New paper: How strong are mangrove trees?

I did a thing: I published my first PhD paper! The official version is here. Too long? Here’s my short take:

We wanted to know if we can use mangrove forests for coastal flood protection. So we measured how strong mangrove trees are and if storms can break them.

We need more dikes but they’re expensive

A lot of us humans across the globe, myself included, live near water. I guess it’s nice to live near the coast. Beaches are fun. Probably also because historically it was handy, and still is, to ship stuff over water.  But we’ve been a bit dumb causing climate change and now we have to find ways to deal with changing sea levels and changes in storms patterns. It’s probably going to be worse. Oops. Yep. So we will probably get more floods, which sucks for obvious reasons.

Good thing we’ve been building dikes for centuries. Bad thing is that dikes are expensive and require a lot of maintenance, and if you want to make them higher you also have to make them wider (otherwise you get really peaky dikes). There’s not always enough money or space to do that.

Mangrove trees can break the waves and help the dikes

Not to worry, we are a creative species and we can find solutions. One of them might be to use mangrove trees. Mangrove forests grow in the sub- and tropical world and they can break waves. It works like this: if a wave rolls through a forest and keeps running into all the tree trunks and roots and branches, it loses a bit of its energy (it’s a bit like trying to make your way through a busy shopping street on a Saturday afternoon, they are just so many people in the way that you can’t be that fast. But if there is no one in the way, like that same shopping street on a Monday morning, you’re gonna get your shopping done so fast). A whole nice big mangrove forest in front of the dike can be a nice wave breaker, so the dike doesn’t have to do all the hard work itself.

If you have a mangrove forest attenuating some waves you might be able to get away with a smaller dike. Might. We’re still figuring out the details.

But storms can damage mangrove trees too…

Ok so mangroves are cool and may be useful. But we should probably figure out how useful. If a  storm hits a mangrove forest, how many trees will be killed by the wind and waves? Is there going to be enough forest left to recover and protect again next time? Not that storms happen so often, but still, it would be nice to know. Like knowing that your insurance policy is still valid. So it might be smart to know what mangroves can handle. And that’s where we come in!

Time for some measurements

We went into the field to figure out how strong a mangrove tree is. It was tiring but quite fun. Long days in the muddy mangroves under the hot Chinese sun and long lunch breaks to recover with lots of yummy Chinese food.

We collected branches and leaves of five mangroves species. These species kind of followed a gradient, where some of them generally grow closer to the exposed, seaward edge of the forest, where it is perhaps a bit stormier; and on the other side, species generally closer to sheltered landward edge of the forest. We figured that the ones on the exposed side might be stronger, so we wanted to see if that’s true.

A big and strong Sonneratia apetala tree, not native to south China but happily growing there anyway.

How strong is a mangrove branch?

We didn’t actually break entire trees because that’s hard and also seemed unnecessarily damaging. Instead we took the branches we collected to the lab, stripped them of all their side branches and put them in a machine to break them. This ‘universal testing machine’ could tell us exactly how much force was needed to do that. This way we had really controlled measurements, which we converted to a size-independent measurement of strength called ‘Modulus of Rupture’. That let us compare branches of different sizes and different species. So that exposed to sheltered gradient that we were interested in? We found that the exposed species tended to be stronger.

Collecting some branches in Hailing Island, south China. My legs are not normally this short, I had a weird size waders on 😉

How strong should a branch be if it doesn’t want to break?

So we knew how strong the branches were. But with that info alone you can don’t much. Because we didn’t know how strong the branches needed to be. Maybe they were waaay stronger than the waves that hit them?

We got some more branches, this time with all the side branches and leaves still attached, and took them to another lab. This lab had a big flume (kind of like a really long bath-tub that makes waves). We placed the branches inside and attached them to a force meter. We tried to make the strongest waves possible to measure how much force the branches were experiencing. This way we could compare to strength of the branches to the force they were experiencing.

Pulling some leaves of trees in our most muddy site.

Does it help if a branches loses all of its leaves?

We were wondering if it helps if a tree to lose its leaves. Because it effectively makes the tree smaller, and leaves are cheaper to grow than branches (less material = less energy = cheaper). So it might be a smart strategy, you lose some leaves so you have less major storm damage.

So we wanted to see how strongly attached the leaves were. We made a simple device with a luggage scale that we attached to the base of leaves and pulled them of trees. It’s not perfectly similar to a storm (none of our measurement were), but hey, you gotta start somewhere. And we found something really cool! It seems like the species that were more exposed on that exposed-sheltered gradient had weaker leaves that detached more easily. Can’t say for sure it’s true, but it perhaps these exposed trees are really a bit more suited to deal with storms.

Do we now know more stuff?

Do we now know perfectly how to design an optimal mangrove forest that will last a hundred lifetimes and can we predict how many storms it can withstand? No. But we do know a little bit more? Yes! There are of course still a lot of ifs and buts, but that’s ok. We discovered a tiny bit more knowledge and we figured out which directions are best to explore next. For a first PhD paper, I’m pretty happy with that!

Naturally I had to get creative with powerpoint to show off the entire paper. So many pictures! So much colours!

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