Research Blog: the Tropics and El Niño part 3

Research Blog: the Tropics and El Niño part 3 Dr Erika Berenguer
13 Jul
Was last year’s El Niño a practice run for future climate change? In part three of a blog series for Geographical, Laura Cole shadows the University of Oxford’s Global Ecosystem Monitoring Network and its work to discover the impact of the world’s most well-known weather phenomenon on the planet’s tropical forests

Click here for Research Blog: The Tropics and El Niño Part 1 and here for Part 2

During 2016’s El Niño – the natural climate event characterised by unusually warm sea temperatures in the Pacific Ocean – much of the world warmed by about 1ºC. In tropical forests, the carbon sink effect of trees ‘switched off’, while there was a surge in the rate of rise of CO2. Luckily, researchers at the University of Oxford had set up field stations all over the tropics, from which they were able to observe the weather phenomenon’s different symptoms such as drought and fire. Here are notes from Dr Erika Berenguer, Dr Terhi Riutta and Dr Sam Moore on their field sites in West Africa, Malaysia and South America:

Forest Fires
Dr Erika Berenguer, Brazil

‘I see fires every year but I have never seen anything like those of 2015. Many days there was so much smoke in the air we couldn’t see the sun, swarms of animals were running away from the burning and the air smelled industrial even though we’re in some of the most undisturbed tracts of rainforest. Logistically we had a big loss of data as half of my monitoring plots were burned, but emotionally it was a very real, very visual loss of biodiversity. If this El Niño was a window into the future, it offered a grim view.

The drought meant the forest became drier and hotter, with more dead, dry debris on the forest floor. The sites became more and more susceptible to fire – a tinderbox – which was then set alight by traditional agricultural practices. Six out of nine Brazilian states are in total forest and for three of those six states, 2015 was a record year for fire. Many of these were parts of the forest that had never burnt before.

The rainforests aren’t used to fire. Unlike the forest fires that we often study at school, fire causes trauma to tropical species. They have a thin bark and fire does not help them disperse their seeds or stimulate growth. What we see after burning is a multiplication of pioneers, scrubby species that thrive in increased levels of sunlight. Our preliminary data shows that 66 per cent of trees died in those plots, compared with six per cent of trees in the unburned plots. The unburned plots were still experiencing drought, but the impact of fire was ten times worse.

In fact, the area burned was larger than the area deforested for that year in Brazil. So it has to start to be taken more seriously as a symptom of drought. Especially as we are likely to experience more droughts and El Niño events with climate change.’

Dr Terhi Riutta, Malaysia

‘In Malaysia there are a lot of logging operations and we have been monitoring a recently-felled site while using an unfelled, old-growth site as our control. During 2015 to 2016, we were conducting an intensive measurement campaign in one of the selectively logged, which turned out to be a great El Niño monitoring site. We used it to look at the immediate response of the trees.

What we saw on the ground was a very clear El Niño drought. In fact, it was so dry that we had to close the camp. The river had dried up and that’s where we get all our water for washing and cooking. In order to keep the monitoring going, we could only keep a very small team there.

We had a lot of leaf fall, which was picked up by our leaf litter traps. Also, the flow of sap in the trees basically collapsed, which was recorded by our colleague Alex Cheesman, from James Cook University. Sap is needed to transport nutrients and water around the plant, and it is s good indicator of how well the tree is functioning. What was really interesting was how quickly they recovered from the drought – the sap flow completely bounced back, it was incredible and a big surprise. It shows the short-term resilience of the tree species we have in Malaysia. That being said, it could be that the El Niño impacts take longer to become visible. If there is a general weakening of the trees from drought conditions, it might be that higher tree mortalities don’t show up until the next census.

Malaysia is one of the most logged areas in the tropics, and now there are now more logged areas than undisturbed areas. Nonetheless, most of the literature that we have comes from old-growth forests, which have different species composition, micro-climate and tree density. So when we are are talking about the carbon cycle in Malaysian trees we are actually missing data from half of the forest – we don’t know if their responses are actually comparable. Logged forests are already drier and hotter than old growth so it could be that they are closer to the tipping point of drought. Or perhaps they cope with drought better than old-growth. It is still an unknown. Along with researchers at the Stability of Altered Forest Ecosystems (SAFE) project, together with the Royal Society’s South East Asia Rainforest Research Program (SEARRP), we are trying to answer these questions. So far, we have seen that logging has had a far bigger impact on these trees than anthropogenic warming. Of course that may change depending on how they respond to increasing El Niño events and a warmer climate.’

treefallFallen trees in the Gabon plots – could they be attributed to El Niño? (Image: Sam Moore)

Sam Moore, Gabon and Ghana

‘When I went out to our Gabon plots in April, I didn't recognise one of them. Upwards of 28 trees had fallen down and big ones too, up to 30 to 40 metres high. We traced the fallen trees back to the first to come down. It looked as though it had basically uprooted, its stem had broken and then it fell onto several other trees, causing a domino effect. What had probably happened was that a large wind speed would have knocked over the first tree and created this effect. It wasn’t that 28 trees had died, it was that one had died and had wiped out other trees, some even bigger than itself.

Tree falls happen, El Niño or not. But the question here is: could it be that the conditions of El Niño increase the rates of tree fall compared to other years? Would the tree have fallen had the soil not been so dry? It is a little too easy to go out to these places and attribute every observation with the weather, but we just can’t tell that yet. Had this tree fallen another 200 metres to the left, there wouldn’t be any sign of this tree mortality at all. It’s chance. El Niño may just be increasing that chance.

But what we can do is start to link these events with contributing factors such as soil moisture content and rainfall for the preceding few months. Then we could start to look at whether more tree falls occur when the soil moisture drops beyond a given threshold. Another factor I’m more confident can be attributed to El Niño is tree growth. We put bands around all types of tree species to measure their growth over time. What we saw from the end of 2015 – a few months after the onset of El Niño – was shrinkage, or negative growth. We saw this almost uniformly across different trees sizes and across different species. So to me this is clearly something on a larger scale. It is something we see in Ghana and Gabon and across different types of forests: wet evergreen and drier semi-deciduous forests.

Why are the trees responding this way? It could be that they are decreasing stem growth so that they can invest more carbohydrates into the next bunch of leaves. Or they could be investing in the roots because the water is deeper in the soil. However, stem growth by far takes in the most carbon. So when I see a decrease in the dendrometers, it’s no surprise that their overall biomass.’

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