Tag Archives: forestry

Oops there goes another rubber tree

Rubber trees bend, and they also break. This is one of the CATAS plantations in China being battered by strong winds.

Frank Sinatra credited an ambitious ant with shifting a rubber tree, which was a rather implausible achievement. The presumptuous ant might have been taking undeserved credit for a much more normal occurrence, which is rubber trees blowing over in strong winds. That happens all the time with no help from ants.

Rubber tree plantations are found in the tropics, these days predominantly in Asia (despite the rubber tree Hevea brasiliensis being originally a South American species, as its name suggests). Many of the main regions of production are also subject to periodic hurricanes which can cause serious damage to the trees and therefore economic losses to the producers. We can’t prevent hurricanes, but could we manage plantations to make them more wind resistant?

This is a question which we approach in a new paper (led by my collaborator Yun Ting at Nanjing Forestry University*) using terrestrial laser scanning and some novel computational methods. The study site was a research station in southern China where a number of different clones have been planted, each of which has a distinctive branching architecture. We already know which of these is most vulnerable to blow-down from the evidence of past hurricanes. What management options are likely to help prevent damage to the trees?

At present simulations are the only way to approach this problem. It’s not just that it’s far too risky to carry out detailed environmental measurements in the middle of a hurricane (not least because the trees often fall over), but the equipment is likely to blow away too. It’s hard enough to find equipment that’s capable of withstanding hurricane conditions anyway. And then there’s the issue of not knowing when or where a hurricane will strike. Our approach can potentially circumvent all these limitations.

The study involved a large team all of whom had incredibly specialised skills and I don’t pretend to fully understand all of them. The first step was using a terrestrial laser scanner to record the full three-dimensional structure of several rubber trees. This creates a high-resolution point cloud which could be used to reconstruct virtual trees which aim to capture every leaf and branch on the original tree. These were then used to create plantations which were subjected to hurricane-force winds in a simulation environment developed for testing the aerodynamics properties of aircraft and other vehicles. This allowed us to evaluate what the implications of crown structure were for the wind forces experienced by trees during a hurricane. No-one had to put themselves at any risk.**

Computer visualisations of two rubber trees reconstructed from terrestrial laser scanning data. Notice how the one on the left has broader, spreading branches, while the one on the right has a more upright form.

There are many caveats to this study; don’t bother pulling it apart because we already know a good few of them. Most of the assumptions and simplifications were enforced by the computational demands of such high-resolution simulations. For example, plots were relatively small in size (nine trees at a time).*** More problematic is that adding natural flexibility to the trees wasn’t an option. What we are measuring is therefore the wind resistance generated by something approximating a tree made of platinum. Of course we know that this isn’t realistic, but I’d suggest that you look at the qualitative outcomes rather than expecting the quantitative predictions to be precisely accurate. This is a first demonstration of the approach, not a complete realisation of every possible feature. It’s good enough to reveal some interesting differences.

Wind velocity around clone 1

Wind velocity around clone 2

What did we find? Well, clones with larger, denser crowns put up greater wind resistance and generate higher degrees of turbulence, making them more likely to be damaged when exposed to hurricane-force winds. Reassuringly, these are the same ones that blow over more commonly in the real world. So we did all this sophisticated work… and found out something that everyone already knew.

That’s not really the point though. Having shown that the simulations generate reasonable outcomes that match with experience, we can now start to tweak the models and explore the impact of management strategies. Manipulating tree spacing or thinning of tree crowns can all be done virtually, more quickly and with less effort than establishing another plantation and waiting for the next hurricane to discover whether it was effective. What we have is a framework for trying out almost any combination of tree sizes, shapes and arrangements.

Rubber plantations are one of the simplest types of forest, which was a deliberate choice. We can imagine further applications of our method in more complex habitats, where a mixture of tree species could be put together to see how real-world forests cope in the face of one of nature’s greatest destructive forces. That’s our eventual aim, anyway. High hopes, as Sinatra would have put it.

* This was a massive collaborative study in which my own role was very minor. I haven’t even visited the field site.

** No trees were harmed in the preparation of this paper. It’s not even in a print journal.

*** When I suggested some minor amendments to a late draft I thought Yun Ting was about to cry. It’s hard to convey quite how much computational effort goes into generating just one of these figures. Simulations are not easy research!

Tree diseases harm people too

What trees would we plant to maximise carbon uptake?

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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.

Field notes from Mexico 4 – tree farmers

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Pico de Orizaba looms large over the landscape of Veracruz State, Mexico. From this vantage point we’re still 3000 m from the summit.

There are two main crops grown on the northern slopes of Pico de Orizaba, a dormant volcano and the highest mountain in Mexico*. Unsurprisingly, one is maize, the standard subsistence crop in this region. The other is the pine tree Pinus patula.

