The Isle of Arran is often called “Scotland in miniature,” a place where rugged mountains meet rocky shores. But tucked away on this island is a secret not many know; Arran is home to three endemic whitebeam species, three types of tree found nowhere else on Earth.

I first set out to visit Arran with one goal: to see these elusive endemics for myself, as part of my research for Endemic, my book about species unique to Britain.

The Whitebeams

The three whitebeams unique to Arran are the Arran Whitebeam (Sorbus arranensis), the Arran Service-tree (Sorbus pseudofennica), and the Catacol Whitebeam (Sorbus pseudomeinichii).

Two of these species are reasonably common; there are around 400 Arran Whitebeam across the island, and about 500 Arran Service-trees. Well, I say common, both of these trees have smaller populations than the Giant Panda which has around 2,000 individuals in the wild. But the really rare one is the Catacol Whitebeam, which, for a time, was arguably the rarest tree on Earth with only a single known individual. The situation is a little better today, as a small sapling has since been found, doubling the known wild population to two.

I visited Arran in August 2023. Me and my friend Jack got the ferry across and he spent the first afternoon snorkeling off the coast near Lamlash whilst I sat on the shore and double checked the notes and maps I’d made to help me track down the three whitebeams.

We camped at Lochranza that night, and the next morning parked up in a small car park near the river just south of Catacol. The easiest way to see all of Arran’s whitebeams is to park here and then walk 50 metres up the river to a small enclosure where all three species have been planted out. This is a great way to get your eye in on their differences, though they are pretty distinct from each other.

I did stop here for a look, but - for me - seeing the trees planted here was a bit like seeing an animal in a zoo. They’re amazing, beautiful trees, but I wanted to see them growing in the wild. Also, it would have been a pretty underwhelming chapter of my book if I stopped here.

We crossed the footbridge over the river and followed the path as it began to climb into Glen Catacol. It was a bright morning and we had the place to ourselves. The sea slipped out of view behind us, and soon we were walking through a classic glen - steep-sided, rough-edged, dotted with boulders and heather, though, as we walked upriver, the glen was conspicuously empty of trees.

I was starting to grow concerned at the lack of tree cover, when we suddenly spotted that the steep banks cut by the river were home to occasional trees, presumably the only place they can escape the overgrazed nature of the glen. Knowing this, it didn’t take us too long to find the first two species we were looking for by searching close to the river, the Arran Whitebeam and the Arran Service-tree.

The Arran Whitebeam arose, originally, as a hybrid between two completely different looking trees; the Rock Whitebeam (which has ‘entire’ oval leaves) and the Rowan (which has leaves divided into around 15 separate leaflets). The leaf of the Arran Whitebeam looks exactly like you’d imagine for a species trying to reconcile these radically different ancestors - a leaf that is broadly oval, but deeply jagged around the edge as though trying to split into separate leaflets.

The Arran Service-tree is even more odd. It originally arose as a hybrid between Arran Whitebeam, and, once again, a Rowan - meaning that it has Rowan in its ancestry twice. In this case, the leaves of the Arran Service-tree are separated into a couple of pairs of discrete leaflets at the base, like its Rowan ancestor, with the upper half of the leaf a stubby, jagged oval.

As we walked further up the glen we saw more of both species, but it’s the third one that is incredibly difficult to find, with only a single mature tree left in the wild on the entire planet. The Catacol Whitebeam originated originally as a hybrid, this time between the Arran Service-tree and, once again, the seemingly-promiscuous Rowan. In this species, with Rowan appearing three times separately in its ancestry, the leaves are closer to that of Rowan; mostly composed of neat, paired leaflets, but with the terminal lobe a jagged oval as a testament to its distant Rock Whitebeam ancestry.

The search for the Catacol Whitebeam was more involved. I had a rough grid reference, gleaned from an old report, and a few scribbled notes I’d compiled over weeks of desk research. But there was no signpost to this tree, no worn trail leading the way, just the knowledge that it was somewhere out there, growing wild.

We stayed close to the river as it climbed further up into the glen, threading our way along narrow tracks through knee-high heather and into pockets of soft, spongy peat fringed with carnivorous sundews. It was the kind of walking that made your boots squelch with every step. The heather thickened in places, and the occasional outcrop forced us up and around, but the river was always there beside us.

Now and again we’d spot a tree tucked in a sheltered hollow or hunched beside the water, a scrubby Aspen here, a wiry Rowan there, and each time we’d pause, scrutinising the leaves, willing one of them to show the right signs. Most were clearly wrong; too serrated, too rounded, too familiar. Still, we checked them all.

My notes pointed to a spot near a rocky bend in the river. I’d spent hours poring over aerial photos, trying to triangulate that tree’s lonely presence from the vaguest of clues. It felt a bit like hunting for a ghost.

And then, just as we reached the bend, I saw something growing down in the low gorge below us. Could this be it? I fumbled my binoculars out of my bag and…

Well. That’s where I’m going to leave things, because the full story of what we found, and how it felt to stand face to face with what might be the rarest tree on Earth, is told properly in Endemic, my book about the unique species that live nowhere else but Britain.

In the meantime, if you’re heading to Arran, I’d encourage you to take a moment to look out for the island’s rare trees. You don’t need to scramble through peat or puzzle over grid references to appreciate them. Just south of Catacol, a small fenced enclosure beside the river brings all three whitebeam species together in one place. They’re young, planted specimens, but they offer a rare chance to see and compare the trees side by side.

The whitebeams are easy to overlook, unless you know to look for them. But once you know they’re here - once you realise that these three species grow nowhere else in the world - it changes the way you see the landscape. Arran isn’t just beautiful. It’s botanically unique. And that, in itself, is worth the journey.

To read the full story of the search for the Catacol Whitebeam - and to meet more of Britain’s unique and most remarkable species - look out for my new book, Endemic.