Curious Questions: Why have our oak trees produced so many acorns in 2020?

It's not your imagination: there really were far more acorns on the ground than usual this autumn. Martin Fone looks at the phenomenon of the 'mast year'.

Over the last year I have taken the opportunity to discover the attractions of my local area, shamefully neglected as hitherto I had been all too ready to answer the siren call of far-off, exotic lands. My quest for daily exercise has taken me to the extensive woodlands just a stone’s throw from my house.

Part owned by the Ministry of Defence and the local council, it is a mix of pine, oak, and sweet horse chestnut, interspersed with a carpet of bracken and heathland gorse and an avenue of rhododendrons. Large enough to allow an escape from the madding crowd, it does present some surprises. Used by the military for training exercises, more so now as even their ability to travel has been restricted, the sylvan calm can be punctured by the popping of gunfire, the sight of camouflaged soldiers and urgent exhortations to get out of the way. Its dense pine woods make it eerily atmospheric, perfect for attracting film crews who nervously protect their elaborately designed sets from the unwary boots of walkers.

“An oak will not start to produce acorns until it is around 50 years old and, over the span of its lifetime, will produce around ten million of them”

While the display of rhododendrons in springtime is spectacular, my favourite season is autumn with its vivid palette of colours, a mix of vibrant golds, rusts, browns, reds, and greens. The wind’s soft summer susurration has given way to a more sinister whistle and the deciduous leaves, forming a thick carpet which increases in depth and composition by the day, rustle and crackle under my feet. Even if only momentarily, the cares of the world seem a million miles away.

I have been surprised by how many fallen acorns and sweet chestnuts with their green spiky cases have littered the tracks, adding an extra dimension to the soundtrack of my walk. The gentle thuds as they hit the ground are intermingled with the sharp, intensely satisfying cracks, as their casings split under the weight of my boot. Is this apparent abundance of nuts, sufficient to satisfy even the most voracious army of frugivores, normal or yet another unusual feature of the year that was 2020?

The technical term for the nuts and fruits that our woodland trees produce is mast, derived from the Old English word ‘mæst’, which was used to describe nuts that had accumulated on the ground and then foraged by domestic pigs.

There are two forms of mast; hard, like acorns and beech nuts, and soft, like catkins and rose hips. By extension, the term mast year is used to describe a year when our woodland trees produce bumper crops of fruit. They occur infrequently, 2013 being the last for oaks, but 2020 looks as though it will be another one. Our forefathers, whose livelihood and well-being were interlinked with the successful fattening of their livestock, were acutely attuned to the peaks and troughs of mast production. However, to this day its mysteries are still not fully understood.

Trees on farmland near Spennithorne, Wensleydale, Yorkshire Dales. The tree in the foreground is oak.. Image shot 05/2010. Exact date unknown.

An acorn is how an oak propagates itself, each nut containing usually just one seed, enclosed in a tough leathery shell, sitting inside a cup-shaped cupule. If the seed germinates, it pushes out a taproot that will anchor the tree for the duration of its existence. The problem, though, is that each seed is full of energy-rich starch, which, while designed to give the seedling its first vital nutrients, presents an irresistible snack for the squirrels, beetles, jays, and the occasional deer that make the wood their home. If the tree is to improve its seeds’ chances of survival, a clever strategy is to overproduce so that there are more acorns than the hungry foragers could possibly devour.

For the oak over production brings a further benefit. Adult trees can grow up to 45 metres tall and spread almost as wide and can take up to seven hundred years to reach old age. Its longevity and size mean that it needs to ensure that its seeds travel some distance away. When there is an abundance of acorns, squirrels will obligingly carry them away and bury them rather than eat them immediately, thereby increasing the range over which the seeds are dispersed. The seed only needs to fall into the hands of an amnesiac squirrel or one that falls into the clutches of its own predator to increase its chance of survival. An oak will not start to produce acorns until it is around 50 years old and, over the span of its lifetime, will produce around ten million of them. Even so, precious few make it even to the sapling stage.

Producing a bumper crop of acorns comes at some cost to the tree, making significant inroads into its store of sugars and starch. To replenish its store of starches, the oak needs a period of recuperation, forcing it to concentrate on less demanding activities, such as increasing its production of leaves and wood. If 2021 follows the precedent of 2014, then acorns are likely to be thin on the ground. While the branches of oaks were bowing under the weight of their crops of acorns in 2013, the following year the Forestry Commission were reporting that there were barely any.

“That 2020 is a mast year is a reason to celebrate, a welcome relief from the doom and gloom”

As well as providing the oak with a well-earned rest, the masting cycle also has an impact on the population of those creatures that feast on acorns. Although a mast year will mean that predator populations increase, a run of lean years will put pressure on numbers so that by the time the next mast year occurs, there are fewer predators to eat the seeds, thereby further enhancing the chances of the seeds germinating. Clever, really.

It is still a matter of some debate as to why mast years occur precisely when they do and why all the trees seem to synchronise their behaviour. One theory is that weather conditions have an important part to play in the process. A cold spell during spring will freeze the tree’s flowers, upon whose pollination it depends to produce acorns. Similarly, a dry summer will kill the developing seeds and force the oak to close the pores in their leaves to conserve water, reducing its ability to make carbon dioxide for photosynthesis. This spring was dry and warm and the summer wet, ideal conditions for the oak to develop its crop of acorns. As all the trees in a particular area experience the same weather conditions, it makes sense that these climatic triggers will influence their behaviour in the same way.

Intriguingly, some scientists think that the trees may synchronise the mass production of fruits by releasing some form of chemical signal, as they do to kick start defensive mechanisms when neighbouring species are damaged. Whatever the reason, our local frugivores have had a bumper crop of acorns to feast on and, who knows, some seeds might even survive to produce the next generation of the mighty oak, that most distinctive and ancient symbol of our woodlands. Let us hope so.

That 2020 is a mast year is a reason to celebrate, a welcome relief from the doom and gloom that has accompanied much that has occurred in this most unusual of years.