Some intervention, such as removing competing plants or grazing animals, may be needed to give natural processes a kickstart. It does not necessarily involve sitting back and letting nature take its course. For this reason, the greatest potential for natural regeneration is in areas next to existing forest, according to the UN's Food and Agricultural Organization. Trees grow from seeds blown in by the wind, carried there by animals or birds, or from plant parts such as stems, leaves or roots. Distinct from active tree-planting, trees are allowed to grow back spontaneously, or with limited human intervention, on land where the original forest cover had been cleared for uses such as agriculture or destroyed by fire.

The method described by Tree is known as natural forest regeneration. The pre-Incan technology revived by Peru.The trees' growth was aided by thorny scrub that had also been allowed to grow at the farm, which acts as 'nature's barbed wire', protecting the saplings from nibbling deer and the estate's free-roaming cattle and ponies. 'Not a single tree was planted, no saplings were bought from commercial nurseries, no tanalised wooden stakes, no polypropylene tubes and plastic ties, no direct financial or carbon costs – no effort,' says Isabella Tree, co-owner of Knepp Castle Estate. Birds such as jays can disperse as many as 7,500 acorns in four weeks. Instead, the trees at Knepp Castle Estate in southern England were allowed to spread naturally. But they might be surprised to learn of the secret to this farm's success – none of these trees were 'planted' here at all. The transformation is the kind of story that many countries are aiming for with large-scale tree planting programmes, from India to the US to Ethiopia. Twenty years ago, these trees weren't here at all. Alder, hornbeam, ash and birch trees are also thriving. At a former intensive dairy farm in Sussex, England, oak trees now tower up to 20 feet tall (6 metres), sucking in carbon from the atmosphere, providing habitat for birds, mammals and insects, purifying air and water, and protecting land from flooding.

Wilding tree