“All those trees that were blown over. Are you going to do anything with them?”
It’s a question I occasionally get asked by visitors to the ranger hut. Storm Arwen did for quite a few big old trees along the Dee, and almost two years later they still lie where they fell. Folk are always really nice about it, enquiring politely and sincerely, but you know what they’re essentially asking is:
“When are you going to tidy up?”
I do understand how, from a conventional aesthetic point of view, dead trees might look a bit jarring in an otherwise ‘ordered’, picture-postcard landscape of upright conifers, flanking a sparkling Cairngorms river.
But this way of seeing things, borne of our cultural desire to ‘beautify’ and tame the great outdoors, even in our wilder places, underestimates (or perhaps overlooks) the importance of death in the landscape. And how death, in turn, brings life.
The first thing to say about ‘deadwood’, is that the word is entirely inadequate for describing something so full of life. Yep, the wood itself might be dead. But it’s far from lifeless.
The next thing to say, is that what makes deadwood such an important habitat for all manner of life, is decomposition – the process whereby natural materials are broken down into their constituent elements.
Wood is difficult to break down, because as we all know, it’s generally very tough. It’s made from a number of substances, but most notably cellulose, hemicellulose and lignin. Together they give wood cells their structure and strength.
Lignin, an organic polymer, is particularly tough, and only a small number of organisms are capable of breaking it down – some bacteria, but mostly fungi.
Fungi are therefore the primary colonisers of deadwood. They will already be present in living trees, exploiting weaknesses or damage left by other animals or the elements. But in deadwood they are uninhibited, secreting enzymes that break the tough wood cells down into simpler compounds that they can consume.
In so doing, fungi make deadwood more hospitable or digestible to other organisms, kick-starting a process of colonisation. If you’re a sci-fi fan, think of the fungi as terraformers.
Invertebrates, such as beetles, then lay their eggs on the now decaying wood. Their larvae eat their way through the deadwood, boring tunnels as they go. Other invertebrates, such as mason bees, then nest in the holes the beetles leave behind.
Invertebrates that depend upon deadwood to complete their life cycles are termed ‘saproxylic’ (sapros = rotten, xylon = wood), and some of them are among my favourite Scottish animals. In fact, I’ve been known to deliberately hang around by dead logs, on the off-chance I might see a horntail or a sabre wasp.
Also called a wood wasp, the horntail is a harmless sawfly. The fierce-looking ‘sting’ at its rear end is actually an ovipositor, an appendage used for drilling into wood and laying eggs. Horntails’ massive wings buzz like spitfires and make for memorable encounters, and they couldn’t exist without deadwood.
In turn, without horntails there would be no sabre wasps. At almost 10cm in total length, even longer even than the horntail, the sabre wasp flies from log to log, sniffing for the horntail larvae inside the wood. When she finds one, she bores into the wood using her needle-like ovipositor, and lays the egg actually on the horntail larva. I won’t go into what happens next, but it’s not pretty.
Sabre wasps are, again, completely harmless to us, and they’re miracles of evolution. If you’re wondering how a hair-like appendage can bore into wood, it’s because the ovipositor is partly made of zinc and manganese to give it strength. Amazing eh?
Moving further up the food chain, plenty of other creatures prey upon deadwood invertebrates. Birds, amphibians, reptiles and mammals, they all benefit in terms of food, but also in terms of shelter, refuge, hibernation sites and nest space.
Deadwood also provides new, bare substrates for lichens and mosses. Mosses especially benefit from larger pieces of wood, whose surfaces are far enough away from the ground to free the moss from competition on the forest floor. Similarly for tree seedlings, a new, decaying substrate devoid of any competition might be the only opportunity a sun-loving scots pine has to get above the dense heather.
But at a finer level, species composition on deadwood can vary enormously from one piece to the next, because one piece of deadwood is not necessarily like another.
Consider first, all the wood that has fallen onto the ground: sticks, branches, perhaps whole trees. Is that deadwood old or new? In sunlight or shade? Wet or dry? Warm or cold? Is it humid? Sheltered? If it’s on dry land, is it exposed or buried? If it’s in water, is it partially or fully submerged?