A maize field on the valley floor

A developing plantation of Pinus patula. Here, close to the village, they also graze sheep beneath them.

We’ve spent two days this week getting to know the northern face of Pico de Orizaba, which is the side where the majority of coniferous species are found. As in so many parts of the world, our explorations are complicated because anywhere that’s accessible has been transformed by humans, which means that even if we can spot intact forests through the telescope, reaching them would be nigh-on impossible without a guide, climbing gear and a lot more time.

The first day was spent on the ridge tops, trying to get a view down into some of the valleys and find a promising route. This was a race against time as the cloud falls rapidly over the course of the day, shrouding everything in thick fog. Eventually we spotted some tall old-growth stands and reached a friendly village where they showed us the trail to reach them. We scouted a short way up the ridge but heavy rain put paid to any further adventures.

The next morning we set out to climb the narrow gorge which led to the forests we wanted to reach. On several occasions we hit waterfalls and had to turn back to climb around them. It also poured with rain for a large part of the day. Our determination was rewarded, however, when we finally emerged into some grand, full-stature forests of Pinus patula, mixed with some P. ayacahuite and Abies religiosa. It was a breathtaking sight and made the long climb worthwhile.

Hiker: “What a gorgeous waterfall!”. Forest ecologist: “How am I supposed to get up this gorge now?”

Bedraggled, but you can see the tall pines just upstream in the valley behind me.

We weren’t the first to reach them though. All the way, our trail had been pock-marked by the hooves of donkeys. It soon became apparent that what we had reached was not an isolated remnant of forest but the current front line of an ongoing, small-scale logging operation.

The dominant cottage industry in this region is the manufacture of low-grade crates and pallets, the kind that are used to transport fruit and vegetables. Many households are surrounded by mounds of sawdust; often someone (usually a woman) is sat outside knocking together an endless series of crates. The improvised sawmill providing the planks is around the back. Their dominant raw material is Pinus patula.

Even in this remote location, the chainsaws get here first.

The forest remains but the giants are being lost.

The end product: stacks of crates.

Charcoal production is another cause of forest loss.

Which brings me back to my comment at the start of the post about farming Pinus patula. Government grants have provided landowners in this area with thousands of seedlings of Pinus patula, which are now being planted all through the valleys. Men can be found peppered across the slopes, clearing the brash and shrubs away in order to plant yet more. The scheme has obviously been running for several decades because, in a few places, the trees are now reaching harvestable sizes.

I will confess to having mixed feelings about this. There is no doubt that the main driver of loss of these magnificent ancient forests has been the manufacture of cheap pine products. On the other hand they are, in some sense, being replaced, which means that the activity could in the longer-term become sustainable. Farmers are at least planting a native tree species, and one which clearly belongs in these valleys. It will act to reduce erosion, prevent flooding, and although not as good as old-growth stands, plantations will still provide habitat for many of the species that formerly inhabited the forests. The waste products — bark and other off-cuts — can be used as fuelwood to reduce their dependence on other sources such as charcoal. Finally, making crates provides a stable income for communities who have lived on and farmed these slopes for generations.

This landscape is so rugged, the topography so steep, that there will always remain some places where the native vegetation persists, out of reach of both donkey and chainsaw. These may only be small fragments but they are crucial in providing continuity, seed sources and safe redoubts from the encroachment of civilisation. I may never reach them or survey them, but I am glad to see them from my telescope, and know that they still exist.

This beautiful forest is under threat. But somewhere in these mountains others like it will remain, simply by virtue of their inaccessibility.

UPDATE: in between writing and posting this piece, the aftermath of Storm Earl led to multiple deaths in the area around Coscomatepec, where we were staying, due to flooding and landslides. It’s worth remembering that behind the headline figure of fatalities is always a larger story of survivors who have lost houses, crops, livestock; many villages will have been cut off. Mexico is a beautiful country but one with great disparities in wealth, and in this tragedy the heaviest burden will fall on those least able to cope.

* Its total height is 5,636 m, but most striking is its prominence, rising 4,922 m above the surrounding landscape. It really does stick out.

Two lumps please

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Here’s a quick thought experiment. Imagine you have a spare flowerbed in your garden, in which you scatter a handful of seeds across the bare ground. You then ignore them, and come back some months later. What will have happened?* Your expectation might be that you will have a healthy patch of plants, all about the same size. Some might be larger or smaller than average, but overall you’d expect them to be pretty similar. This is known as a unimodal size distribution. They have after all experienced identical conditions.

