The dog walkers arrive early at Cardinham Woods. It's still another hour before the cafe in the old woodsman's cottage starts serving walnut cake and bowls of stew, but already the car park is filling with vehicles containing excitable dogs yelping to be released on to the muddy forest trails heading up towards the steep valley flanks. The dense rain cloud resting on the tree canopy overhead doesn't appear to be dampening their zeal for fresh air and freedom.

Stephen Lees pulls on his raincoat, locks his car and strides towards the sign marked "Lady Vale Walk". As recreation and public affairs manager for the Forestry Commission in south-west England, he is a regular visitor to Cornwall's largest recreational forest – a 263-hectare site just east of Bodmin, which offers 15km of trails to the 80,000 people who visit each year.

"Here in the south-west, the Forestry Commission manages 15,400 hectares across 200 woods," he says, looking up at a plot of regimented 60-year-old pines overlooking him. "Each year, Cardinham produces 1,500-1,800 cubic metres of timber, which is broadly equivalent to 75-100 truckloads. We have good quality larch and spruce here, much of which is used by the construction industry locally. The low-grade 'tops' get sold for firewood or chipboard. And the thinnings get used for fencing and paper."

Cardinham is the quintessential modern forest: part playground, part nature reserve, part plantation. Across the whole of the UK, the Forestry Commission – the government department "responsible for the protection and expansion of Britain's forests and woodlands" – owns or manages 814,000 hectares of woodland, which, in 2009-2010, generated £24.5m from timber sales.

But, according to reports over the weekend, half of the commission's properties in England – including Cardinham – could be put up for sale over the coming decade as part of the coalition government's attempts to reduce the budget deficit. It was spared being used as kindling in the recent "bonfire of the quangos", but the commission could yet be a high-profile victim of the 30% cuts earmarked for the Department for Environment, Food and Rural Affairs. The environment secretary Caroline Spelman is expected to soon spell out plans to raise as much as £5bn from the sale. (Control of the commission's assets in the other regions has been devolved.) If confirmed, it would amount to the largest change of land ownership since the second world war and could, some claim, see previously protected woodlands make way for golf courses, housing developments and a wave of new Center Parcs-style resorts.

The news has been met with near-universal disgust and shock. Caroline Lucas, the Green party MP, described it as an "unforgivable act of environmental vandalism". The Labour party said it feared developers would "cherry-pick the most profitable land", while the Woodland Trust said the planting of much-needed new native woodland could be jeopardised if the revenue from the sales, as looks highly likely, does not directly benefit the commission. By yesterday, more than 10,000 people had signed an online petition urging the government not to proceed with the sale.

The news seems to have stirred a raw, emotional reaction in many people. Our inner treehugger leads most of us to recoil at the notion of a tree being cut down. And yet, as a nation, we are a ferocious consumer of timber and paper products, so much so that we currently import about 75% of the wood we consume each year. It is this glaring deficit that led to the creation of the Forestry Commission in the first place.

"In 1919, when the Forestry Commission was established after the first world war to create a strategic timber reserve, just 4% of the UK was covered in woodland," says Lees. "Now it's about 12%. But this is still very low compared to the rest of Europe where the average is nearer 30%."

The relentlessly cheap price of timber abroad means our forests have shifted in recent decades away from being the much-maligned monoculture conifer plantations of old. Today, by comparison, they are increasingly a blend of native broadleaf species, such as larch, oak, willow and ash, with neat rows of Douglas firs and Sitka spruces. We now often refer to our state-owned forests as "conservation areas" and "carbon sinks" to recognise the fact that their value has diversified and moved away from simply being viewed as timber farms.

"There is now a 'presumption not to fell' and a particular emphasis on preserving ancient woodland," explains Lees. "These are our woods that date back to at least 1600 but, in reality, go back to the last ice age. We don't 'clear fell' now. Instead, we still harvest trees, but we aim to maintain continuous cover. It looks better and helps the wildlife. For example, we have lots of dormice here at Cardinham, as they seem to like conifers."

The commission, says Lees, now aims to make as much of its woodland accessible to the public as possible. In fact, it is obliged to; the Countryside and Rights of Way (CRoW) Act 2000 ensures the public can walk freely on mapped areas. "Bridle and cycle paths are increasingly popular. And, here at Cardinham, we are planning to install mountain bike trails as we want to appeal to the more adventurous visitor and help extend the tourist season here in Cornwall with winter activities. We also now actively encourage kids to build dens in the woods."

The shift in how the British public uses its state-owned forests is, perhaps, best reflected by the changes seen at Haldon Forest near Exeter in Devon. Five years ago, it was attracting 20,000 visitors a year – typically, the traditional mix of ramblers and dog walkers. Today, however, the figure is nearer 300,000 due to the introduction of mountain bike trails and a "Go Ape" zip-wire adventure course.

The commission is also looking to the forests of the future, says Lees. "We conduct forestry research, particularly surveying new species in light of potential climate-change impacts. We are looking at species that fare better further south into Europe. Small areas are being planted with experimental species. Sweet chestnut and beech might be contenders for wider use, but beech trees are loved by grey squirrels. There's always a balance."

