The two forests the path runs through are old. Heverleebos and Meerdaalwoud were medieval hunting reserves of the Dukes of Brabant; some of the beech and oak still standing date back three or four centuries. The Dikke Eik - the Fat Oak - that the trail passes around its midpoint was planted in 1680 and is still very much alive, although a steel cable now helps support its main limb. A second venerable oak farther along was planted around 1800. Between them runs a network of straight forestry rides - the dreven - laid out in the eighteenth century when the woods belonged to the Croy family. The arrow-straight avenues, the quiet, the soft duff underfoot in autumn: it is the kind of European forest that feels both manicured and ancient, neither wild nor entirely tame. Roe deer cross the path at dusk. Wild boar are common enough that the Flemish forest service publishes hunting calendars. Above all there are mushrooms - the rust-brown bracket fungi, the yellow chanterelles, the white-spotted scarlet of fly agaric. One of Wouters' sculptures, three slender wooden stalks meant to evoke the shaggy mane mushroom, is named simply Coprinus.

Most of the figures along the path are carved from single fallen oaks, weathered now to silver-grey. There is the Director, the first piece you reach inside the Heverlee Arboretum, sitting in a chair as though about to call a meeting to order. There is Ignatius nearby, and a friendly owl that is genuinely easy to miss if you do not know to look up into the Y-shaped trunk of a particular tree. There is a giant horsetail plant - Equisetum arvense - added in 2015 along a new cycle path. There is a Common Kestrel, large enough to serve as the mascot of the trail's welcome zone. Baloo the bear sits in a clearing of birch saplings. Bosprotter - the local Brabant Flemish word for a fat little forest spirit - has a comfortable belly. The Bird Whistling for Cheer Joy lives up to its name. None of these were commissioned. Wouters made them because he wanted them in the woods and because he believed sculpture belongs outdoors where ordinary people walk.

Roughly two-thirds of the way around the loop, on the shore of a small pond called Geertsvijver, stands a piece that breaks completely with the rest of the trail. It is called Drowning. It is not carved from wood. The figure - a man waist-deep, arms reaching up, mouth open - is welded from steel mesh and filled with garbage. Plastic bottles. Cigarette packets. Crisp packets that crinkle in the wind. Wouters made it in the spring of 2014 from litter he and volunteers picked up along the very trail his other sculptures sit on. The figure rusts a little more each year. The plastic does not. It is, perhaps deliberately, the least photogenic piece on the path. Stand in front of it long enough and the metaphor becomes uncomfortable - the man is sinking into the same material we used to make him.

Leuven is the nearest city, about thirteen minutes by train from Brussels Airport, which makes Ad's Path one of the few proper forest hikes in Europe you can do as a day trip from a transatlantic flight. From Leuven, take De Lijn Bus 1 or 2 down Naamsesteenweg to the corner of Leopold III-laan, walk five hundred meters south to the E40 motorway viaduct, and the entrance to the Heverlee Arboretum begins just after. Light hiking shoes are enough; there are only a few low hills. The full loop is 25 kilometers, which most walkers will not complete in a single day - but the trail crosses itself, intersects bus lines, and offers official shortcuts that bring it down to as little as 16 kilometers. Two primitive free campsites along the way let serious hikers split the walk over two days. Lyme disease is endemic; check yourself for ticks in the evening. And carry a map. The Flemish tourist office has scattered numbered directional signs across the forest, but they describe a different walking network entirely, and following them will not take you past the sculptures.

Ad Wouters could have sold these sculptures. There is a market for outdoor art in Belgium and the Netherlands, and his pieces are skilful enough to fetch real money. Instead, he donated them to the forest. His sole stipulation was that they remain outdoors and free to visit. He kept making them well into his seventies - the giant kestrel and the horsetail were added in 2015, the year he was already past pensionable age. There is something quietly Flemish about the whole enterprise: a craftsman who did honest work for honest money for half a lifetime, then spent the second half giving his work away in a forest where most visitors will never know his name. The path is not famous. It does not appear in many guidebooks. On a weekday afternoon in autumn, you can walk most of it without seeing another person, and that, on the evidence, is exactly how the artist meant it to be.