You’d be wrong. In fact, it’s more likely that your plants will have separated into two or more size groupings. There will be a set of larger plants, spread apart from one another, and which dominate the newly-formed canopy. In between them will be scattered other plants of smaller size. This results in a bimodal (or multimodal) size distribution. There isn’t a standard, expected size; instead there will be different size classes present.

A normal, unimodal distribution of sizes (left) is what you might expect to see when all plants are the same age and growing in the same conditions. In fact it’s more common to see a bimodal size distribution (right), or something even more complicated.

This observation is nothing new. Much was written about the issue from the 1950s through to the 70s, particularly in the context of forest stands. The phenomenon was widely-recognised but remained paradoxical.

I stumbled upon this old literature back in 2010 when I published a small paper based on a birch forest in Kamchatka which showed a clearly bimodal size distribution. I didn’t need to go all the way to Kamchatka to find a stand with this feature; but since I had the data it made sense to use it. I used the spatial pattern of stems to infer that the bimodality was the result of asymmetric competition (i.e. that large trees obtain disproportionately more resources than small trees, which is definitely true in terms of light capture). All the trees were the same age, but the larger stems were spread out, with the smaller stems in the interstices between them. Had the bimodality been the result of environmental drivers we would expect there to be patches of large and small stems, but in fact they were all mixed together.

This is the stand of Betula platyphylla with a bimodal size distribution that was described in Eichhorn (2010). If it looks familiar, it’s because the strapline of this blog is a picture of us surveying it. The white lights on the photo aren’t faeries, it’s the reflectance of mosquito wings from the camera flash. So many mosquitoes.

Three things struck me when I was reading the literature. The first was that hardly anyone had thought about multimodal size distributions in cohorts for several decades**. This was a forgotten problem. The second was that the last major review of the phenomenon back in 1987 had concluded that asymmetric competition was the least likely cause — which conflicted with my own conclusions. Finally, I had no difficulty in finding other examples of multimodal size distributions in the literature, but authors kept dismissing them as anomalous. I wasn’t convinced.

Analysing spatial patterns is all well and good but if you want to really demonstrate that a particular process is important, you need to create a model. Enter Jorge Velazquez, who was a post-doc with me at the time but now has a faculty position in Mexico. He built a simple model in which trees occupy fixed positions in space and can only obtain resources from an the area immediately around themselves. Larger trees can obtain resources from a greater area. When two trees are close to one another, their intake areas overlap, leading to competition for resources.

When there are two individual trees (i and j), each of which obtains resources from within a radius proportional to its size m, the overlap is determined by the distance d between them. Within the area of overlap the amount of resources that each receives depends on the degree of asymmetric competition, i.e. how much of an advantage one gets by being larger than the other. This is included in the model as a parameter p described below.

This is where asymmetric competition is introduced as a parameter p. When p = 0, competition is symmetric, and resources are evenly divided between two trees when their intake areas overlap. When p = 1, each tree receives resources in direct proportion to its size (i.e. a tree that’s twice as large will receive two thirds of the available resources). Increasing p makes competition ever more asymmetric, such that the larger competitor receives a greater fraction of the resources being competed for. In nature we expect asymmetric competition to be strong because a taller tree will capture most of the light and leave very little for those beneath it.

We applied the model to data from a set of forest plots from New Zealand which have already been well-studied. Not only did we discover that two thirds of these plots had multimodal size distributions, but also that our model could reproduce them.

We then started running our own thought experiments. What if you changed the starting patterns, making them clustered, random or dispersed? That turned out to have very little effect on size distributions. What about completely regular patterns? That’s when things started to get really interesting.

By testing the model with different patterns we discovered three important things:

Asymmetric competition is the only process which consistently causes multimodal size distributions within simulated cohorts of plants. Nothing else we tried worked.
Asymmetric competition is the cause, not the consequence of size differences in the population.
The separation of modes is determined by the length of time it takes for competition in the cohort to start, which usually reflects the distance between individuals.
The number of modes reflects the effective number of competitors that each individual has.

What does all this mean? Given that asymmetric competition is normal for plants, I would argue that we should expect to see multimodal size distributions everywhere. In fact, seeing unimodal size distributions should be a surprise. Don’t believe me? Grab some seeds, give it a go, and tell me if I’m wrong.

You can read our new paper on the subject here. If you can’t get hold of a copy then let me know.

* Luckily this is a thought experiment, because in my garden the usual answer is ‘everything has been eaten by slugs’.

** I should stress here that I’m specifically referring to multimodality in size distributions of equal-aged cohorts. When several generations overlap then the distribution of sizes reflects the ages of the individuals. If multiple species are present this adds additional complications, and in fact size distributions of species across communities have been a hot topic in the literature of late. This is very interesting but a completely different set of processes are at work.

Trees In Space

Boldly going where no forest ecologist has gone before