Tag Archives: trees

What trees would we plant to maximise carbon uptake?

Fangorn Forest as represented in Lord of the Rings: The Fellowship of the Ring. This is fantasy fiction, not the type of habitat that we should expect to find or create in the real world.

One of the reasons often put forward for growing more trees is that it’s a method to draw down carbon from the atmosphere and lock it up in wood. Afforestation is far more efficient and straightforward than any currently imagined ‘carbon capture technology’. Photosynthesis is the original carbon uptake mechanism, evolved and perfected over more than a billion years, and human ingenuity isn’t going to design anything better, at least not on the scales required by rapid climate change. Trees have done it all before.

Before going much further, and for the avoidance of doubt, planting trees (even a trillion of them) isn’t going to solve the climate crisis. It’s one potential tool but no substitute for massive reductions in emissions. At its very best, tree-planting would only remove the carbon which has been released from land-use change, not that from the burning of fossil fuels, which represent stores of carbon generated hundreds of millions of years ago. That’s even before we get to the contentious question of where we might plant the trees, which usually turns out to be someone else’s country.

The other problem with tree-planting is that it assumes the trees either stay in place or are continuously replenished. Forest fires, logging, land clearance, droughts, pest outbreaks… all the potential causes of tree mortality will eventually lead to this carbon returning to the atmosphere. A tree is at best a temporary carbon store, albeit one that can last a few centuries. We’re not laying down any new coal deposits, they just don’t make it any more. I’m therefore sceptical of any off-setting program which justifies current emissions on the basis of anticipated long-term carbon storage through tree planting. It’s not something we can rely on.

All these caveats accepted, we can begin to ask the question: if we really were planting forests with the primary objective of taking up carbon, and we planned to do it in the temperate countries which are overwhelmingly responsible for global change, what should we plant?

A recent newspaper article asked this question with the UK in mind*. By coincidence I had a Twitter exchange on the same subject shortly before the article came out. As a thought experiment it’s a reasonable discussion to have, and by doing so publicly it forces people to contemplate the implications of the arguments that are so often made for planting trees.

The right tree species should be (a) fast-growing under local conditions, (b) tall, preferably forming dense forests with as little space between the trees as possible, and (c) of high wood density, maximising the amount of carbon for a given volume of trunk. Ideally they should also be long-lived and relatively resistant to the many forms of disturbance that kill trees, including extreme weather and diseases. There’s no single species of tree that satisfies all these conditions, not least because high wood density leads to slower growth rates. Some compromise is necessary.

The conclusion of the article, taking into account the assumption that carbon uptake was the sine qua non, was that plantations of fast-growing non-native conifers were the best way forward.** The backlash to this suggestion was immediate, predictable and justified. Such tree species are not only hopeless for conservation (and therefore would lead to a net loss of biodiversity) but also aesthetically undesirable as they would transform familiar landscapes. Yes, say the public, we want more forests, but surely not like this.

I don’t disagree with any of the objections to such a scheme, but it does highlight the inherent problem with making so many claims for the benefits of tree planting that are logically incompatible. It is impossible to design a forest which maximises all the potential functions we want from them: promoting native species, boosting biodiversity, storing carbon, amenity value, aligning with our aesthetic preferences, and maybe also providing some economic benefit to the landowners who are being asked to turn over their productive estates to trees. If we pick just one of these factors to emphasise — in this case carbon — then inevitably we will have to lose out on the others.

Every response to climate change presents us with difficult choices. The trite maxim that we should plant more trees puts people in mind of a sylvan idyll of sun-dappled glades beneath the bowers of mighty broad-leaved giants. Such forests exist in Europe only in the imagination. If real trees are to be used to solve our problems then real forests will be necessary, and they might not be the ones that everyone expected. Be careful what you wish for.

* Full disclosure: the academic whose views the article reports, Prof. John Healey of Bangor University, is also a collaborator of mine (we co-supervise a PhD student). We haven’t shared our opinions on this topic though.

