They are head-turners. Six feet wide, their great trunks riven with deep, hard veins like weathered skin, the 800-year-old Cadzow oaks of Hamilton High Parks prove that youth and beauty don't always go together. Set in a clearing above the gorge in Chatelherault Country Park, they are not particularly tall, but their mighty limbs stretch out for metres on all sides. They make an arresting sight: if there were a Dove Real Beauty campaign for trees, they'd have a starring role.

Yesterday, the trees presided at the launch of Scotland's newest national nature reserve, Clyde Valley woodlands, which is intended to give people in central Scotland access to an outstanding natural habitat. The managment team hope visitors will be drawn by the chance to see the oaks, of which there are about 200, some dating from the 1140s when the woods were planted, others from the 1320s and the reign of Robert the Bruce.

The trees are safe from the feller's axe thanks to a barrage of acronyms protecting the habitat, including an SSSI (special site of scientific interest), an SAC (special area of conservation) and now an NNR (national nature reserve). But many important and ancient trees in Scotland have no such protection. In fact, we don't actually know for sure where they all are. There might be one in your back garden. There might be one in your local woods. There might be one near you that's about to get chopped down to make way for flats.

Earlier this week, the actor Robert Carlyle came out fighting for condemned mature trees in Glasgow's west end. Carlyle, who starred in Trainspotting, Hamish Macbeth and The Full Monty, attacked Glasgow City Council for allowing a developer to cut down nearly 100 chestnut, lime and ash trees in the west end to make way for housing, a move that has already brought many local residents out in protest. Carlyle, who lives with his family in the area, described it as "unbelievable, shocking and disgusting and an act of mindless vandalism". The firm responded that it had carried out a tree survey and that saplings would be planted, a plan Carlyle described as "just patronising". Ouch.

Development is the main threat to trees: a new housing development, a new airport runway, carelessly sited power lines, golf courses and roads may all crush mature woodland in their path. New guidance on planning, issued last year, states that authorities should seek to protect trees or woodland that enhance an area, but it is not binding.

That is not to say there is no protection for trees - far from it. You need a felling licence to cut down a growing tree of more than eight centimetres in diameter, for a start, though there are some exceptions.

Then, if an area is designated as an SSSI, the landowner is very constrained in the action he or she can take. "If the landowners want to undertake any development, they can only do so with the consent of Scottish Natural Heritage," says Roddy Fairley, SNH's area manager for Strathclyde and Ayrshire. "That could mean removing trees, increasing grazing pressure, opening a paintball course - a whole range of things." Planning permission cannot be granted without the consent of SNH, either. Similar regulations apply in the case of special areas of conservation. But where there is no such protection, if you own a piece of land and have the green light to build on it then you can go crazy with the chainsaw and there's not a lot anyone can do to stop you. "Any planning permission that is granted overrides the need for a felling licence," says the Woodland Trust's Jacqui Morris.

There is another line of defence - tree protection orders (TPOs), granted by councils to safeguard individual trees or woods - but these are far from armour-plated. A council can revoke a TPO, and the criteria for giving one out vary drastically from council to council.

Trees, of course, don't make a fuss even when they're being hacked to bits, which makes things that much more difficult when you don't know where the oldest ones are. To get round that, the Woodland Trust would like to see a statutory register of important trees developed - and, in the absence of one, recently launched a campaign to track down all of Scotland's ancient trees (a tree doesn't have to be ancient to be important - it could be a local landmark or loved by local people - but the Trust chose ancient trees as a starting point). Called the Ancient Tree Hunt, it encourages people to look out for trees that might be particularly old. How do you tell? Hug it - the fatter it is, the older.

But while threats to native woodland, including ancient forests, do remain, it's far from being all bad news. The establishment of the new reserve yesterday is a case in point: the Clyde Valley woodlands is an important habitat for a huge range of species. Malcolm Muir, countryside and green space manager for South Lanarkshire Council, is delighted: "They may not have a huge number of eagles and raptors, but these woods have more species than any other habitat in Scotland."

Roddy Fairley, for his part, feels that the degree of loss of native woodland in Scotland is sometimes overstated. Walking back from the Cadzow oaks across the gorge, he says the cover from native species has increased significantly since the end of the First World War. "I would almost go so far as to say that if you looked at almost anywhere in Scotland 80 years ago, the difference you'd see today is more trees." He's not talking about conifer plantations, either, but native unplanted trees. Pointing across the bridge to the ruins of Cadzow Castle, now half obscured by trees, he says: "There's a picture of the castle on a grassy bank and it wasn't that long ago. It's easy to get the negative stories out of proportion. Individual trees are being lost, but there are more trees overall."

www.ancient-tree-hunt.org.uk