- Drought and high temperatures amplify the destructive effects of deforestation and wildfires.
- Across the Amazon Basin, tree species adapted to drier conditions are becoming more prevalent, and in the Central Amazon, savannas have replaced floodplain forests in just a few decades.
- While deforestation remains a main concern, the impacts of forest degradation are becoming increasingly important.
The climate in the Amazon has been changing over the last few decades. The average temperature in the basin rose about 1º Celsius (1.8° Fahrenheit) between 1979 and 2018, with increases of up to 1.5ºC (2.7°F) in some regions. And there have been three “one-in-a-century” episodes of extreme drought in the last 15 years: in 2005, 2010 and 2015.
When the forest becomes too hot, trees need more water to cope. Extreme temperatures have a huge impact on the forest: When temperatures exceed 32.2°C (90°F), forests begin to lose biomass and release carbon, a large international study has found. The authors estimate that these temperatures will affect most of the Amazon by mid-century, even if greenhouse gas emissions are curbed.
As the region becomes hotter and drier, the forest is beginning to adapt. Recent research shows that trees that thrive in moist environments are dying and being replaced by species that are more adapted to dry conditions.
“If you stop to think that forests that don’t have a direct human action are changing in such a drastic way that we can detect them with data from 30 years, that by itself is something worrying to me,” says Adriane Esquivel-Muelbert, an ecologist at the University of Birmingham who was the lead author of the study. She also notes that these new species grow faster and die earlier than the ones they are replacing. “With that increased turnover rate their carbon stocks will likely be much lower because they are smaller and accumulate carbon for less time.”
The biggest changes to the climate are occurring along the southern and eastern regions of the basin, where precipitation is lower and where agriculture has historically encroached into the forest. In some of these areas, the average temperature is now up to 3ºC (5.4°F) higher during the hottest months, and the dry season is becoming longer, increasing the likelihood of droughts and fires.
In this region, there is evidence that deforestation is driving some of these changes in climate. One study estimated that in the state of Rondônia, in the southern part of the Amazon, deforestation alone is responsible for a delay in the onset of the rainy season of about 11 days.
But what has scientists worried is that the impacts of temperature, drought and wildfires are also beginning to be felt in remote and well-conserved areas, where human impacts are still low.
“Trees in Central Amazonia are not adapted to fire so potentially it could have a very large impact,” says Aline Pontes-Lopes, a researcher at the Brazilian National Institute for Space Research (INPE), who has just authored a study that looks at how forest plots in Central Amazonia recovered from wildfires caused by the 2015 drought.
Although humidity in this region has rendered fires less destructive historically, new research has found that humidity may not protect the region from “higher intensity fires arising from climate change.”
Already, repeated fires in the region in recent years have created white-sand savannas. The process takes place in floodplain forests locally known as igapós. These forests experience intense annual pulses of drying and flooding. During the wet season, water levels cover large parts of the trees. However, when the dry season comes, water levels drop more than 5 meters (16 feet).
The soils of these forests are relatively rich in nutrients because the tree roots form a mat that protects the soil from water erosion. But during the dry season, the root mat can become exposed and flammable. If fires occur repeatedly over a short period, the areas are unable to recover. Soils lose nutrients and become sandy. And over the next few years, the area is colonized by savanna tree species, which are more adapted to these new open spaces.
“Nobody expected this could happen in just 40 years,” says Bernardo Flores, a researcher at the Federal University of Santa Catarina and co-author of the study. Flores also noted that the wildfire regime is “increasing abruptly.”
“During the 2015-2016 drought, which was the strongest of the century, the area burned in my field site was seven times higher than all the area burned during the previous 40 years.”
Wildfires and deforestation have long-lasting effects in the regions where they occur. Because the structure of the forest is changed, many areas become vulnerable and progressively less resilient to new disturbances. These effects are amplified by droughts and high temperatures, and are often grouped under the term “forest degradation.”
More vegetation is lost today in the Amazon due to forest degradation than to direct deforestation, according to two recent studies.
While significant amounts of carbon are released during wildfires, the majority of carbon released actually occurs in the years after fires have stopped burning. This happens through delayed mortality and decomposition, and is only partially offset by regrowth in the burned areas. A study estimated that plots that had experienced a single fire store about 25% less biomass 30 years later than undisturbed plots.
Something similar happens with deforestation, which also has a long-lasting effect on the trees that are spared. When an area is logged it becomes fragmented. This increases the exposure of trees that were once surrounded and protected by other trees to drier air, intense winds, and more solar radiation, a phenomenon called “edge effect.”
This edge effect has important consequences. A 2020 study showed that these areas lose carbon for five to six years. When they became stable, their biomass stores 37% less carbon than it did originally.
Tackling deforestation and improving land planning are two key points to address forest degradation, says Luiz Aragão, a researcher at INPE.
“We already have a framework of tools for territorial planning that would allow us to minimize these impacts, mainly from carbon emissions,” he says. “It is obvious that many areas need infrastructure, but it must be done in such a way as to minimize the damage to the forest’s environmental services.”
Yet politicians do not seem to be following these frameworks.
The government, for instance, plans to repave the BR-319, a major Amazonian road that runs more than 800 kilometers (500 miles) across Indigenous lands and protected areas and lacks an environmental impact study. According to recent modeling, it would increase the rates of deforestation in the region fourfold.
“We can’t continue to follow development plans from the ’60s and ’70s,” Aragão says.
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Banner image: Aerial view of burned areas in the Amazon rainforest in the city of Porto Velho, Rondônia state. Image by Victor Moriyama/Greenpeace.