** My pick, if we’re playing tree Top Trumps, is the Nordmann fir, Abies nordmanniana, which is so much more than just a great Christmas tree. It’s also the tallest-growing native tree in Europe and as a montane species tolerates a wide range of challenging environments.

From tiny acorns

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My father planted acorns.

This is one of those recollections that arrives many years after the fact and suddenly strikes me as having been unusual. As a child, however, it seemed perfectly normal that we should go out collecting acorns in the autumn. Compared to my father’s other eccentric habits and hobbies, of which there were many*, gathering acorns didn’t appear to be particularly strange or worthy of note.

In our village during mast years the acorns would rain down from boundary and hedgerow oak trees and sprout in dense carpets along the roadside. This brief flourishing was inevitably curtailed by the arrival of Frankie Ball, the local contractor responsible for mowing the verges. His indiscriminate treatment shredded a summer’s worth of growth and ensured that no seedlings could ever survive.

Sprouting acorn (Quercus robur L.) by Amphis.

Enlightened modern opinion would now declare that mowing roadside verges is ecologically damaging; it removes numerous late-flowering plants and destroys potential habitats for over-wintering insects. I’m not going to pass such judgement here though because it was a purely practical decision. Too much growth would result in blocked ditches, eventually flooding fields and properties. Frankie was just doing his job.

My father, however, couldn’t allow himself to see so many potential oak trees perish. His own grandfather had been a prominent forester back in the old country (one of the smaller European principalities that no longer exists), and with a family name like Eichhorn it’s hard not to feel somehow connected to little oak trees. He took it upon himself to save as many of them as he could.

And so it was that we found ourselves, trowels in hand, digging up sprouting acorns from the roadsides and transporting them by wheelbarrow to the wood on Jackson’s farm. Here they would be gently transplanted into locations that looked promising and revisited periodically to check on their progress. Over the years this involved at least hundreds of little acorns, perhaps thousands.

They all died. This isn’t too surprising: most offspring of most organisms die before they reach adulthood. Trees have a particularly low rate of conversion of seedlings to adults, probably less than one in a thousand. That’s just one of the fundamental facts of life and a driving force of evolution. Why though did my father’s experiment have such a low success rate? He’d apparently done everything right, even choosing to plant them somewhere other trees had succeeded before**. It’s only after becoming a forest ecologist myself that I can look back and see where he was going wrong.

First, oak trees are among a class of species that we refer to as long-lived pioneers. This group of species is unusual because most pioneers are short-lived. Pioneers typically arrive in open or disturbed habitats, grow quickly, then reproduce and die before more competitive species can drive them out. Weeds are the most obvious cases among plants, but if you’re looking at trees then something like a birch would be the closest comparison.

Oaks are a little different. Their seedlings require open areas with lots of light to grow, which means that they don’t survive well below a dark forest canopy. Having managed to achieve a reasonable stature, however, they stick around for many centuries and are hard to budge. In ecology we know this as the inhibition model of succession. Oaks are great at building forests but not so good at taking them over.

The next problem is that oak seedlings do particularly badly when in the vicinity of other adult oak trees. This is because the pests and diseases associated with large trees quickly transfer themselves to the juveniles. An adult tree might be able to tolerate losing some of its leaves to a herbivore but for a seedling with few resources this can be devastating. This set of forces led to the Janzen-Connell hypothesis which predicts that any single tree species will be prevented from filling a habitat because natural enemies ensure that surviving adults end up being spread apart. A similar pattern can arise because non-oak trees provide a refuge for oak seedlings. Whatever the specific causes, oak seedlings suffer when planted close to existing oaks.

This makes it seem a little peculiar that acorns usually fall so close to their parent trees. The reason acorns are such large nuts*** is that they want to attract animals which will try to move and store them over winter. This strategy works because no matter how many actually get eaten, a large proportion of cached acorns remain unused (either they’re forgotten or the animal that placed them dies) and so they are in prime position to grow the following spring. Being edible and a desirable commodity is actually in the interests of the tree.

A Eurasian jay, Garrulus glandarius. Image credit: Luc Viatour.

