Mast seeding: origin and ecological consequences

Many trees show mast seeding and fruiting in which there is a huge crop one year followed by years or low productivity. Here is discuss how this might arise and why it is important.

2025 is a remarkable year for oak trees — and their acorns. Across much of the landscape, oaks are producing an extraordinary abundance of acorns, a phenomenon known as a mast year. The term “mast” comes from Old English mǣst, referring to the forest food that once sustained pigs. Mast years are where trees synchronise seed or fruit production, alternating between years of scarcity and years of abundance.

But why do trees do this? One explanation is the predator satiation hypothesis: in years of low acorn production, up to 80–90% of acorns may be consumed or parasitised by acorn weevils. In mast years, however, the sheer number of acorns overwhelms predators, leaving many seeds untouched and able to germinate. Synchrony between trees is key, reinforced by environmental cues (the Moran Effect) and natural selection.

Yet the story doesn’t end with predation. Acorns are also dispersed by animals — in Britain, wood mice and jays, and in the Americas, squirrels. These animals bury acorns for later use, and forgotten caches often sprout into new seedlings. Mast years amplify this dispersal, boosting regeneration.

The ecological ripple effects are profound: small mammal populations rise, predator numbers shift, and communities change. Mast seeding is a hallmark of abundant, dominant species, like oaks, influencing forest structure and dynamics at large scales.

Filmed at Cavenham Heath NNR. Thanks to the warden, Chris Hainsworth.