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How can a tree grow on a boulder?

Ben DolphinHigh up in a quiet corner of Glen Etive is a large boulder, and improbably anchored on top of that boulder is a rowan tree. It stands in a beautifully isolated but prominent position on a tongue of land between two burns and, as you climb higher onto Stob Dearg this oddity, bowed in deference towards the Buachaille, feels like a sentinel guarding the entrance to the upper coire.

I have a list of favourite natural oddities in Scotland but this tree / boulder combo is up there with the best. I love it because it just looks plain weird – a fairly hefty trunk doing the impossible by rising out of solid rock. Like an illusory art installation it invites you to walk around it looking for clues as to how it was done, and like all good illusions the solution is hard to fathom.

Hmm. A conundrum to solve in Glen Etive.

The conundrum of the combo is this: trees need soil in order to grow because it contains vital minerals, nutrients and moisture. Soil is essential not only for a tree’s nourishment but also for its stability, because a tree that can’t force its roots into the ground won’t last long in even the gentlest of Scottish breezes. But there is no soil to be seen here. Just rock. The only possible explanation is that the tree somehow has access to soil despite being 5 or 6ft off the ground. But when the boulder rolled down a hillside or tumbled off a melting chunk of ice and came to rest, it didn’t have soil on top of it. So where did the soil come from?

It’s well hidden, but if you look closely at the base of the tree you find a shallow gash in the rock, into which the trunk and largest roots plunge. Somewhere in there must be the all-important soil. And while that revelation might suffice as explanation enough for some people, the conundrum remains because we still haven’t found an answer to the question. How did the soil get up there? It’s a much more powerful question than you might think, for explaining this single oddity unlocks the phenomenon on an infinitely larger scale and explains why Scotland, and indeed pretty much anywhere else in the world you care to mention, looks the way it does.

Perhaps it’s easiest to first ponder exactly what soil is. At its most basic level soil is a mixture of inorganic and organic substances. The inorganic component is primarily made of mineral-rich rock that has been broken down into tiny particles such as sand, silt or clay. The organic component is the decayed remains of plants and animals, but given that plants can’t gain a foothold without soil, how do their remains get up there in the first place? Well, a tiny amount of fine organic matter could be imported from elsewhere by the wind, only to be caught by the boulder’s cracks and ridges, but the first organisms to colonise the boulder don’t actually need soil.

Lichens and mosses are first to move in.

Lichens, algae and mosses are the ‘pioneer’ species on newly exposed rock. I hesitate to go into this level of detail as it can start to fry your brain thinking about it, but lichens are in fact a dual life form made of a fungus and an alga. The fungal part doesn’t need soil, as it is able to extract fine dissolved minerals from the rock, which in turn helps the algal part to photosynthesise, which in turn gives energy back to the fungus. I tie myself in knots thinking about that one! But because the two organisms work together it means the lichen can survive on bare rock and, in so doing it secretes acids that help to break down the rock into very fine particles. In addition, the rock is slowly broken-down by weathering (wind, rain and ice).

As the lichens and mosses die and get broken down by bacteria and other decomposers, organic matter is added to the fine rocky particle mix. Very gradually you start to get the most basic of soils forming in any small depressions in the rock. This opens the door to the next wave of colonisers, the small wildflowers and grasses, which actually need very little soil in order to gain a foothold. As those new species grow, reproduce, die and decompose over and over again on the boulder, the soil beneath them gets steadily thicker and richer with nutrients until there is enough of a foothold for shrubby plants like heather and blaeberry to take hold. They in turn perform the same function as their predecessors, inadvertently creating the necessary conditions for something much larger than them to follow, such as a rowan.

Crucially, with each new wave of colonisation, many of the species that paved the way for those that followed find they can no longer survive in that location, as they are now out-competed for light, space or nutrients, and they die out. This process whereby waves of steadily more complex, larger and generally more long-lived organisms colonise a newly exposed surface or landscape, is called ‘succession’. So-called because different groups of species succeed one another as time goes on.

Succession isn’t confined to boulders though, it occurs on a landscape scale too, but regardless of how large or small the object or landscape is you’re looking at, there comes a point when it reaches the end of the road in terms of what community of species will ever be able to occupy that space under those conditions.

A scots pine woodland at Abernethy NNR. ‘Climax community’ doesn’t just refer to trees, it refers to the whole community of species.

