Naturalist

Mighty oaks from little acorns grow

What happens in spring to the millions of acorns which have spent winter buried beneath the forest floor? Devon Wildlife Trust’s Steve Hussey does some digging to find out.

Against the odds

The odds on any single acorn becoming a mature oak tree are very small. One estimate suggests that an oak will produce around five million acorns during a typical lifetime of around 300-400 years. Of these acorns, only a tiny number – some estimates suggest as few as half a dozen – will develop to become trees.

The first step along the reproductive journey is dispersal from the parent tree. Acorns are a key food for a long list of animals. For many acorns, being picked up by a hungry badger or fallow deer, will signal the end of life as they are quickly crunched and swallowed. However, the sudden autumn abundance of acorns often means there are simply too many to be consumed at once, so some woodland creatures choose to hoard them. The jays and grey squirrels we see collecting acorns are helping to disperse them, taking them sometimes hundreds of metres away from the parent tree to store elsewhere. Storing acorns often means burying them, out of the sight of others. This helps set the acorn on the next stage of its development. Acorns require ‘hypogeal germination’ – that is germination which takes places without light, beneath soil or leaf litter. It’s been estimated that a single jay will bury 5,000 acorns. Most of these they will return to and eat during winter, but a few are overlooked, and these will remain to begin germination in spring.

Spring breakthrough

Without light to generate energy via photosynthesis, and without roots to draw nutrients, each acorn relies on its own food stores. It does this to produce a single embryonic shoot which grows upwards towards the soil’s surface. When it breaks through that surface the shoot becomes a seedling, its first leaves appear, photosynthesis can start, and the young oak takes advantage of spring’s lengthening daylight hours and stronger sunshine. It’s now that the plant begins to establish a root system.

Acorns (Alan Price)

Jay with acorn (Margaret Holland)

The oak seedling encounters a series of other threats as it continues to grow. Drought and disease, grazing animals, fungal infections, frosts, the attentions of hungry insects and human dispensed herbicides, a tree’s passage through its early life is packed with challenges. This risk exposure isn’t fleeting, it takes place day-in, day-out over decades. Growth from seedling to sapling (defined as a tree more than three feet tall) can take around five years. To become a mature oak capable of producing its own acorns takes a further 35-40 years. It all makes being in the presence of a mature and mighty oak an even more wondrous experience.

Watch it happen

Watch the amazing emergence of an oak sapling captured using time-lapse photography by Neil Bromhall. Just search for ‘Acorn becomes oak tree in timelapse video’ in YouTube.

Steve Hussey Devon Wildlife Trust www.devonwildlifetrust.org