Forests in the southern and western Amazon at risk of drought-induced embolisms and mortality

“Deforestation and rising temperatures have disrupted rainfall patterns in a way that has increased the risk of forest mortality.”

Published in the journal Nature, an article arising from a study involving CREAF and led by the University of Leeds reveals that climate change is weakening the forests of the southern and western Amazon. The areas affected include Rio Branco (Acre) and Nova Xavantina (Mato Grosso), in Brazil. Having compared how capable different tree species in the Amazon are of withstanding water stress, the study found that rainfall pattern changes and warming in the south and west of the region have pushed trees to the limits of their ability to adapt, leaving them facing possible embolisms and death. “The southern forests are better suited to dealing with dry conditions, but deforestation and rising temperatures have disrupted rainfall patterns in a way that has increased the risk of forest mortality,” says CREAF-based ICREA researcher Maurizio Mencuccini, who co-authored the article and oversaw the physiological part of the study.. “The western forests are less exposed to drought but are more vulnerable,” he adds. Contrastingly, the study does not envisage forests in the central-eastern Amazon being affected in the same way, despite their tree species being less suited to coping with drought, because, for the time being, there is still sufficient water in the area in question.

The study suggests that the effects of climate change may have been underestimated until now, due to previous scientific research having focused on the central-eastern area, where trees are less exposed to drought.

“That’s why the results are so revealing: they help us paint a more accurate picture of what the Amazon’s forests will be like in the future and how they will be affected in terms of weakening and mortality.”

MAURIZIO MENCUCCINI, CREAF-based ICREA researcher, co-author of the article and supervisor of the physiological part of the study.

The study also found that climate change, deforestation, and intensive crop production and livestock farming are altering rainfall patterns in the Amazon, the effect of which will be a reduction in the ability of the region’s forests to store carbon and remove CO₂ from the atmosphere. “When trees die, they decompose and the carbon they contain is released into the atmosphere; additionally, a smaller forest mass will mean fewer trees capturing CO₂,” explains Mencuccini. “Forests across the Amazon hold between 10% and 15% of the carbon stored by vegetation throughout the world, so their mortality and good health are key factors in the fight against climate change,” he concludes.

To identify the areas most at risk and how each ecosystem is responding to the changes occurring, the team of researchers took measurements and samples, over a two-year period, at 11 different sites in the western, central-eastern and southern Amazon, in Brazil, Peru and Bolivia. In total, they analysed data on 540 trees from 129 species. According to the study’s co-supervisor, Professor Emanuel Gloor from the University of Leeds, “the pattern of resilience and risks identified among the different tree populations in the study will be used to build more effective and accurate climate models of the way the Amazon may change as the region responds to climate change.” The study’s findings are the result of cooperation between 80 scientists from Europe and South America.

Embolism risk

If there is not enough water available in the ground, a tree’s conduits become blocked, its sap cannot flow, and its tissue dies.

The study shows the trees in the southern Amazon to be more resistant to embolisms. The problem is that they have reached their limit, something the scientists determined by studying the flow of water in tree trunks, branches and leaves, a method fundamental to their research. Each species has a particular level of resistance to drought, depending on the conduits via which it transports sap from roots to leaves. If the conduits in a tree and the pores that connect them are large, it is at greater risk of an embolism when water is in short supply. Why? Because when there is not enough water available in the ground, evapotranspiration (the mechanism by means of which trees return water to the atmosphere through their leaves) generates pressure that causes air bubbles to develop in transport vessels, similar to embolisms in human blood vessels. The tree’s conduits become blocked, its sap cannot flow, and its tissue dies. The researchers found that the parts of the Amazon with more exposure to such embolisms have higher mortality rates and a lower carbon storage capacity.

Rainfall pattern disruption

The research team also warn that deforestation and intensive crop production and livestock farming are interrupting the evapotranspiration process, which is essential to rain cloud formation. Trees are fewer in number and the crops (such as soybean) planted where forests used to stand do not have the same ability to cool and moisten the air, resulting in a drier atmosphere and reduced precipitation.

The consequences of the above are not limited to the Amazon. The water vapour that the region’s forests release into the atmosphere is transported, via what are known as flying rivers, to other parts of the planet, where it forms clouds. Disruption to that cycle would pose a threat to rainfall patterns worldwide.

Article: Tavares, J.V., Oliveira, R.S., Mencuccini, M. et al. Basin-wide variation in tree hydraulic safety margins predicts the carbon balance of Amazon forests. Nature (2023).
https://doi.org/10.1038/s41586-023-05971-3