Is it hardwood or softwood? Pine or spruce? Oak or aspen? And then there’s size, and volume. Is it large or small? A log or a stump? A twig or a branch?
Then consider the trees that have died, but not yet fallen. This is ‘standing’ deadwood, or ‘snags’. Here in the Cairngorms there are a lot of them, mainly dead pines that have long since lost their bark, and have subsequently been bleached white into skeletal, bone-like features in the landscape.
Scots pines, due to their high resin content, decay slowly and can remain upright for a century or more, in so doing becoming cherished landmarks. Yep, I have my own favourite snag.
Consider too how deadwood exists both on and inside living trees. Where rot sets in, cavities form. The heartwood itself can decompose, hollowing out the tree from within. Or you can have dead branches that have been caught in the canopy as they fall, held aloft above the forest floor.
There are more deadwood habitats than you can count. And when you consider that each of these habitats, each of these ‘niches’ has its own unique environmental conditions that dictate what can and cannot inhabit it, you start to grasp the extent and value of deadwood biodiversity.
While some plants and animals are generalists, able to inhabit or colonise a wide variety of these types of deadwood, others are much more fussy, only able to inhabit the wood during a specific phase of its decomposition.
Green shield-moss is a good example of the latter, but I’ll leave it to my colleague Andrew Painting to explain why, in an extract from his book Regeneration: Rescue of a Wild Land:
‘It likes wood that is fairly mushy, but not so mushy that it gets too colonised by other mosses. It likes wood that is pretty wet, but not too wet. It needs a constant supply of deadwood, because the stuff that it lives on is only suitable for a few years, before it completely decays or is taken over by other mosses and plants.’
Decay both helps and hinders species like this. If the wood didn’t decompose, if it remained exactly as it was when it died, the community of creatures living on it wouldn’t really change and they wouldn’t need new deadwood to move to.
But the decomposition process, initiated and sustained by the very creatures who make it their home, means that, over time, conditions on and within the wood do change. And as they do so, the species composition of these small communities changes with them. Some species move out as conditions become less hospitable. Others move in, depending on the current phase of decomposition.
It’s difficult to get a precise figure for how many UK species depend directly upon deadwood, but a quick trawl through the websites of various well-known conservation organisations offers various eyebrow-raising stats:
- 7% of all native UK animals
- 13% of all native plants and animals
- 40% of woodland species
- 2000 invertebrates, including almost 700 beetles
- One third of all UK woodland birds ‘nest in holes or cavities in dead trees’.
Given this whole process depends on fungi, it’s little wonder that Buglife describe fungi as ‘keystone species’ in woodlands. i.e. species who hold a whole ecosystem together, without whom it would collapse. We commonly hear that term applied to newsworthy species like wolves and beavers. But fungi?
Ultimately though, the figures and stats aren’t really important here, because whatever figures you find, the message is clear – deadwood is VERY important. If not for the species inhabiting it, then for the role it plays in recycling nutrients such as nitrogen, potassium and phosphorous back into the ground, boosting woodland productivity.
Deadwood is also an important terrestrial carbon sink, especially in temperate climes such as ours where it decomposes slowly. Much of deadwood’s carbon content is released into the atmosphere as carbon dioxide, but studies abound into how much carbon is also transferred into the soil, either by leaching or burying, locking carbon away for the longer term. Recent estimates for the amount of carbon held globally in deadwood is a whopping 73 billion tonnes.
Meanwhile, in rivers, large deadwood structures (such as fallen trees) add diversity to the river system. By forming obstructions and disrupting the flow, deadwood creates pools, retains sediment and oxygenates the water, all of which are good for fish. It can also filter pollutants and alleviate flood risk downstream.
But how much deadwood do we actually need? Have we got enough? How much deadwood should there be in a typical woodland?
They’re knotty questions. Different woodlands in different countries have been managed to different degrees over the centuries, so they vary enormously in deadwood content.
One startling figure that gets banded about, for healthy temperate forests in North America, is that deadwood can account for as much as 30% of all the woody biomass in the forest. Almost a third!
