Driving through part of Northumberland, you might look around at the tall Sitka spruce and imagine yourself in Canada’s evergreen forests, or perhaps, on a sunny day, in northern California. Instead, you are in England’s largest forest, Kielder, often heralded as a success story that balances commercial production with ambitious conservation.

The first trees of this 60,000-hectare forest were planted 100 years ago with one aim: increasing Britain’s timber reserves. Much has changed since then. From a single-use plantation, Kielder Forest has been transformed into a haven for nature and an invaluable environmental asset.

In the spring of 1926, the newly created Forestry Commission was tasked with solving a security issue that had emerged during the first world war: with Britain’s woodland cover at a record low of 5%, and timber an essential building resource, the UK needed more trees.

And so hours of physical labour were poured into planting, with thousands of unemployed ex-servicemen put to work. Eventually, 250 square miles of tree cover were planted.

From the 1960s, people began to realise the forest could serve purposes other than timber production and could be a valuable location for carbon storage and wildlife habitats. This was the beginning of Kielder’s transition to a mixed-use venture no longer about maximising the commercial value of each inch of land.

By planting a more diverse range of trees, protecting rare ecosystems such as peatland bogs, and conducting targeted species reintroductions, Kielder was transformed into a haven for many significant plant and animal species.

Tom Dearnley, the forest’s ecologist, describes the ospreys, goshawks and kestrels that have made homes in Kielder, along with water voles, otters and what is thought to be half of England’s remaining red squirrel population.

Kielder’s wildlife manager, Paul Pickett, helped to make the forest’s first osprey platform in 2008. He describes the moment when he realised a breeding pair had settled on the platform as “magical”.

“It’s a great honour to be involved in things like that,” he says. The pair, which came in 2009, were the first in Northumberland for 200 years.

In his 25 years working at Kielder, Pickett says everything has changed. As trees are planted and cut down, the forest is always shifting, opening up new opportunities for wildlife.

And the benefits go beyond the edges of the wood. “We’ve got a really good population [of ospreys] here that’s starting to filter out to other parts of the north of England,” says Pickett.

The Forestry England team at Kielder has an ambitious vision for what the forest will look like in the future. It involves cutting back the forest border around waterways to create wildlife corridors, and a 6,000-hectare chunk designated for conservation called “wild Kielder”.

At the same time, their commercial operation is being reconsidered in light of the climate emergency. Looking at how disease has struck German commercial forests, Mark Holroyd, Forestry England’s north district director, sees how important it is for the UK’s forests to be resilient.

“As the climate changes, our forests are susceptible to more pests and diseases. When tree disease strikes at the heart of big commercial forests, it can have really substantial economic and wildlife impacts,” he says.

Holroyd and Kielder’s planning manager, Laura Shreeve, are working to plant a more diverse range of trees. “The species that we’re using today may not be the right species for 100 years’ time,” says Holroyd. In any case, he is sure they will have a use as a sustainable building material.

At the same time, they recognise Kielder’s importance in fighting climate breakdown. The forest contains swathes of 10,000-year-old peatland that stores more carbon than all the trees combined. Despite covering only 3% of the world’s land surface, peatlands are the largest natural terrestrial carbon store.

Rowan Hickman, peatland restoration project manager at Kielder, is responsible for looking after this vital ecosystem.

Historically, parts of the peatland were drained to prepare it for tree planting. When this happens, the top layer of peat starts to decompose and release its carbon, undoing thousands of years of carbon capture.

So Hickman and her colleagues are trying to reverse this damage and restore the waterlogged ecosystem that suits peat-forming plants such as sphagnum moss by blocking the old drainage areas to raise the peatland’s water table. They have already blocked more than 50,000 metres of drainage area.

Trampling through this spongy bog every day is not easy, but Hickman loves the job.“They’re fabulous places,” she says, squeezing water out of sphagnum moss to demonstrate how much it can soak up. “The rare flora is really interesting. There are loads of specialist species that only live in peatlands. Specialist insects as well that feed this whole food chain. It’s really underappreciated.”

Every member of Kielder’s team spends their days in and around nature, and their enthusiasm for its protection is palpable.

Although the UK imports 80% of the timber it uses, second only to China, Shreeve believes Forestry England can set a positive example with its management of Kielder. “As a country, we want to supply our own timber and know that it’s come from a source where the land has been looked after.”