That end-point is called the ‘climax community’ and is determined by factors such as geology, topography and climate. In most cases in the UK the climax community of the landscapes, if they were left alone to do whatever comes naturally, is mature woodland and forest. Even many of our lochs would reach this point eventually as they’d slowly fill with sediment, become bogs or fens and ultimately woodlands. That could be oak woodland, scots pine woodland, beech woodland, there are many possible outcomes. That’s not to say that mature woodland doesn’t change in other ways thereafter, as it is a dynamic environment constantly seeking a natural balance between its constituent parts. Trees die or are blown over, and in so doing create new open spaces for pioneer species once more.

In some places, however, conditions are never going to be conducive to woodland. Think of mountain summits where very little could ever grow higher than a few inches. On our highest hills the climax community could be limited to lichens, mosses and a few hardy, horizontally-growing plants. Or think of sea stacks where wind, salt and nesting birds would similarly restrict what could grow there. Or consider vast tracts of blanket bog, where conditions are so waterlogged and acidic that most trees just can’t survive, and where the climax community would likely be a mix of sphagnum moss, grasses and sedges rather than shrubs or trees.

The key point to take away is that succession is an ongoing process and it doesn’t just happen in the ‘countryside’. It takes place across every square inch of the country in one form or another and on every possible surface. You therefore don’t need to go all the way up into a high coire in Glen Etive to see succession taking place. If you have a garden or driveway as unruly as mine you need only step out the door of your home.

From grey to green. Succession on my patio begins in the cracks between slabs. Moss first, then grass.

My patio won’t win any prizes for neatness but it might for its use as an educational resource! It was laid down bright and new perhaps a decade ago, when it formed a solid impenetrable barrier between the soil below and the world above. And yet there is life to be seen – small blotches of lichens on the slabs, and mossy straight lines running along the cracks between them. Small particulate matter has accumulated in those cracks over time and created a rudimentary soil in which grass, nettles and willowherb have found the most tenuous of anchors. Slowly but surely, the grey stone is turning green.

Picture that patio on an infinitely larger scale and turn the clock back 15,000 years to when the glaciers that covered Scotland were melting, and suddenly we’re able to imagine how the barren, rocky landscape that was revealed from underneath the ice came to become the country we inhabit today. It would have taken a very long time to reach the climax community of woodlands and forests, though. Initial colonisation of a bare surface where there has never been any prior soil or vegetation (called Primary Succession) can be fairly speedy with pioneer species arriving quickly. Just look at the shale bings throughout the Central Belt and you’ll see how quickly this can happen. But depending on the location, succession can still take hundreds or even thousands of years to reach its climax.

In other instances, where prior colonisation has occurred and has subsequently been disturbed or damaged in some way, the climax community can be restored extremely quickly. Think of where a forest has been felled or where there has been a landslide, or where there has been a fire, or where a factory has been left derelict. In those examples, where well-established soil and a healthy seed bank already exist below the surface you might find waist high vegetation after just a year or two, and a climax community after as little as 100 years. Nature, if left alone to its own devices, is excellent at reclaiming its old haunts.

A shale bing is a bit like a newly formed lava flow. Barren and inhospitable, but life finds a way.

You’re under arrest!

Of course, in most modern societies nature isn’t allowed to reclaim its old haunts. It’s clear when you look around you, pretty much wherever you are in Scotland, that the process of succession still has some way to go before the natural climax communities can be reached. That’s not because succession has stopped of its own accord. On the contrary, nature is an irrepressible force that can out-live and out-wait whole dynasties, empires and civilisations. No, it’s rather because we as human beings stop succession from getting beyond a certain point, either deliberately because it suits our needs to do so, or inadvertently as an inevitable consequence of our actions.

The list of examples is endless: we graze livestock in fields; we plough and sow so that we can feed ourselves; we fell trees so that we can print on paper; we quarry hillsides so that we can make concrete; we mow grass so that our parks look neat and tidy; we spray herbicides to kill plants on our high streets; we keep railway edges clear of trees; we burn moors so that we always have different ages of heather in which grouse-shooting can thrive; and we maintain red deer at such high densities that any young saplings get quickly eaten and woodlands cannot naturally regenerate. If those all feel a bit remote and impersonal, consider something as simple as pulling ‘weeds’ from our gardens to stop them becoming overgrown. Yep, we all stand in succession’s way.

None of this is to say that arresting succession is good or bad in itself, as it depends on the context, the objective and how sustainable a practice it is. I’ll leave it to you to decide, but the next time you encounter a tree growing from a boulder in a remote Scottish glen, spare a minute to ponder the remarkable chain of events that have enabled it to grow up there, and another to the considerable impact we have on the natural processes of this world. And who knows, you just might become a bit more tolerant of an unruly patio as a result!




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