The bracken-clad hills are marked “Dundreggan forest” on the map but this Scottish glen is mostly stark Highland scenery: open, beautiful, and almost totally devoid of trees.

On a steep-sided little gully, 40 years ago, a few baby silver birches escaped relentless browsing by red deer and grew tall. Now, the nearby path through the bracken is dusted with thousands of brown specks: birch seeds.

“Each year, this ‘forest’ produces trillions of birch seed,” says Doug Gilbert, the operations manager for the charity Trees for Life at Dundreggan. “Until we reduce the deer pressure, not a single one has grown into a tree. Once we get the deer population right, this forest will absolutely take off. It’s starting to do that now.”

The charity purchased the Dundreggan hunting estate 11 years ago. Slowly – “at tree speed,” smiles Gilbert – it is rewilding 4,000 hectares (10,000 acres) of this degraded Highland landscape, restoring a diversity of native trees, scrub and associated life, from the dark bordered beauty moth to black grouse and, yes, red deer.

Trees for Life is one of four charities chosen for the Guardian and Observer’s 2019 climate emergency appeal, each of which is committed to renewing nature and the planet by planting and protecting trees, forests and woodland.

Increasingly, Trees for Life’s aims – to revive Scotland’s native forests with practical rewilding that includes Scottish people at its heart – are chiming with the times. During the general election campaign, politicians desperately tried to outbid each other with tree-planting pledges. Who doesn’t love a tree? More trees can tackle the climate crisis – absorbing carbon dioxide – and the biodiversity crisis. But Trees for Life’s efforts reveal it is not quite so simple.

Since Victorian times, when the sheep estates that followed the Highland clearances were replaced by more lucrative deer hunting estates, the landscape, and economic model, has been shaped by red deer. Around Dundreggan there are also non-native sika and roe deer. What many see as a wild, natural landscape – the treeless mountains – is engineered by deer, whose grazing of tasty young shoots prevents old trees being replaced by young. Less than 2% of Scotland’s native Caledonian forest – dominated by Scots pines – remains.

The first step at Dundreggan has been to increase deer culling. Ecologists calculate that a red deer population of five per sq km in the wider landscape will allow natural regeneration; in many Highland regions it is 20. But culling deer is controversial because the value of stalking that estates base on deer numbers. Trees for Life has proceeded slowly with culling, seeking positive dialogue with neighbouring stalking estates. They’ve also tried non-lethal methods such as bagpipe-playing volunteers acting as nocturnal deer scarers. Trees and deer can coexist and Dundreggan’s deer population is now at a level where some young birches, pines, rowans and junipers will grow tall.

“We would like to let the trees come on their own,” says Steve Micklewight, the chief executive. “But because this land has been so bare for so long, there isn’t the diverse seed source to make this happen.”

So, helped by volunteers, Trees for Life has planted nearly 2 million native trees so far at Dundreggan and on its other Scottish projects. It wants to plant millions more.

All the trees come from Scottish seeds – meaning they are suited to Highland climates and species, as well as being free of novel diseases. Half have been grown from seeds collected around Dundreggan. Its on-site nursery bristles with 94,000 saplings.

Doug Gilbert and Steve Micklewright carry seedlings destined for Dundreggan’s slopes. Photograph: Murdo MacLeod/The Guardian

Seed-collecting is not as simple as it sounds. Seed must come from a wide variety of individual trees to ensure genetic diversity. Cones from Scots pines have to be harvested before they drop to the ground, so specialist tree-climbers are employed. Trees for Life specialises in growing non-commercial high-mountain species such as woolly willow and dwarf birch. Surviving specimens are often only found on cliffs and crevices – with seeds or cuttings only retrievable by specialist climbers.

Because of the deer grazing, every sapling is planted within a fenced enclosure (costing £10 per metre). Fencing is “a little bit of an admission of failure,” says Gilbert. In the long term, when reducing deer numbers becomes less controversial, trees won’t need fences. Gilbert hopes the fences will last 30 years, when the well-established trees and scrub will survive browsing deer.

At first, Dundreggan’s biggest fenced enclosure looks dominated by bracken. But crouch down, and the horizon is fuzzy with young trees: downy birch, rowan, and plump little Scots pines. There are no straight lines – it is emphatically not a “plantation” like the rigid blocks of non-native sitka spruce on the far side of the glen. (Trees for Life are concerned that politicians’ tree-planting pledges will lead to new non-native commercial plantations, which are disastrous for most wildlife).

At Dundreggan, life is beginning to return. A black grouse shoots up and a raven scuds on the wind, prospecting for the small mammals and young birds in the burgeoning scrub. Gilbert has a better word for scrub: elfin forest.

“It’s getting to the blue touch-paper stage,” says Micklewight. “You plant the trees and not much visible happens for seven years as the roots slowly build. Then it suddenly moves quite fast. We’re just about to see that at Dundreggan. The trees in the ground enable the landscape to take off.”