Estimates vary across Europe, depending on whether the forest is broadleaf or conifer, but there can be as little as 5 cubic metres (m3) of deadwood per hectare, and as much as 500 m3, depending on the level of management.
NatureScot says that ‘unmanaged forests accumulate annually 50-200 m3 of deadwood per hectare, whereas in a conventionally managed forest this volume could be as low as 1-5 m3 (Albrecht, 1991).’
Few of the woodlands in this country have been around long enough, in a state of non-interference, to know exactly what the natural average here is, or indeed should be. But Forest Research (the research division of the Forestry Commission) suggests that at least 80 m3 of deadwood per hectare is a favourable amount for a UK woodland. That probably doesn’t sound like much, and in 2020 the National Forest Inventory reported that only 6% of UK woodlands could be considered as being in favourable condition, using that metric.
That’s most likely because certain woodland habitats are, by their nature, largely devoid of deadwood. Younger plantations, for example, where the trees are the same age and don’t grow old enough to drop limbs or become snags.
At Mar Lodge Estate over the years, as indeed elsewhere in Scotland, considerable effort has been expended to create deadwood in plantations like these. Clumps of trees are ‘ring-barked’ using hand axes – removing a band of the outer bark and some of the vascular tissue underneath, which interrupts the transfer of nutrients and slowly kills the tree. In time this creates both standing and fallen deadwood, as well as open glades within the woodland, to the benefit of the woodland structure.
Thankfully though, it’s generally more common these days to see deadwood on forest floors by virtue of it not having been cleared away. Fallen trees are generally cut up and left where they fell. Brashings are left in piles. But simply creating deadwood as a one-off exercise isn’t enough in the long run. A healthy woodland needs a constant supply of deadwood, of the kinds illustrated above, to allow continuity of species and allow them (and especially the fussier ones) to ‘emigrate’ to a suitable new home.
This can and should occur naturally, but isn’t necessarily a steady process. Large woody matter will occasionally fall, if old or diseased, but en-masse it’s only likely to be deposited during sporadic big events such as storms, floods, or under heavy snow.
Continuity of deadwood habitats and species therefore depends, to some extent, on having a diverse age structure within a woodland. i.e. veteran trees and mature trees, who create deadwood as they age and die, but with younger trees coming up below who will, one day, take their place.
But even in a healthy woodland with a diverse age structure, that continuity of deadwood, from either a continual or a periodic drip-feed of new material, can be hindered or halted when wood is removed. Fussy species by nature have fewer colonisation options nearby, so can go extinct locally if, after their old home becomes inhospitable, there’s no suitable new habitat to move to.
The perceived need to ‘tidy up’ or, as is often the case in the Cairngorms, trawling the forest floor for firewood, can have a detrimental impact locally upon the supply of deadwood. I’d be the first to admit that it can seem, if you’re on the receiving end, like an overreaction on our part to be gently chiding you for collecting deadwood from around a bothy, when you’re only there for one night and have gathered a rather modest pile of sticks and logs. After all, it’s just one fire, on one night, in one corner of a large woodland.
True, if it’s just you visiting that place, once in a blue moon, it’s really not going to have much of an adverse impact. But as with so many things in the outdoors these days, where your impact can be small or non-existent in and of itself, it’s never just you in isolation.
Our impact on places, and on popular places especially, is cumulative. Today it’s just you, but tomorrow it will be someone else. The day after that, someone else again. And so it goes on, every day of the week, 52 weeks a year. Little wonder that there are some popular spots where the woodland has been stripped bare of any deadwood resource whatsoever, or it never accumulates to any useful degree. So in that context I would appeal to people to bring their own fuel to bothies, unless of course the landowner provides wood on-site, as some obviously do.
It just needs a slight shift in perception, for us to be able to see and understand the value of deadwood beyond what it can do for us, especially in places designated for its natural heritage. And so in that spirit, in returning to the opening question……are we going to tidy up?
Well, assuming that the wood in question hasn’t blocked a path and doesn’t pose a hazard to people or estate operations……then no. Absolutely not.