Why Oaks Sometimes Drown the Wood in Acorns

Walk through a mature oak wood in late September and you will hear it before you see it: the soft tap of acorns striking leaves, then the crunch of them underfoot. Some years there is a scattering. Other years the ground is paved with them, and every gust brings down another wave.

That variation is deliberate. Pedunculate oak and sessile oak do not crop evenly. They produce a heavy mast roughly every two to five years, sometimes with longer gaps, and trees across a whole region tend to synchronise. The reason is simple maths. Squirrels, jays, mice and weevils can eat a modest crop many times over. In a mast year, the sheer volume overwhelms them, and a proportion of the crop survives simply by being surplus to requirement.

A warm, dry spring and a sunny summer help. So does the synchrony itself: when every oak in a valley flowers at once, pollination is better and the resulting acorns are more likely to be viable. Lean years are not failures so much as part of the rhythm that makes the fat years work.

Who Eats Acorns, and Who Buries Them

An acorn is a packet of fat and carbohydrate with a tannin coat. Plenty of animals want it, and they do very different things with it afterwards.

• Jays are the most important dispersers. A jay can carry several acorns at once in a throat pouch, fly a few hundred metres or more, and hammer each one into the soil as a winter cache.
• Grey squirrels scatter-hoard, burying single acorns in shallow caches. They often bite out the embryo tip first to stop it germinating, which kills that acorn but leaves others to sprout.
• Red squirrels cache too, frequently in damper, more reliable spots where acorns stand a better chance.
• Wood mice and bank voles move acorns only a few metres, into burrows and under logs, and lose a fair number along the way.
• Deer — fallow, roe and muntjac — eat acorns where they fall. They disperse nothing, and heavy browsing can wipe out natural regeneration across whole woods.
• Domestic pigs still do the job in the New Forest, where commoners turn out pigs during pannage, usually from late September into November, to clear acorns that would otherwise poison ponies and cattle.
• Insects get in early. Acorn weevil larvae develop inside the nut and chew a neat round exit hole before it falls. Knopper galls twist and deform acorns on pedunculate oak, and the acorn moth and various gall wasps complete the list.

The Accidental Foresters

Jays have arguably done more for the British oak than any forester. After the last ice age, oaks spread north across the land faster than they could have done on their own, and jays are the likeliest reason. A single bird may cache several thousand acorns in an autumn and recover only a fraction of them. The rest are planted.

What makes jays so effective is where they choose to bury. They favour short turf, hedgerow edges, open woodland margins and rough grassland — precisely the places where an oak seedling can survive. Acorns that simply drop beneath the parent tree usually land in deep shade and thick leaf litter, where germination is poor and competition is fierce. An acorn carried a few hundred metres to an open field edge has far better prospects.

Squirrels contribute too, in a messier way. Their shallow caches dry out, get raided, or are simply forgotten, and enough survive to matter.

From Acorn to Sapling: The Odds Stacked Against It

Acorns are awkward seeds. Unlike many, they cannot be dried and stored — they need to stay damp, and they lose viability quickly once they desiccate. This is why burial is so valuable: it keeps the acorn moist, hides it from mice and tits, and shelters it from hard frost.

Germination begins in spring, with a root pushing down first and the shoot following. From there the hazards pile up: browsing by deer, rabbits and livestock; competition from grass, which is a surprisingly fierce enemy of young oaks; drought in the first summer; and the shade of everything above.

Of the thousands of acorns a single tree drops, only a tiny number ever reach the canopy. That is not a failure of the system. It is how a woodland replaces itself slowly, in fits and starts, often in the decades after a gap opens up.

Reading the Signs on an Autumn Walk

Once you start looking, an oak wood in October tells you a great deal about what is going on.

• Acorns with a single neat hole: weevil larvae have left.
• Crinkled, sticky, misshapen acorns on pedunculate oak: knopper gall.
• Acorns with the tip nibbled off and the rest discarded: squirrels.
• A jay flying low between an oak and a hedgerow with a visibly full throat: caching underway.
• Clusters of small oak seedlings in rough grass well away from any mature oak: caching that worked.

Growing On the Next Generation

If you want to give the process a hand, autumn is the moment. Collect acorns in October and discard any that float or carry an exit hole — both are signs of weevil damage or an empty shell. Plant the rest straight away, about two to three centimetres deep, either in deep pots of soil-based compost kept outdoors, or direct into a prepared patch of ground.

Then protect them. A mesh guard or tree tube against rabbits and voles, and a stake if deer are about, will see most seedlings through. Keep the grass and weeds cleared around each one for the first few years; that single task does more for a young oak than anything else.

For anyone managing a wood, the lesson from the jays is worth remembering. A lack of new oaks is rarely a shortage of acorns. It is usually heavy browsing, dense shade, or a lack of open ground and hedgerow nearby where a caching bird will choose to bury. Leave a seed source, keep deer pressure in check, and leave some rough, unkempt edges. The jays will do the planting.