Earlier this year the government announced plans for a new northern forest. Under the auspices of the Woodland Trust, an area that stretches 120 miles from Hull to Liverpool will be planted with 50 million trees over the next 25 years. In March, the first trees were put in the ground on hills above Bolton, in Greater Manchester, by an “army of planters” composed of local community members, volunteers, funding bodies and MPs.

There is, certainly, an urgent environmental need to get trees back in our landscape. We lost more ancient woods – tens of thousands of them – in the 40 years after the second world war than in the previous 400 years. The UK has only 13% woodland cover, compared with the European average of 37%. Quite apart from being beautiful things, trees – like all vegetation – absorb CO2 from the atmosphere, and the northern forest is intended to help the UK government meet its carbon emissions target by 2050.

Better for biodiversity and the taxpayer would be to allow insects, mammals, the wind and birds to do the tree-planting

But there’s a much more effective way of re-establishing trees in the wider countryside than putting a spade in the ground – and one that has far greater benefits for biodiversity, carbon sequestration and the trees’ health, and that would save us a fortune in financial and environmental costs. And that is natural regeneration – let the trees plant themselves.

The reason this doesn’t happen nowadays is our demonisation of thorny scrub. Hawthorn, blackthorn, dog-rose, gorse and bramble are nature’s barbed wire. These species provide saplings with protection from browsing by deer and rabbits; shelter from the wind; a buffer against drought and excessive rain; and nutrients via mutual mycorrhizal connections underground. Thorny scrub is also one of the most biodiverse habitats – providing a rich source of berries, seeds, nectar, pollen and nuts for small mammals, insects and birds, as well as protection from predators.

But we’ve forgotten about this precious resource in recent times. In the days before plastics and mass manufacture, thorny scrub provided us with charcoal, medicines, dyes, food, fodder, thatching, furniture, gunpowder and tools. Above all, it was valued as a nursery for trees. “The thorn is the mother of the oak” is an ancient forestry saying. Thorny scrub was considered so valuable that a 1768 statute in the New Forest imposed three months’ forced labour and lashes of the whip on anyone found extracting it. Now, however, we consider scrub “wasteland” and eradicate it wherever it appears, and even on nature reserves, volunteers spend weekends“scrub-bashing”.

The modern system of establishing trees relies entirely on humans, and saplings produced in nurseries. Even with a volunteer workforce this is expensive. Ultimately the northern forest is expected to cost £500m. And it is manifestly inefficient. Commercial saplings – bare-root “whips” – are vulnerable, and can easily dry out and die before, or even after, they’re planted. They are not as well connected to the soil as naturally established seedlings, and often lack the appropriate fungal associates. They need to be individually protected by tree guards, almost invariably carbon-intensive polypropylene cylinders tied with plastic ties to treated wooden stakes. Even if the areas to be planted are protected from deer by a perimeter fence, tree guards are poor protection against burrowing rabbits and voles, which cannot be fenced out. Humidity inside the tubes can induce rot and mildews. Using saplings from commercial nurseries also carries a risk of importing disease. Chalara, or ash dieback, was spread into the UK this way.

Facebook Twitter Pinterest ‘Thorny scrub, like gorse, is one of the most biodiverse habitats there is – providing a rich source of food for insects and birds, and protection from predators.’ Photograph: Sheree Palmer/Alamy Stock Photo

And anyway, is one enormous plantation what we want for our northern forest? Closed-canopy woodland is remarkably species-poor. Even ancient woodland is not the biodiverse wonderland it is often held to be. Species monitoring in Lady Park Wood in the Forest of Dean and the Mens Nature Reserve in Sussex – considered to be some of our oldest surviving stands of trees – shows a downward trajectory for biodiversity. There may be saproxylic (dead-wood-specialising) fungi and beetles in these densely shaded woods, but there is precious little else. With no sunlight, there’s no birdsong, no thrum of pollinating insects, no wildflowers.

Herein lies the great “forest” misconception. Underpinning the northern forest project is the idea that Britain was originally ubiquitous closed-canopy woodland. Before human impact, it’s said, a squirrel could have run from John o’Groats to Land’s End on the tops of trees. Dense forest tends to be the state we aspire to when aiming for nature restoration. But this is the stuff of fairytales. Many of our shrubs and trees – including hazel, wild apple, wild pear, wild cherry, rowan, dogwood, common privet, Scots pine and oak (that quintessentially English tree) – are light-demanding species that cannot regenerate in closed-canopy conditions. They point to a much more open landscape – a wood-pasture system driven by free-roaming herds of grazing and browsing animals, where trees regenerated out in the open, within thorny scrub. This is the landscape described by Oliver Rackham, our late, great woodland expert, as the “wildwood”. It is the landscape of the New Forest and all the other hunting forests of medieval England – a dynamic mosaic of gigantic open-grown trees and groves of all species and ages, grazing “lawns”, water meadows and thorny scrub. Paradise for wildlife.

Far better for biodiversity and the taxpayer would be to allow small mammals, the wind and birds to do the tree planting for us. A single jay can plant 7,500 acorns in four weeks. Carrying these highly calorific oak nuts in its gullet, six at a time, the jay flies away to bury them a commodious distance from the parent oak tree, often in, or near, protective thorny scrub, as a store for feeding itself through the winter and its young in spring. The acorns that are not recovered by the jay end up developing into saplings.

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Rewilding – enabling natural processes to function as they are meant to – allows the jay and all the other seed dispersers to do their job. But this is not something that organisations like the Woodland Trust have embraced with any real enthusiasm. Their fundraising model depends on grant aid, and the messy, competitive, variable responses of nature do not fit within a grant system that requires precise costs, targets and predictability. The appeal of physically digging a hole and planting a tree is also a powerful way of recruiting donors and volunteers. If charities simply left it to nature, a major means of forging those relationships would vanish.

It’s encouraging that in June the government appointed its first “tree champion”, William Worsley, the former president of the Country Landowners Association. As well as protecting existing trees he will spearhead the planting of millions of new ones. But I urge him to rethink how we do this – and to consider areas where we could return to the richer forest systems of old, to the wildwood, a landscape naturally imbued with spirit of place, where the thorn can once again be the “mother of the oak”.

• Isabella Tree is the author of Wilding: The Return of Nature to a British Farm