Oops there goes another rubber tree

2.10

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.**

image3

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.

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!

 

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