Contrary to most expectations, squirrels turn out to be pretty poor dispersers of acorns. Although they move acorns around and bury them nicely, they don’t put them in places where they are likely to survive well. Jays are much better, moving oaks long distances and burying single acorns in scrubby areas where the new seedlings will receive a reasonable amount of light along with some protection from browsing herbivores. My father’s plantings failed mainly because he wasn’t thinking like a jay.

My father’s efforts weren’t all in vain. The care shown to trees and an experimental approach to understanding where they could grow lodged themselves in my developing mind and no doubt formed part of the inspiration that led me to where I am today****. From tiny acorns, as they say.

* His lifelong passion is flying, which at various points included building his own plane in the garden shed and flying hang-gliders. It took me a while to realise that not everyone’s father was like this.

** One possible explanation we can rule out is browsing by deer, which often clear vegetation from the ground layer of woodlands. Occasional escaped dairy cows were more of a risk in this particular wood.

*** Yes, botanically speaking they are nuts, which means a hard indehiscent (non-splitting) shell containing a large edible seed. Lots of things that we call nuts aren’t actually nuts. This is one of those quirks of terminology that gives botanists a bad name.

**** Although I’m very sceptical of teleological narratives of how academics came to choose their areas of study.

It’s not easy being a tree

Are conkers getting smaller?

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A selection from this year’s unimpressive crop of conkers.

It’s a sign of age to notice that many things appear smaller than they used to be: chocolate bars, coins… and conkers.

Hold on. Conkers? Surely the same trees, growing in the same places, can’t have suddenly started producing smaller fruit? Well it seems that they have, and I’m not the first to notice, although hard data on sizes over time has proven elusive. Even so, it’s pretty obvious to me that this year’s crop provides relatively few conkers that would be worth putting on a string. One wonders how the Conker World Championships (and yes, they do exist) are going to respond to this threat*.

Conkers come from the horse chestnut tree, Aesculus hippocastanum (I have a video about it). It’s not native to the British Isles, coming instead from the Balkans, although like many immigrants it has embedded itself in the culture of these islands such that it would be strange to imagine it not being here. There are something like half a million horse chestnut trees in the UK, but they aren’t often found in our woodlands. The overwhelming majority are planted in parks and urban streets, and therefore we encounter them frequently in our daily lives. They also naturally spread into disturbed habitats around towns such as railway sidings or abandoned land.

In the last few years horse chestnuts have started to suffer from two problems. One is the horse chestnut leaf miner, a species of moth that was first discovered in 1985 in northern Greece, and only described as a new species the following year. It then spread rapidly throughout Europe, reaching the UK in 2002, and since then has reached all parts of the country. This is the main reason why the foliage of horse chestnuts starts to turn brown in August (or sometimes even earlier), long before it would naturally begin to yellow at the onset of autumn. The caterpillars actually live inside the leaves, eating away at the green tissue while protected by the tough upper and lower layers.

The leaf miners aren’t a risk to the tree’s survival — afflicted trees still generate a fresh flush of leaves the following year. What the leaf damage does, however, is to restrict the resources available to trees just when they’re getting round to producing the year’s crop of conkers. For a tree, the year is like a marathon; they spend many months storing up energy, biding their time, and around the end of August launch into their sprint finish, culminating in the release of thousands of fruit. The arrival of the leaf miner is like hobbling them halfway through the race. They can still make it to the line, but they don’t have enough energy left for the grand finale.

Evidence of the leaf tunnels created by the horse chestnut leaf miner. This tree is right outside my building, and every tree on campus looks much the same.

If your trees have leaf miners, then there are some things you can do, such as removing the leaves that fall and composting them, burying them (at least 15 cm / 6 inches deep) or, if it’s the only option, burning them. This will kill the pupae of the moth which are overwintering, but it won’t prevent reinfection from other trees in the neighbourhood, which is highly likely to occur. Some birds such as blue tits have learnt to recognise and eat the leaf miners, but this is unlikely to control them effectively because the main damage is done later in the summer once the fledgelings have already left the nest, so there is less feeding pressure.

