Mast years

The park’s oak trees have produced more acorns this year than any of us can ever remember. These periodic bumper harvests are called mast years.

Mast years, which happen every 5 – 10 years, have a major evolutionary advantage for the oak tree. While producing so many acorns is costly work and reduces the tree’s growth that year, it’s worth the effort if some of the acorns germinate into new saplings that colonise new ground.

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But all those acorns fall straight down into the shadow of the parent tree, sometimes, like this year, in a layer so dense that they crunch underfoot. How does the tree ensure that the acorns do not germinate where they fall, growing into light-deprived saplings and competing with the mature tree for resources?

The tree has a plan: the tree is exploiting the food-caching habits of squirrels.

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Squirrels store acorns by burying them, and in mast years they store many, many more than they can eat. They also cache acorns further away from the source tree when there is less competition with other squirrels: there are not going to be fights for food when the acorns are centimetres thick on the ground. A squirrel will have time to carry its tasty find away, out of the sight of other squirrels, and bury it in, for instance, the middle of Sleeper Field.

For the energy that the tree has put into this year’s bumper harvest, it will be rewarded with uneaten caches of its seeds, carefully planted in open ground, ready to germinate when the winter is over. The squirrels will be rewarded for all their acorn-planting with a well-fed baby boom next spring

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For this symbiosis to work properly, all the oak trees in an area have to have a mast year at the same time. If each oak tree in the park was on a different timetable, the squirrels would just move from one bumper-cropping tree to another, leaving no surplus acorns and maintaining their population at a level that might eventually damage all the trees. Between these shared mast years, squirrel populations drop back to a sustainable average.

 We used to think that the signal for a mast year came from weather patterns, rainfall and temperature ranges, but now that we know that trees can communicate with each other through the mycorrhizal fungi that live around or in their roots, we are not so certain.

We love the thought that the park’s oak trees might have planned this mast year together.