They are seen as one of the best natural solutions to deal with climate change. The catch? Global warming could deal a serious blow to many of them. What future for trees in a warmer world? The subject was discussed at the 89th congress of the Association francophone pour le savoir (Acfas), presented until Friday.

What happens when a tree faces a scorching drought? He may be faced with a painful choice: to starve or to die of thirst. This is indeed the physiological dilemma that trees face during a drought, explained researcher Hervé Cochard at the Acfas congress.

Basically, when it lacks water, a tree has to make a choice. Either he does what is necessary to collect as much water as possible to ensure his survival. In this case, its ability to absorb carbon is reduced and it could starve. But if he favors the carbon option, he could die of thirst. This is the deadly dilemma that trees will face as the planet warms.

“The risk of [tree] mortality is going to be greatly increased with climate change,” Cochard said during his presentation titled The Physiological Basis of Drought Resistance of Trees. And this is not good news considering their importance in the fight against climate change.

The researcher explained in particular the large difference in the death rate, depending on the scenario where the world succeeds in limiting warming to 1.5 ℃ or even 2 ℃ or a scenario of high emissions leading to warming above 2 ℃. For trees, the first scenario is clearly preferable to the second.

However, if more trees die due to global warming, there will be fewer opportunities for carbon sequestration on a global scale. It will get even hotter, which will amplify the tree mortality rate, and so on…

“This problem is still little taken into account in the forecasts of the IPCC [Intergovernmental Panel on Climate Change], reports Hervé Cochard to La Presse. Part of the problem comes from the bias of certain models that are sensitive to the fertilizing effect of CO2, he adds. With the increase in CO2, the stomata of the leaves close and, as a result, consume less water and also capture less carbon. These models are therefore rather optimistic, as they predict that plants will grow more and consume less water. But these models do not take into account the VPD effect, i.e. the fact that the hot air is also much drier, and that with the increase in CO2, we must also expect an increase in the air warm. This will strongly impact the ability of trees to store carbon. »

A study recently published in Nature Review also shows that models that incorporate the VPD effect demonstrate that the gain related to CO2 fertilization is not enough to offset this effect.

In short, as Zachary Richard sang, The tree is in its leaves. But the song doesn’t say what would happen if the tree had no more leaves…


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