This isn’t the only issue affecting horse chestnuts though. A new disease known as horse chestnut bleeding canker was discovered around ten years ago, and is now also widespread. It may have infected around half of British horse chestnuts. This causes scars on the stem which ooze sap. Not all trees are killed by the disease; some survive, while others might be immune. Nevertheless, many trees are weakened by it, and a number have already died. This hidden problem might also be part of the reason why our trees are struggling to produce conkers of the same quality as before.

There is some debate over whether the two problems, the leaf miner and the bleeding canker, are related to one another. Some early experiments suggested that seedlings with leaf miners were more vulnerable to the effects of the disease, but more recent and large-scale surveys have found no association between the two. The balance of evidence at the moment therefore indicates that they are independent problems. There is at present no cure for the disease, although some very recent work published earlier this year implies a role for the bark microbiome in regulating the severity of the disease.

A citizen science project has been tracking these new arrivals throughout the UK, and in 2013 found that parasites of the leaf miner have been catching up. This is good news, as it might mean that natural biological control could eventually restrict the impact of the miners. They are still asking for data and there are lots of nice resources on the website which could be used in classrooom teaching to engage schoolkids in observing an interesting phenomenon**. Children might not be able to play conkers in quite the same way, but why not use this as an excuse to teach some exciting ecology!

* Interestingly, the game of conkers predates the arrival of the horse chestnut tree in the UK, and was formerly played with snail shells or other objects. This would imply a rather radical shift in the modern rules though.

** You should also look at the OPAL national tree health survey, which involves recording observations on a whole range of trees as well as horse chestnut.

How to avoid the Allee effect, assuming that you’re a tree. Or a barnacle.

How much stuff is in a forest?

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Forest are complex, messy habitats. The tired old saw of not being able to see the wood for the trees has an element of truth, because the tangle of material prevents easy measurement and assessment. And that’s without even mentioning the leaves.

Terrestrial laser scanning (also known as ground-based LiDAR) is a great technology for getting round this because it allows us to create full three-dimensional reconstructions of forests, from which we can extract parameters that would be otherwise unavailable to a surveyor on the ground. In this aspect it’s a major advance over traditional techniques of forest surveying, though challenges remain in turning the vast quantities of data into relevant and meaningful measurements that we can use to understand how forests form and what the implications are.

One of our laser scanners in action in a UK woodland. Photo by Joe Ryding. If FARO want to give us a discount for all this free advertising then I’d be very happy to hear from them.

In this post I’d like to focus on some of the findings of our recent paper in Journal of Applied Ecology that might have been overlooked in the media hullabaloo. For a reminder of the major findings, and what I think the implications are for forest conservation, see my earlier post. In any paper, however, there are always some hidden insights that couldn’t be elaborated on in the space available.

In this post I’d like to ask: how much stuff does a forest actually contain? In answering this, our data is perhaps non-intuitive. We split the whole forest into 1 cm cubes then asked whether each contained wood, leaf or neither. Terrestrial laser scanning can’t see inside stems, so instead we measured the surface area of trunks and branches*. Rather than using LiDAR to simply recreate the metrics we can obtain by other techniques, I think the future will be in learning to use these outputs more directly as indices in their own right.

Vertical distribution of foliage in 40 UK woodlands, split into those with either high or low deer densities, and which were either managed or unmanaged.**

Our scans found that woodland plots contained an average (median) density of leaves of 523 cm³/m³. What that means in real terms is only 0.052% of the total forest volume. In other words, although a forest looks like it’s full of foliage, actually it’s mostly empty space. This is necessarily a minimum estimate because we probably failed to detect many leaves higher in the canopy because our laser beams were blocked by other material getting in the way. Even so, if we only detected half of all the foliage in the forest, it would still be less than a tenth of one percent of the total volume.***

If forests are mostly empty space, this has a number of interesting implications, one of which is for the movement of species. For large, lumbering mammals like ourselves, we perceive forests to be difficult to move through, but for small insects they contain vast distances which need to be traversed. Organisms that can fly find this easier, others will depend on physical linkages to help them move around. The amount of stuff in a forest, along with its density and distribution, will have major influences on the mobility of organisms of all sizes.

