. To produce a healthy crop, the trees need the right combination of temperature and rainfall in the spring. Phenology – the study of the timing of natural events in relation to the weather – can help us better understand this.