Amazon and Atlantic Forest could become dry forests – 06/15/2023 – Fundamental Science

Much of the work of ecologist Catarina Jakovac consists of delving into degraded forests. One day, she and her team were in an area in Rondônia that was beginning to regenerate – a place “still quite ugly”, she says –, when they suddenly came across a family of tapirs walking with their babies towards a lake to drink water. Everyone stopped to watch. “There are these surprises”, says the scientist, who hopes to discover others in her research on the regeneration of forests in the face of human action.

Jakovac’s hypothesis is that, in this forest regeneration process, given the impact of activities such as livestock, agriculture and extractivism, species from dry forests tend to prevail over those from humid forests. In other words: you know that image of the Amazon and the Atlantic Forest as very green and dense forests, with high air humidity and lots of rain? Perhaps it is already becoming an inaccurate representation of these biomes.

Species that occur in drier environments are better able to withstand climate change and fires. It’s just that disturbances like these favor those who invest more in surviving than in growing fast. And, in humid forests, growing fast is essential to win the competition for light, which largely governs the dynamics and productivity of the forest. “The selection of certain species due to human impact can lead to a cascade of effects that will homogenize the forest formations”, says Jakovac, a professor at the Federal University of Santa Catarina (UFSC).

It is worth mentioning that forests tend to be quite resilient and have the capacity to return to what they were before degradation. But, depending on the pressure suffered, going back to what it was before may become impossible. And then the so-called transition of biomes could happen: they become another ecosystem. Tropical forests like the Amazon, for example, could become savannas – this, incidentally, is one of the most studied hypotheses by scientists in recent years, although the most recent evidence suggests a transition to degraded forests or dry forests.

Analyzing the distribution of forests around the world, we see that more humid forests prevail in climates with rainfall above 2 thousand millimeters of rain a year, and savannas predominate in regions with less than 1,500 millimeters a year. But there is a midway zone, between 1,000 and 2,000 millimeters, where it is possible to have both. “These are bi-stability regions, where you can find humid forest or savannah depending on factors other than the climate, since the latter allows for both”, explains Jakovac.

Disturbance is a crucial effect to determine the prevalence of savanna in certain situations, such as in regions of Africa, in the Brazilian Cerrado and in the pampa of the south of the country. But, between humid forest and savannah, dry forest is something in between. It includes species that are in transition between the Cerrado (mainly characterized by the savannah biome) and the Amazon; between the humid and evergreen Atlantic Forest, and drier vegetation, with a preponderance of species that lose their leaves.

This happens because nature is more complex than the definitions we create for it. Within what we call the Atlantic Forest biome, for example, there are different plant formations: the humid Atlantic forest, which can be seen in Serra do Mar; the semi-deciduous, towards the interior and up the mountain range; the Araucaria forest, specific to the south of the country; the rupestrian fields, located in high places like the Serra dos Órgãos.

The savanna itself, although characterized by fields dominated by grasses, also has species such as the ipê, which loses its leaves during the dry season in order not to transpire and save water. And, in the Amazonian biome, there are patches of Cerrado that are native, related to the type of soil and water availability in those areas, and not the result of degradation.

When researchers went to study the hypothesis of savannization in the Amazon, they saw that the species that dominated the new open ecosystems in the region were not typical of the savannah, but native or exotic invasive grasses and fire-resistant trees typical of drier regions. This phenomenon occurs because there is an overlapping of species from wetter forests and from drier forests in the border regions of the Amazon. And the latter are more likely to survive and persist in the landscape in a scenario of climate change, successive fires in pasture areas and degradation in general.

A consequence of this forest homogenization, which perhaps sounds obvious, is the loss of biodiversity. And, in more equal environments, resilience tends to be lower. “In the current situation – of climate change, an increase in extreme events and even a certain amount of unpredictability –, a smaller variety of species can reduce the chances of that system regenerating”, explains the scientist.

Another consequence would be the reduction of carbon sequestration – the process of removing carbon dioxide from the atmosphere and transforming it into oxygen, naturally carried out by forests, oceans and soils –, since humid forests have much higher carbon sequestration rates than of dry forests. One of the reasons is that, in dry environments, trees have slower growth, as their wood is denser. In practice, this change in the dynamics and growth of forests can reduce the rate of water renewal and affect the local microclimate.

To measure the regeneration of forests, Jakovac sets up a kind of chronosequence: because she cannot spend 20 years monitoring the same area to find out what will happen there, she measures one that is five years old, another that is ten, another that is 20. “I do a lot of forests to get a good estimate of the growth rate, because there is a whole range of environmental variations that influence their characteristics.” In the forest, she takes the diameter of the trees, collects wood samples to measure their density, and seeds to test their tolerance to drought.

All this, of course, is done with the help of many people – for example, botanists who specialize in identifying trees. But participation in research goes beyond scientists. “One of the coolest parts of being in the countryside is the contact with a very rich forestry culture, not only indigenous, but also cabocla, caipira. We met people who know everything about the plants and are capable of taking us to any species. We learned a lot and we depend too much on them. In the Amazon, many times we live in the houses of riverside people.”

Born in the city of São Paulo, Catarina Jakovac, a climber and hiker, says that it was leisure that led her to science. As a teenager, she was more interested in the sea and thought she was going to be a marine biologist, but over time she was seduced by the forests. She ended up even marrying a biologist, with whom she has worked together. “Fun joined work, which made me have even more contact with nature. For me it was very good.”

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Clarice Cudischevitch is a journalist, coordinator of the blog Ciência Fundamental and Communication manager at the Serrapilheira Institute.

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