Another implication is that we can treat the three-dimensional surfaces of forests as area in much the same way as in less complex habitats. This is actually an old idea, with Southwood coining the term ecospace to describe the effective area presented by a habitat, and proposing that this increased area might be responsible for differences in diversity between habitats. For the first time we have a straightforward way to measure the amount of leaf in the forest, which is a metric of how much habitat space there is for the many organisms that feed on or move across them. We could potentially compare this between sites.

We can do the same thing with stems, remembering that our measure is of surface area rather than woody volume. The area of stems in these woodlands was an order of magnitude lower than of foliage, at 49 cm³/m³. This is interesting because it tells us that for species that forage on tree trunks and branches, there is approximately ten times less area available to them than there is for those utilising leaves as habitat.

The Species-Area Relationship (the closest thing ecology has to a law) tells us that the number of species doesn’t increase linearly with area****. Instead, we would expect a 90% reduction in area to mean a roughly 50% reduction in species richness. Is that true? Do we find half as many species of insects, lichens or gleaning birds on bark as we do on leaves? I’d be interested to look. Unfortunately right now we don’t have an easy way to tell live and dead wood apart, which is important because they form habitats for completely different species.

In short, capturing the three-dimensional structure of these forests is just the beginning, and there are all sorts of new avenues which we can explore with these data.

* A range of algorithms exist to convert these into volumes using cylinder fitting, which is the standard approach used if you want to determine either the amount of timber or carbon in a forest. These weren’t part of our objectives so we didn’t, and actually as a measure of habitat structure as experienced by other organisms, surface area might be more relevant.

** Note that these figures are flipped relative to those in the original paper. This is a more intuitive way of looking at the patterns, whereas in the paper the orientation was set to match the statistical analyses.

*** Strictly this is ‘number of 1 cm cubes per cubic metre which contain foliage’. Leaves are flat and much thinner than 1 cm, of course, so the true volume of foliage is much, much lower than this.

**** The actual relationship is S = cA^z, where z is a parameter determining the sensitivity of species richness S to area A, and c is a mathematical constant representing the theoretical richness of a single unit of area.

Barnacles are much like trees

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I am not a forest ecologist. OK, that’s not entirely true, as demonstrated by the strapline of this blog and the evidence on my research page. Nevertheless, having published papers on entomology, theoretical ecology and snail behaviour (that’s completely true), I’m not just a forest ecologist. Having now published a paper on barnacles, one could suspect that I’m having an identity crisis.

When a biologist is asked what they work on, the answer often depends on the audience. On the corridor that hosts my office, neighbouring colleagues might tell a generally-interested party that they work on spiders, snails, hoverflies or stickleback. Likewise, I usually tell people that I work on forests. When talking to a fellow ecologist, however, the answer is completely different, as it would be for every one of the colleagues mentioned above*.

If you walked up to me at a conference, or met me at a seminar, I would probably say that I work on spatial self-organisation in natural systems. If you were likely to be a mathematician or physicist** then I’d probably claim to study the emergent properties of spatially-structured systems. I might follow this up by saying that I’m mostly concerned with trees, but that would be a secondary point.

What I and all my colleagues have in common is that we are primarily interested in a question. The study organism is a means to an end. We might love the organism in question, rear them in our labs, grow them in our glasshouses, spend weeks catching or watching them in the field, learn the fine details of their taxonomy, or even collect them as a hobby… but in the end it is the fundamental question that drives our work. The general field of study always takes priority when describing your work to a fellow scientist.

Behold the high-tech equipment used to survey barnacles. This is the kind of methodology a forest ecologist can really get behind.

