2025 may be a mast year - but is that the whole story?

This autumn looks set to be spectacular for crops of acorns and other nuts, due to a natural phenomenon known as a mast year. Masting occurs every few years when certain tree species produce a superabundance of seeds to increase the likelihood of successful establishment. In all other years, these same species produce very few seeds. Such synchronous cycles of boom and bust have a significant impact on seed-eating animals and woodland ecosystems as a whole.

To mast or not to mast?

Though we tend to attribute all bumper crops in abundant autumns to masting, only around 0.1 per cent of global plant species have mast years. The reason mast years are particularly noticeable in woods and gardens in the UK is because three of our most common trees – pedunculate oak (Quercus robur), ash (Fraxinus excelsior) and beech (Fagus sylvatica) – are masting trees. Some conifers such as Norway spruce (Picea abies) and larch (Larix decidua) have mast years too, whereas others like Scots pine (Pinus sylvestris) tend to crop more regularly.

Why do trees synchronise mast years?

The advantage of synchronised mass seed production is explained by predator satiation – whereby seed-eating species are so overwhelmed with the quantity of food that they are unable to eat it all, so some is left behind to germinate. Then, during poor cropping years, seed-eaters struggle to find enough to eat, so populations sizes decrease. This leads to better survival rates for seeds in subsequent mast years.

Long-term studies on UK beech trees have found that in non-mast years, seed-eating moths, birds and mammals can consume 100 percent of the crop. On the other hand, in beech mast years, over half of the seed crop might remain to germinate and develop.

How do trees synchronise mast years?

The truth is that we still don’t know for sure how tree species synchronise mast years over large areas. Forest scientists believe weather cues are partly responsible for triggering mast years. Those trees that respond successfully pass on their genes to more seedlings and the trait spreads through the population via natural selection. Scientists also believe it depends on the internal dynamics of plants, in particular, how quickly they are able to build up their food stores again after the demands of a mast year.

Other botanists, such as Robin Wall Kimmerer, believe that communication via systems like chemical signals released into the air or mycorrhizal networks might also be responsible for trees coordinating their masting times to get the most benefit from the surplus of seeds they've all produced.

Why is it happening this year?

The last mast year was in 2020, with a smaller masting event in 2022. Normal cycles would suggest the next mast year is likely to be this year or next year. This year’s warm spring weather may have triggered a mast year in oak trees, as studies have shown that high spring temperatures synchronise oak’s flowering period, which then maximises pollination and seed production.

Is this the whole story?

There have also been reports of bountiful fruit and berries crops this year in gardens, woodland and hedgerows. After the UK’s hottest summer on record, fruiting trees like hawthorn and blackthorn have produced bumper crops that ripened earlier than usual. In our garden, the ‘Fiesta’ apple produced the most abundant harvest in nearly a decade and the National Trust has reported getting superb apple crops that have also ripened early. But is this bounty really down to a mast year?

While trees that produce nuts benefit from masting as a survival strategy, many species that produce fruits need animals to eat the fruit in order to disperse the seeds. For these plants, it makes more sense to balance their cropping to avoid fruit and berries being left on the tree. So the heavy apple and berry crop is likely a result of excellent growing conditions, rather than any masting strategy employed the trees themselves.

What does it mean for your garden and wildlife?

The mast year in 2025 means the crop next year is likely to be far smaller. However, this won’t necessarily be the case for plants that have cropped heavily for other reasons. In the coming months, mammals like mice, bank voles, squirrels and badgers will feast on the heavy nut crop, leading to population increases in some species. High numbers of small mammals like voles could then have a knock-on effect, boosting populations of predators such as kestrels and stoats.

Deer and birds like jays will also benefit from the abundance. And where moth larvae feed on the nuts, the abundance of these caterpillars could benefit other invertebrates and some bird species. In this way, the effects of mast years can be seen across ecosystems. In subsequent non-mast years, some populations will reduce in number again.

Is climate change affecting mast years?

Climate change is having a significant effect on the patterns of mast years. Beech is already showing signs of a reduced ability to synchronise nut production, likely because of changing weather cues, and this is leading to fewer seedlings becoming established. At a time when forest regeneration is more vital than ever, it's concerning that changing weather patterns are having detrimental effects on the synchronisation of mast years.