Not thirsty at all: when trees get too much water

Much of the recent research examining likely effects of climate change on tropical forests has focused on the threat of more frequent and severe drought in tropical regions. This focus isn’t surprising. Tropical forests can receive up to four metres of rainfall per year. I think it’s natural for us to be concerned that such wet ecosystems will be highly sensitive to drought.

But a recent study from Esteban et al. (2021) has demonstrated that wet extremes (lots of rainfall at once) can decrease tree growth and increase tree mortality far more strongly than drought. The reason for this remains an open question, but my work on the physiology of rainforest trees in Borneo suggests that tropical trees could frequently suffer from waterlogging – a situation when the soil is so saturated that a tree can’t get oxygen to it’s roots. Here, I want to tell you a short story about what that means in the life of a tree and how clever trees can be in trying to counter the negative effects of excess water.

An energy crisis

Trees growing in waterlogged soils have a very hard time. The lack of oxygen in the soil affects their ability to generate the energy needed to grow, roots and leaves can start to rot and become more vulnerable to disease and to pests, and the risk of death sky rockets.

We know that waterlogging triggers a chain of events, starting in the roots. When the soil is full of water, it is much more difficult the oxygen to diffuse from the air into the soil. This difficulty can lead to hypoxia – a critical drop in oxygen levels that starts to affect the metabolism of the tree. Even when the hypoxia is only mild, some tissues inside the tree roots can be totally deprived of oxygen.

Under normal conditions, with oxygen in the mix, trees use what is called ‘mitochondrial respiration’, which allows them to make 36 lots of energy for every lot of sugar they produce during photosynthesis. But when hypoxia sets in, there isn’t enough oxygen around for mitochondrial respiration to continue. The clever tree activates an alternative. Our tree starts fermenting the sugar instead. This fermentation doesn’t need oxygen, but it is wildly inefficient. Now, the tree can only make 2 lots of energy for every lot of sugar produced during photosynthesis.

But that’s not the only compromise in play. Fermentation also makes waste products that are toxic to the tree. Together with the critically low oxygen levels, these toxic compounds make it more difficult for a tree to breath.

Under normal conditions, trees draw up water from the soil, which travels upwards through the stem and out through tiny pores in the leaves. This flow of water from the leaves is an essential part of photosynthesis, wherein trees absorb carbon dioxide from the air and use energy stored in water molecules to turn this carbon dioxide into sugars – yummy! The toxic compounds built up by fermentation increase the resistance in this essential pathway, so the flow of water out of the leaves becomes hindered and photosynthesis starts to shut down (ref1, ref2, ref3, ref4).

Lastly, waterlogged trees can also suffer for lack of nutrients. Because roots have to become less permeable to water when they’re flooded, it probably becomes much more difficult to absorb nitrogen from the soil – an essential ingredient for photosynthesis and regular metabolism. 

Together, the switch to fermentation, the build up of toxic compounds, the nutrient shortage, and the shutdown of photosynthesis create an energy crisis for our tree. This only gets worse when the roots start struggling to grow and trees end up top-heavy – a smaller volume of roots needs to supply the same amount of wood and leaves with the nutrients and water they require (ref1, ref2).

A clever tree (a tree with the adaptations that make it able to respond to waterlogging) can make a lot of adjustments to its body and its inner workings. And the cleverness is all in the roots.

Photo credit: Mohsin Kazmi. Source: https://www.rainforest-alliance.org/articles/5-rainforest-tree-species-we-love

Trees under stress from waterlogging do a few things. First, they can produce roots with air cavities inside – called aerenchyma. These cavities allow the tree to truck oxygen from the stem and leaves aboveground, which have direct access to the air, down into the roots. What’s more, some clever trees can form a tight barrier on the outside of their roots, to stop this oxygen escaping. Think of it like a tunnel or a pipe – the tree is pumping oxygen down to the roots, and builds a tunnel to make sure the oxygen travels all the way down to the tip of each root. Alternatively, the clever tree can also start building roots that are more dense, which makes it harder (literally) for fungus and insects to infect the roots.

In a study of tree seedlings in Borneo, Born et al. (2015) showed that trees growing at fractionally higher micro-elevations survived better. Our forthcoming study suggests similar. Despite taking measurements from trees that have been periodically droughted with an interest in the effects of increased drought under climate change, we see adjustments in chlorophyll, leaf nitrogen and plant water use that all suggest the shutdown of photosynthesis on saturated soils. And survival is lower for trees in this setting too.

Let us go back to the study we started our story with – Esteban et al. (2021) observing reduced growth and survival for trees during extreme wet events in the Amazon. These scientists don’t conclude that waterlogging is the cause of these negative effects. Instead, they point out that heavy rain brings heavy clouds and strong wind with it. They suggest that thick, persistent cloud cover reduces the availability of light for photosynthesis and increases the likelihood of high winds that can be truly destructive for tall trees in tropical forests. In the context of their study, this conclusion makes sense. But given that targeted studies of the effects of waterlogging show the consequences are so significant, perhaps we should start asking more frequently–as Esteban and co. did–whether the tropical forests we study are waterlogged?

What does this all mean? It means, in short, that I agree with Esteban and co.– the increased severity of wet events under climate change, alongside drought – are likely to give tropical trees a very hard time.


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