The work on barnacles was done by a brilliant undergraduate student, Beki Hooper, for her final-year project***. The starting point was the theory of spatial interactions among organisms most clearly set out by Iain Couzin in this paper****. His basic argument is that organisms often interact negatively at short distances: they compete for food, or territorial space, or just bump into one another. On the other hand, interactions at longer ranges are often positive: organisms are better protected against predators, able to communicate with one another, and can receive all the benefits of being in a herd. Individuals that get too close to one another will move apart, but isolated individuals will move closer to their nearest neighbour. At some distance the trade-off between these forces will result in the maximum benefit.

Iain’s paper was all about vertebrates, and his main interest has been in the formation of shoals of fish or herds of animals (including humans). I’m interested in sessile species, in other words those that don’t move. Can we apply the same principles? I would argue that we can, and in fact, I’ve already applied the same ideas to trees.

What about barnacles? They’re interesting organisms because, although they don’t move as adults, to some extent they get to choose where they settle. Their larvae drift in ocean currents until they reach a suitable rock surface to which they can cling. They then crawl around and decide whether they can find a good spot to fix themselves. It’s a commitment that lasts a lifetime; get it wrong, and that might not be a long life.

If you know one thing about barnacles, it’s probably that they have enormously long penises for their size. Many species, including acorn barnacles, require physical contact with another individual to reproduce. This places an immediate spatial constraint on their settlement behaviour. More than 2.5 cm from another individual and they can’t mate; this is potentially disastrous. Previous studies have focussed on settling rules based on this proximity principle. They will also benefit from protection from exposure or predators. On the other hand, settle too close to another barnacle and you run the risk of being crushed, pushed off the rock, or having to compete for other resources.

Barnacles can be expected to interact negatively at short distances, but positively at slightly longer distances. This disparity in the ranges of interactions gives rise to the observed patterning of barnacles in nature.

What Beki found was that barnacles are most commonly found just beyond the point at which two barnacles would come into direct contact. They cluster as close as they possibly can, even to the point of touching, and even though this will have the side effect of restricting their growth.

Furthermore, Beki found that dead barnacles had more neighbours at that distance than would be expected by chance, and that particularly crowded patches had more dead barnacles in them. There is evidence that this pattern is structured by a trade-off between barnacles wanting to be close together, but not too close.

On the left, the pattern of barnacles in a 20 cm quadrat. On the right, the weighted probability of finding another barnacle at increasing distance from any individual. A random pattern would have a value of 1. This shows that at short distances (less than 0.30 cm) you’re very unlikely to find another barnacle, but the most frequent distance is 0.36 cm. Where it crosses the line at 1 is where the benefits of being close exceed the costs.

Hence the title of our paper: too close for comfort. Barnacles deliberately choose to settle near to neighbours, even though this carries risks of being crowded out. The pattern we found was exactly that which would be expected if Iain Couzin’s model of interaction zones were determining the choices made by barnacles.

When trees disperse their seeds, they don’t get to decide where they land, they just have to put up with it. The patterns we see in tree distributions therefore reflect the mortality that takes place as they grow and compete with one another. This is also likely to take place in barnacles, but the interesting difference lies in the early decision by the larvae about where they settle.

Where do we go from here? I’m now developing barnacles as an alternative to trees for studying self-organisation in nature. The main benefit is that their life cycles are much shorter than trees, which means we can track the dynamics year-by-year. For trees this might take lifetimes. We can also scrape barnacles off rocks and see how the patterns actually assemble in real time. Clearing patches of forests for ecological research is generally frowned upon. The next step, working with Maria Dornelas at St. Andrews, will be to look at what happens when you have more than one species of barnacle. Ultimately we’re hoping to test these models of how spatial interactions can allow species to coexist. Cool, right?

The final message though is that as an ecologist you are defined by the question you work on rather than the study organism. If barnacles turn out to be a better study system for experimental tests then I can learn from them, and ultimately they might teach me to understand my forests a little bit better.

* Respectively: Sara Goodacre studies the effects of long-range dispersal on population genetics; Angus Davison the genetic mechanisms underpinning snail chirality; Francis Gilbert the evolution of imperfect mimicry; Andrew MacColl works on host-parasite coevolution. I have awesome colleagues.

