An edited extract from Matt Collins’ Forest: Walking Among Trees, published today by Pavilion. With photographs by Roo Lewis.

The winds sweeping up through Montseny’s steep gorges are hushed the moment I enter the tree. The bitter January air they carry also abates once inside, stopped short by a foot of weathered wood; here is a shelter on the edge of a cliff, as protective as a cave and natural as stone. Besides its low doorway, which requires me to stoop to pass through, I find the interior to be complete with a level floor, window and smooth wooden walls; it is decidedly homely and, rather surprisingly, not in the least bit dank. The inner face of the trunk is worn smooth on most sides except for the section in front of me where it has been charred black by fire. Standing fully upright I come nowhere near reaching the ceiling, the apex residing another few feet above my head. The effects of microbial processing furnish this once living tissue with a variety of colouration, ranging from a corky orange to damp, mottled brown, and the whole surface is illuminated by a second, higher-level window further up. Around this small opening dangle strands of cobweb and dry grass, which I presume to be the remnants of temporary nests made by summer migrants. The window lower down is formed of a much larger opening, shaped like an arch and reminiscent of an imposing church window above an altar. Its smoothed edges perfectly frame a solitary conifer outside in the clearing with a backdrop of mixed forest beyond, the freshness of which carries into this wooden chamber an appealingly clean mountain air.

In the dim light I scan the rest of the tree’s interior. To my left, at shoulder height, the wall curves back before continuing upwards, forming a natural protrusion one could imagine repurposed as a shelf. I find a few brittle fragments of bark resting on the ledge amongst a deposit of dry dust, but picture in their place a candle, a book, an ornament perhaps. The floor is also dry and powdery, and surprisingly wide: there is ample room to lie down should one choose to; room, in fact, for a bed. But these ideas of habitation are more than fantasy, and there is sense in speculating how one might indeed make this hollow a home. It is believed this cavernous chestnut was once occupied in such a way, many decades ago, by a charcoal maker of Montseny’s mountain forest. Little is recorded of his tenancy in the tree but its substantial girth is rumoured to have accommodated a table and chairs during its occupation. Castanyer d’en Cuc, as the chestnut is known (castanyer being Catalan for chestnut), is a fine example of an ageing sweet chestnut and the largest of its kind in Catalonia.

There is no trunk to speak of – that single distinguishing feature by which we classify trees as trees – instead, wandering out from the peculiar central bulk, is a collection of aimless limbs that thrust into the air like tentacles belonging to some urchin of the seabed. They vary in breadth and have a weightless quality, as if lacking any particular purposeful direction. Two stout segments remain of the original trunk; having died off at the top both now stick out as cylindrical chimneys. And at the very bottom, protruding from under the folds of molten bark, brand new shoots appear, half an inch thick, carrying the chestnut’s signature fat, red buds that are every bit as healthy and exuberant as a nursery- grown sapling. These many components together – old, ageing and new – form an organism emblematic of struggle and survival: it is a tree wholly un-treelike and devoid of conformity, a character sculpted by the demands of its challenging environment. Fire, most likely caused by lightning strike, has altered its torso dramatically, cutting the plant down and hollowing out its centre. Drought, in turn, is most likely responsible for the waning limbs, especially given the tree’s exposed positioning at the mercy of furious wind and a blazing sun. The chestnut may even have been pollarded at one time too, or cut from at intervals by local foresters. But this is speculation; who, at a glance, can really account for the many adversities faced by a specimen of this maturity and isolated existence? We merely see the contortions that have resulted from their infliction.

As I stand admiring the chestnut, I find myself drawn to that incongruous church window at the centre of its trunk. I’m reminded of the enormous cedars I once met on an estate in Devon whose holes had been filled up with tar. Ageing trees are so often treated in this way amid fears for their lasting stability; packing the exposed centre (usually with concrete) is a measure intended to keep them upright and alive. Though this is relatively harmless to the tree itself it is generally a cosmetic measure; with all ventricle tissue located on their outer layers trees can live on quite happily lacking any substance to their core. There is no better testament to this than Chêne Chapelle in France, an oak over 800 years old that has withstood similar excavation by lightning. The immense tree stands in a small farming village outside the northern city of Rouen, and has become a destination for religious pilgrimage ever since a chapel was erected in the entrance of its hollow. The lightning strike took place sometime in the 17th century and, taken as a sign of divine intervention, prompted a local abbot to construct within it a shrine to the Virgin Mary. Today, Catholic mass is still conducted twice a year from within the cavity, which is fitted with a paved brick floor, wood-panelled interior and an ornate altar against its back wall. There is even a second chamber higher up known as the hermit’s room, accessed via a staircase that spirals around the outside of the resilient trunk. Parts of the exterior are clad in wooden shingles (in lieu of interior concrete), to cover the natural crevices and holes that have broken through over the years, a kind of curious domesticity added to an already adulterated organism.

But give me Castanyer d’en Cuc over Chêne Chapelle any day. If I were a hermit I’d far rather take the chestnut’s empty, earthy shell for the quiet of its hillside forest; give me a few leaky holes in the trunk if they let in the winter sky, and leave the walls unclad if they present a nook for the odd nesting bird. This is a chapel as nature intended; a tree far more fitting for a hermit, religious or otherwise.

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Matt Collins is a freelance garden writer, author, and Head Gardener at London’s Garden Museum. You can read his contribution to our ‘Pleasures’ column here.

Visit Matt’s website here.

‘Forest: Walking among trees’ is out now and available here.