Branch out this winter and try tree-skiing

My first experience of tree-skiing was certainly memorable. Early in the winter of 2001, in the Montana ski destination of Whitefish, I headed to the upper slopes with a group of skiers and boarders in a tracked vehicle. Our intention was to ski down several forested pitches before moving onto the groomed areas. Most of the group did exactly that. What I did, while trying to keep with up with skiers considerably more skilful than myself, was "skiing" only in the broadest sense. The others waited for me at the bottom of the first pitch. It was quite a long wait. Finally I emerged from the trees, sweating profusely, shame-faced, and with my pockets full of snow.

My second experience was in most respects the same. But this time – breaking one of the cardinal rules of tree-skiing – I skied alone. I got to the bottom of the slope feeling very pleased with myself. Without the pressure to keep up with anyone, I found the experience very gratifying.

My tree-skiing style is, frankly, idiosyncratic. There's none of that darting from side-to-side that you might see in a Bond-movie ski chase; for me, tree-skiing is hardly more athletic than a game of chess, each move being thoughtfully plotted in the glorious, meditative silence of a snowy glade, away from the lifts and the crowded pistes. That I am a self-taught tree-skier is, however, a blessing. When, belatedly, I got chapter and verse on tree-skiing from an instructor, it was apparent that, on the slopes, he would have steered me away from any trees.

The estimable Steve Angus is a Val d'Isère-based instructor with the TDC school who in his spare time helps his partner to run their chalet, called Fresh. "With people who want to ski the trees, I test their aptitude on a clear area by just getting them to do a series of turns varying in radius and speed," Angus said. "If they can't do the whole range, they'll get into trouble."

Give someone the benefit of the doubt, he added, and it'll be obvious if they can't hack it in the trees, and they'll want to get out. But all being well, Angus takes wannabe tree-skiers on to a concave slope (on which visibility of any hazards is good) with well-spaced trees, tells them to look for a target in the middle distance – maybe five turns away – and asks them to plot their route to it. "The idea is to rehearse the journey mentally, so that you prepare for a series of turns, in the same way that you would in a mogul field," he explained.

That all seemed straightforward; but I knew from the fairly sparse literature that tree-skiing demands specific safety precautions. Checking your speed is an obvious one: a range of hazards – root, rock or low-hanging branch – may lie behind the next tree. Another seems so obvious that it hardly warrants mention: "Aim for the gaps between the trees." Even I don't make the mistake of aiming for the trees. But it is a genuine problem, Angus assured me: "Some skiers find their eyes get fixed on the tree, their brain gets suckered, and they panic – it's not uncommon."

Less common, but much more alarming, is the danger from tree wells. Around the base of an evergreen tree there is usually a depression where soft, unconsolidated snow may collect, to a depth of six feet. A skier who falls into the well can have great difficulty getting out, and a snowboarder – with both feet firmly attached to a wide plank – even more difficulty; and the unluckiest, falling head first, may die of asphyxiation.

There were two deaths in tree wells last season in the same week at Whitefish; and such events occur with sufficient frequency to warrant the creation of an acronym, Narsid ("Non-Avalanche-Related Snow Immersion Deaths"). If you want the full, frightening story of tree wells, visit the website treewelldeepsnowsafety.com.

Why is skiing with a buddy another requisite of tree-skiing? See above for details. Get into trouble among the trees, where there are very few skiers and no ski patrols, and you'll want to have help at hand.

Angus says that he likes nothing more than skiing trees. So it's a pity that he spends his ski seasons at Val d'Isère, where the great, bald head of the Espace Killy protrudes way above the tree line. There is, he says, some tree-skiing in the Le Fornet area and at Tignes Les Brévières; but it's too little and too low. In the Alps the tree line barely climbs beyond 2,000 metres. To get a proper taste of tree-skiing you have to travel a few thousand miles to Colorado, where the trees grow 1,000 metres higher.

It was actually in Utah, at the resort of Park City, that I became a convert. In McConkey's Bowl, a delightfully quiet corner of the ski area, I made what was for me a discovery: that if you find a good, wooded face there is no need to go chasing after new terrain, because that slope will continually provide new challenges. Even if you tried, you probably couldn't make the same descent twice.

But it is in Colorado that I have found the best tree-skiing. There are many ski areas that have tall stands of huge, mature trees – lodgepole pine, fir, spruce – growing high on the ski areas, reasonably spaced and without the new growth you'd find lower down: Crested Butte, for example, and Telluride. It is in Steamboat, however, up in the north of the state near Wyoming, where the most striking forest glades are to be found. Get off the resort's top lift at 3,200m, and you can immediately ski into a magnificent stand. Anyone who likes tree-skiing will love Steamboat.

But here's the rub: tree-skiing in Colorado is under threat. Not, as you might expect, from aggressive lawyers or cautious health-and-safety officers, but from a small beetle about the size of a grain of rice. This tiny "mountain pine" or "bark" beetle has ravaged four million acres of forest in Colorado and Wyoming since 1996, turning entire mountainsides from a healthy green to the rusty red of leaves on a dying pine tree.

The way the beetles go about their work is chilling. They drill through the bark and create a cavity in which to lay their eggs; the hatched larvae feed on the sweet layer of the tree which provides its nutrients. The tree's normal defence mechanism would be to drown the larvae by producing sap; but these are smart larvae, and they inject a fungus to prevent the flow of sap. Trees can also defend themselves by producing a white resin which traps the beetles. But listen to this: in that situation a beetle can call for reinforcements with a pheromone distress signal, and an army of beetles will swarm on to the tree.

This type of beetle is not a new species. The reason why it has become such a successful predator is that it is not itself being killed – as has been the case, historically – by cold weather. Though drought, the age of the trees and fire-prevention have all contributed to the forests' frailty, experts are agreed that it is a succession of warm winters that have enabled the beetles to proliferate and to spread further north – to Canada, where they have infested almost 40 million acres of forest.

What can be done? Trees can be sprayed with insecticide, but the cost is prohibitive. A fake "pheromone" mimicking the scent beetles give off when a tree is fully infested has only a limited effect. Taking out dead trees may help younger, stronger specimens to survive; and anyway the hazard of falling trees means they must be cut down. (They are a cash crop, too, especially now that pine tinged blue by larvae fungus has become fashionable for kitchen finishes in Colorado.) A really effective solution? There isn't one, except for a sustained cold spell the like of which the state hasn't seen for many years.

Already the beauty of the mountain landscapes has been compromised; and bare patches have appeared in the glades (something that the ski resorts don't necessarily mind, if they can create more beginner/intermediate terrain). But there are still trees to be skied, and some arborists believe that Colorado's forests will eventually be stronger, when pines are not so dominant. It's not all bad news.

Travel essentials: Tree-skiing

More information

* Steve Angus, Val d'Isère (00 33 630 41 9969; steveangus.com).

* Park City, Utah (001 800 453 1360; visitparkcity.com).

* Steamboat, Colorado (001 970 879 6111; steamboat.com).

* Crested Butte, Colorado (001 970 349 2222; skicb.com).

* Telluride, Colorado (001 970 728 7335; tellurideskiresort.com)