** I’ve just had an abstract accepted for a maths conference, which will be a first for me, and slightly terrifying. I’ve given talks in mathematics departments before but this is an entirely new experience.

*** Beki is now an MSc student on the Erasmus+ program in Evolutionary Biology (MEME). Look out for her name, she’s going to have a great research career. Although I suspect that it won’t involve barnacles again.

**** Iain and I once shared a department at Leeds, many years ago. He’s now at Princeton. I’m in the East Midlands. I’m not complaining…

What is a forest?

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Well… it depends. You might think that I’m being facetious here. When I ask this of my students, they immediately suspect that it’s a trick question. After all, I do describe myself as a forest ecologist. But if you pinned me down and forced me to respond then I would struggle to give a clear answer. In this post I’ll try to explain as clearly as I can, and thereby demonstrate that we have a real, serious problem.

Let’s get the etymology out of the way first. In Medieval English the word forest did not refer to a place with trees. Its original meaning was one of the King’s hunting grounds which were managed particularly to maintain populations of deer. Many of these contained trees, but that wasn’t their distinguishing feature. Some were actually heathlands. To this day Exmoor forest hardly contains any trees. Contrary to popular misconception, that’s not because they were cut down — they were never there in the first place.

Here in Nottinghamshire I live only a short drive from Sherwood forest. Myths of outlaws in the wild woods are doubly fictional; neither Robin Hood nor his primeval haunt have any firm basis in historical fact. This has been a managed landscape for many thousands of years. A quick look at everyone’s favourite tree, the Major Oak, gives the game away.

Major Oak in Sherwood Forest in 2006 by Marcin Floryan.

This is not a tree that grew beneath a closed, dark canopy. Oaks in such situations grow tall and slender, forced upwards by strong competition for light. The north-east of the United States is carpeted in young oak forests, none of which contain trees shaped anything like the Major Oak. Its low spreading branches tell of a lifetime spent in a wide open area, surrounded by heathland, which has only in the last century been filled in by the mass of birch trees which now surround it. These birches are themselves a signifier; birch only grows in bright, open habitats when no browsers are around to eat its seedlings. If you wished to restore Sherwood Forest to how it looked at the time of Robin Hood, the first thing to do would be to cut down most of the trees.

The origin of the word ‘forest’ is therefore not much help in deciding what one is. The modern understanding is of an area covered in trees, so that’s a better place to start. But what is a tree, and how many of them does it take to make a forest?

Defining a tree isn’t straightforward either. Botanists, foresters and ecologists will all give different answers. Here’s my rather technical definition: a tree is any plant actually or potentially forming a free-standing monopodial stem of at least 1.3 m in height, with secondary thickening, and reaching a height of at least 5 m.

Let me parse that quickly so you can see where I’m coming from. The opening caveat ‘actually or potentially’ is necessary because it allows for the fact that many plants are capable of growing into trees, but haven’t got there yet (they might be young) or are prevented from doing so (e.g. by regular grazing or coppicing). A bonsai oak is still the same species as a giant oak, it’s just been constrained by a tiny pot. Free-standing is an attempt to keep out (most) lianas, which are structural parasites. Monopodial means having a single, dominant stem; this separates trees from shrubs, which have many stems and tend to branch below 1.3 m. As for the secondary thickening, that implies the existence of bark and other woody tissues, and excludes giant herbs such as bananas. The height of 5 m is arbitrary and intended to filter out any remaining shrubby species.

As an aside, almost any plant species is capable of evolving into a tree. Nowhere is this more obvious than on oceanic islands, where the chances of reaching them are slim, and therefore whichever plant is lucky enough to get there first can evolve into a dominant tree. On the Juan Fernández islands or in Macaronesia the native trees are actually lettuces, while tree sunflowers dominate forests in St Helena and the Galápagos, and tree silverswords in Hawaii show little resemblance to their weedy ancestors.

Scalesia pedunculata. It’s a Galapagos tree, and also, basically, a daisy. By Haplochromis CC BY-SA 3.0 via Wikimedia Commons.