But will this stewardship and open access really be sacrificed should large tracts ofcommission woodland be sold off to the highest bidder? John Clegg, a senior partner at John Clegg that handles the sale of about two-thirds of the woods sold in the UK each year, believes not.

"Buying a forest or woodland now tends to be very much a whimsical buy," he says. "Most of our buyers in southern England will have a green tinge behind their reasoning as well as thinking about it being a safe haven for their money. Land of any sort is seen as an investment sanctuary at the moment due to the low interest rates. Both agricultural land and forestry prices have fared well in the recession. On average, depending on the maturity and quality of the timber, a forest or wood can give you an annual rate of return of between 1% and 5%. However, it's a very different sort of investor who's attracted to the commercial timber plantations of Wales and Scotland."

Clegg says the various restrictions placed on a buyer now mean it highly unlikely they will ever get permission to take a chainsaw to a woodland and build on the resulting plot.

Mountain bikers in Haldon Forest park, Devon. Photograph: Seb Rogers/Alamy

"The idea that all these forests will be clear-felled is simply unrealistic," he says. "You have to apply for permission to do so and there is a stipulation to replant the site with the Forestry Commission returning after five years to check it has been done. Also, the CRoW Act means that 'on foot only' access must be guaranteed. There is a huge presumption against developing by planners. Our guide prices at John Clegg & Sons would be at least five times higher if we could sell plots with development potential. All this talk this week of building golf courses and resorts on woodland is just ridiculous."

If this sale had taken place in the 1980s – when the Tory government last tried to sell off commission assets before it was abandoned due to public condemnation – it might have attracted a very different sort of investor. Back then, Schedule D tax relief was available to people planting up a forest. A rush of wealthy investors quickly piled into forestry in an attempt to gain the tax break with the rather unlikely result that the likes of Steve Davis, Richard Attenborough, Terry Wogan and members of Genesis all became owners of large forests in Scotland. (Earlier this year, Attenborough sold his 1,700-acre forest estate on the isle of Bute to a collective of islanders for £1.4m.)

Prices today reflect a very different market, says Clegg: "High-quality broadleaf woodland is the most valuable. A small, southern England wood costs about £10,000 an acre. Whereas, in Northumberland, a large commercial forest larger than 100 acres might expect to fetch £1,750-£3,000 an acre. Consider that farmland is currently worth £4,000-£6,000 an acre and many might see planting trees as a way to devalue your land, even with the subsidies available. It's very hard work to make a forest commercially viable. And, perversely, it's a real struggle to get planning permission to plant a new forest.

"Back in the 1980s when the tax relief was available, we had about 23,000 acres being planted a year. So the idea – as has been suggested this week – that someone will want to buy, say, the New Forest for commercial reasons just isn't viable. It's just not a commercial proposition. And even the major overseas buyers are not going to be interested, because our forests are just too small for them to consider. The biggest forests that typically come on the market today are worth no more than £500,000. Even talk of their future value as 'carbon sinks' [forests commercially maintained due to their tradable worth as absorbers of carbon dioxide] is hugely overrated. There is lots of talk at the moment about the 'carbon rights' of forests, but it is still a really undefined market in this country. I can't think of a sale yet where we've put a value on the carbon rights."

At the general election in May, all the main parties made manifesto pledges to plant more trees. The Tories, for example, pledged to plant 1m trees over the course of this parliament. Last year, a report commissioned by the commission – the so-called Read Report – concluded that if an extra 4% of the UK's land was planted with new woodland over the next 40 years, it could lock up 10% of the nation's predicted carbon emissions by 2050.

But our politicians need to be held to their promise, says Colin Tudge, the science broadcaster and author of The Secret Life of Trees: How They Live and Why They Matter. "We urgently need more trees in the UK," he says. "Trees are the best thing we currently have to lock up carbon. We should really be pushing agro-forestry, which is where you farm under the cover of trees. Other countries are experimenting with it. For example, cows, chickens and pigs all fare much better under the cover of trees and greatly benefit from eating leaves as opposed to just a grass- or maize-based diet."

Tudge believes our forests must now enter a new "transitional phase": "From the start of the 18th century, we basically knocked over all our trees. Then, after the first world war, we started to grow trees commercially. Now we urgently need to enter a refurbishment phase and reach the kind of forest cover seen in Europe. It's to say so, but we are spiritually connected to trees. Their size and age puts us in our place. We adore them, aesthetically. To cut down a tree and not replace it has long been judged in many cultures to be an offence against nature. But, sadly, civilised thinking has been put on hold while we pay the bankers back. It's still far cheaper just to import our timber from Finland than to plant up well-managed forests ourselves."

At the car park in Cardinham Woods, the first cyclists of the day have arrived and are busy unstrapping their bikes from their cars. Talk quickly turns to the looming prospect of the Forestry Commission sell-off. "Our children love places like this," says a mother as she fixes the helmet on her daughter. "They can go pond-dipping, search for insects, explore the woods and get lots of exercise. Whenever we go on holiday, we always search out the nearest forest for a walk or cycle. To think our woods could ever be threatened is just terrible."

• This article was amended on 29 October 2010. The original referred to bridal paths. This has been corrected.