'A phenomenon that has shaped Britain’s landscapes and wildlife for millennia': A five minute guide to mast years

Mast year are one of nature’s cleverest survival strategies, turning quiet woodlands into unexpected treasure troves.

If you’ve recently noticed the ground carpeted with acorns or beech nuts then you’ve witnessed a phenomenon that has shaped Britain’s landscapes and wildlife for millennia. But what exactly is a mast year, why do they happen, and why is 2025 set to be especially abundant?

Every so often, trees seemingly go into overdrive, producing far more nuts and seeds than usual. These bumper harvests, known as mast years, are not random quirks, but carefully evolved strategies to ensure the survival of the next generation of trees.

Every species of tree and shrub has its own way of reproducing, but oak and beech are among the most striking examples. Every few years — the last one of note was in 2020 — they produce a superabundance of nuts — collectively called mast. When they drop, it's all rather dramatic: woodland floors covered with acorns or beech nuts, a spectacular and irregular display that adds to autumn’s pageantry.

So, why does it happen? One leading theory is predator satiation. By holding back in lean years, trees naturally regulate the numbers of animals that feed on their seeds, such as squirrels, jays and mice. Then, in a mast year, they produce more than predators could possibly eat — guaranteeing that some seeds are left untouched to germinate into the next generation of woodland.

Such bounty, of course, comes at a price because producing such a yield is exhausting, diverting energy away from growth. That is why mast years tend to appear only once every five-to-ten years. The short-term setback is outweighed by the long-term advantage of ensuring seedlings survive to maturity.

‘Trees have evolved this strategy to outsmart hungry animals, ensuring some seeds are left to grow. Remarkably, they don’t do this alone: underground fungal networks help entire woodlands coordinate their mast years in unison,' says Graham Makepeace-Warne, the CEO of Manx Wildlife.

For wildlife, mast years are a banquet: small mammals thrive, while birds and deer also benefit from the glut. For trees, it is a vital means of survival, a cycle honed by evolution over centuries. And for us, mast years are a reminder of the intricate rhythms of the natural world — one of autumn’s most fascinating spectacles, hiding in plain sight on the woodland floor.