There will always be some in-between cases but we’ll leave them to the specialists. Some lianas start their lives as small trees; some species can grow as either trees or shrubs; no-one really knows what to call a hemi-epiphyte like a strangler fig. Nature doesn’t easily fit into our boxes. Still, this gives us something to work with. It also ends up including species like oil palm, which are farmed as cash crops, and no-one would want landscapes dominated by them to be defined as forests. But we have to draw the line somewhere and specific chauvinistic exclusions would make a mockery of scientific impartiality.

It’s worth pausing briefly to note that the definition does create some perverse consequences. It’s not too dissimilar to that used by the FAO (Food and Agriculture Organization of the United Nations). A major bone of contention in their assessment of forest cover is the acceptance that any land which could grow trees is still a forest until its land use category changes. Completely burnt to the ground? Still a forest. Clear-felled? Still a forest. So long as trees are expected to grow there again one day, the FAO will call it a forest, even if there aren’t any there right now.

How many trees makes a forest? One common approach is to look at how much of the land surface area is covered by tree crowns, and I would personally draw the line around 40% (based on Sasaki & Putz 2009). Note that this is more restrictive than the definitions used by the UNFCC (United Nations Framework on Climate Change) or FAO.

All this causes further problems. My criterion includes things like orchards and rubber plantations, which are not forests by any folk definition. Likewise it excludes vegetation types with low tree densities (e.g. savannahs or dehesa) and shrublands. Some will get very sensitive about this because of the popular perception that conserving forests is the most crucial issue. I do not mean to devalue these landscapes, but would argue instead that these deserve separate treatment rather than attempting to cram them into an all-inclusive definition of forests.

South African fynbos. Undeniably a conservation priority, but calling it 'forest' doesn't help.

South African fynbos. A conservation priority, but calling it ‘forest’ doesn’t help.

Why does this matter? Well, deciding what a forest is makes a major difference in determining how much forest there is in the world. A new paper by Sexton et al. in Nature Climate Change points out that varying your criterion for tree cover from 10 to 30% changes the amount of forest recognised globally from satellite imagery by 6%. In other words, it’s a difference in area the size of China. In the tropics alone it represents a difference of 45.2 Gt of carbon. That’s a much bigger discrepancy than anything caused by arguments over defining a tree, or whether we should include rubber plantations. All those fine details drift into irrelevance.

While different countries are allowed to decide what they call ‘forest’ there will be political motivations to move the goalposts. If we’re trying to battle climate change by tracking the amount of carbon locked up in forests, or save species, or maintain ecosystem services, it’s pretty crucial to find out how much forest we have. When NGOs bemoan ‘forest loss’, or governments declare successes in ‘forest protection’, you should maintain a healthy degree of scepticism about what they’re actually recording and where their figures are coming from. If we are to meet the Aichi Biodiversity Targets or use forest stocks to compensate countries for storing rather than releasing carbon through the REDD+ program then what we call a forest matters. It matters politically, financially, socially.

Global forest cover as detected from satellite data depending on whether one uses 10% (top) or 30% (bottom) land cover by trees as the criterion for defining forest (originals from NASA Earth Observatory, http://earthobservatory.nasa.gov/IOTD/view.php?id=86986&src=eoa-iotd).

All this argument leads to one basic conclusion, which is that the word ‘forest’ is a blunt and imprecise tool when it comes to describing the things we care about. Forests include areas that are old-growth, degraded, secondary or plantations, all of which have different sets of values depending on your perspective (see Putz & Redford 2009 for a great discussion). It is possible for serious degradation of habitats to take place without them ever ceasing to be called ‘forests’, and a dichotomy with ‘non-forest’ is unhelpful in documenting or preventing this. Forests are not merely present or absent.

Let’s just agree that many things can be called forests and stop arguing over the definition. More important is to ask why we care about forests, and start measuring the values that matter to us such as species, carbon, or services. Then, at last, we might be able to move past this distraction.

Trees In Space

Boldly going where no forest ecologist has gone before