Sonian Forest - Grippensdelle B

Grippensdelle B is a strict forest reserve within the Sonian Forest (Zoniënwoud/Forêt de Soignes), located on the south-eastern edge of Brussels, in the municipality of Watermael-Boitsfort / Watermaal-Bosvoorde. It belongs to a small network of so-called integral reserves — parts of the forest deliberately left to develop without any human intervention.

What is this place?

Grippensdelle is one section of the wider Sonian Forest, which stretches across more than 4,400 hectares spanning three Belgian regions. The Grippensdelle reserve itself covers roughly 83 hectares, set aside as a non-intervention zone: no logging, no planting, no removal of dead wood. As a result, visitors can observe every stage of a forest's natural life cycle, from young seedlings to fallen trunks left to decompose on the forest floor.

Why is the reserve significant?

The defining feature of Grippensdelle is its stand of very old beech trees, some well over a century old, forming the tall, even canopy typical of this part of the Sonian Forest. Because the forest is left unmanaged, deadwood and undisturbed undergrowth provide habitat for insects, fungi, and bird species that depend on old-growth conditions rarely found elsewhere in densely populated Western Europe.

Protected status

Since 7 July 2017, the Sonian Forest's reserves, Grippensdelle included, have formed part of the transnational UNESCO World Heritage listing "Ancient and Primeval Beech Forests of the Carpathians and Other Regions of Europe." The designation recognises these forest fragments as living evidence of the ecological processes that have shaped Europe's deciduous forests since the end of the last Ice Age.

How to get there and what to expect

The reserve is easily reached from Boitsfort/Bosvoorde train station, from which trails lead directly into the forest. A loop trail around the Grippensdelle B reserve covers about 8.7 km (roughly 5.4 miles) with modest elevation gain, taking around 2 to 2.5 hours and passing through avenues of mature beeches. Because the area holds strict-reserve status, visitors are expected to stay on marked paths — the whole point of the designation is to minimise disturbance, including from foot traffic.

Frequently asked questions

What makes an "integral reserve" different from an ordinary managed forest? In an integral reserve like Grippensdelle, all forestry management has been halted: trees are neither planted, pruned, nor cleared once dead. The forest is allowed to evolve solely through natural processes.

Can visitors walk inside the reserve itself? Yes, but only along marked trails — staying on the paths is essential to preserving the strict-reserve status and protecting the forest floor from trampling.

What is there to see in Grippensdelle? Mainly the unusually large, old-growth beech trees and stretches of deadwood left to decay naturally — a rare sight in the intensively managed forests of Western Europe.

How do you get to Grippensdelle B? The easiest approach is by train or tram to Boitsfort/Bosvoorde station, from where walking trails lead straight into the forest.

Is Grippensdelle part of a larger UNESCO site? Yes — together with other sections of the Sonian Forest, it forms part of the serial UNESCO World Heritage listing covering Europe's ancient beech forests, inscribed in 2017.