Over a span of approximately 32 years, from 1988 to 2020, 457,474 km2 were deforested in the Brazilian Amazon — an area much larger than Italy’s and almost equal to Spain’s. And the pace of deforestation, which had slowed, has picked up again in the last four years—especially in 2022.
An auspicious fact in this scenario is that 120,000 km² of deforested area, destined mainly for the formation of pastures and then abandoned, have returned to passively regenerate, through natural processes.
At the same time that deforestation and degradation of the remaining areas urgently need to be stopped, the forest offers windows of resilience that can be used intelligently to promote regeneration. The article “Seizing resilience windows to foster passive recovery in the forest-water interface in Amazonian lands”, recently published in the journal Science of The Total Environment, provided substantial information in this regard.
“There are currently many areas under passive regeneration in the Amazon. And, in the region we studied, located in the municipality of Paragominas, in the State of Pará, the forest located on the banks of streams has recovered structural attributes [densidade de indivíduos e de dossel] from 12 years, while the recovery of the basal area occurred in 18 years”, says to Agência FAPESP the researcher Felipe Rossetti de Paula, post-doctoral fellow at the Luiz de Queiroz Higher School of Agriculture of the University of São Paulo (Esalq-USP) and first author of the study.
Rossetti de Paula points out that most of these regenerating areas are located on the edges of water bodies, commonly known as riparian zones or riparian zones. “The importance of having forests in riparian zones is due to the fact that stream ecosystems are narrow and, therefore, almost completely covered by the canopy. Thus, the food resources that sustain the base of the food chain in these watercourses come from leaves, fruits and insects that fall into the liquid medium and are decomposed and used by microorganisms, later consumed by aquatic invertebrates, which will later serve as food for fish”, he says.
This sequence characterizes streams as predominantly heterotrophic systems – that is, they depend on external resources. When riparian forests are deforested, the canopy is eliminated and, along with it, the organic inputs that maintain the system’s heterotrophy. It then becomes autotrophic, having to generate its own energy source to sustain the food chain.
At this point, the role of decomposing fungi in the food chain is replaced by photosynthetic organisms, such as algae, microalgae, and aquatic plants, which use sunlight to produce their food, which will then be consumed by aquatic invertebrates and so on. In this condition, increases in light levels and temperatures in the system can also cause the exaggerated growth of microalgae, increasing the turbidity of the water and making it less suitable for consumption by local populations. In addition, recent studies have shown that high water temperatures decrease the growth of native fish species, which are less tolerant of this condition.
“With the regeneration of the riparian forest, the canopy is recovered, and with it, the supply of organic material and the control of the entry of light into the aquatic ecosystem. The system as a whole returns to heterotrophic status”, summarizes Rossetti de Paula .
The researcher also emphasizes that the large trees that fall into streams have highly relevant ecological functions, such as providing shelter for fish within cavities, providing food and attachment sites for aquatic invertebrates and, most importantly, damming the flow of water, creating small natural pools that are places of reduced flow and retention of organic material and nutrients.
“Without these wells, the availability of food resources and nutrients is reduced, as they tend to be transported more quickly by the water flow. Such pools are also important habitats for fish that use the water column to swim, such as lambaris” , informs Rossetti de Paula.
Therefore, deforestation in riparian zones also eliminates the contribution of trees to streams, and consequently, all their functions within the aquatic ecosystem. And even as regeneration progresses, it takes longer to recover the supply of large trees to the stream than to supply leaves and control sunlight, as the trees take longer to grow in diameter than to develop the canopy.
“A young forest, with trees of small diameter, will even provide trees for the stream, but the wells formed will be small and temporary, as small trees are more quickly decomposed or more easily carried by the flow of water”, argues the researcher, who underlines the importance of taking advantage of the windows of resilience constituted by streams that still have large trees fallen in their course.
“Passive regeneration has an implementation cost practically zero compared to conventional restoration projects, which require preparation, soil recovery, planting of seedlings and management of the area so that the seedlings do not die. Considering the high resilience still present in the Amazon , the chance of riparian forests recovering is very high”, explains Rossetti de Paula.
And he states that many streams still have large fallen trees in their interior, offering shelter and resources for aquatic organisms, which are an important source of biodiversity retention even after deforestation. These opportunities should not be missed.
“If we don’t take advantage of the trees still inside the channel, they will be decomposed and lost, and when riparian regeneration starts, there will be a huge gap until the trees grow in diameter and then fall into the stream. In that gap, the stream will be without many of the functions exclusively provided by the trunks, which will lead to local extinctions and loss of biodiversity”, ponders the researcher.
Considering the enormous diversity of fish in Amazonian streams, it is essential to protect these highly biodiverse ecosystems, which also provide ecosystem services for local populations. In this sense, the enormous potential for passive regeneration of these forests and the large trunks still present in the stream should be taken advantage of to accelerate regeneration at a low cost and with many environmental gains.
Rossetti de Paula points out that in other areas, such as those in the State of São Paulo, passive regeneration may not be as efficient as in the Amazon, due to the long history of deforestation and degradation that possibly depleted the area’s sources of natural regeneration.
“In some areas that we studied in the State of São Paulo, such as the Corumbataí River Basin and the Experimental Station of Forest Sciences in Itatinga, we found riparian forests with approximately 32 years of age, with tree diameter values much lower than those of regenerated plants from our study in the Amazon”, he says.
It is also worth noting that a large part of riparian forests under regeneration is located on rural properties and surrounded by agricultural activities. These can act as sources of disturbances, which delay or eventually prevent regeneration.
For the initial quantification of the age of regeneration, the study initially used a regeneration map provided by the Instituto do Homem e Meio Ambiente da Amazônia (Imazon), based on satellite images with 30 meters of resolution, corresponding to the period from 1988 to 2010. Subsequently, periodization was expanded until 1984, with images available on the Google Engine Timelapse platform. “In addition to expanding the periodization, this allowed a better quantification of the age of regeneration and also of the time the area remained under pastures before the beginning of regeneration”, says Rossetti de Paula.
The study was conducted in Paragominas, in the State of Pará, a municipality that has some important peculiarities. Since its establishment in the 1960s, in the wake of the construction of the Belém-Brasília highway, it has been the scene of intense deforestation, mainly for the extraction of wood and the establishment of pastures. However, many areas quickly became unproductive and were eventually abandoned, which started the process of passive regeneration. In addition, Paragominas has recently embarked on sustainable initiatives, such as the Green Municipalities, which have also contributed to natural regeneration.
“An important piece of data from our study was that it was concentrated in a region farther from the municipality of Paragominas, within a huge forest area under sustainable management, which also helped the process of passive regeneration, since the proximity of the surrounding forests increases the sources of regeneration”, emphasizes the researcher.
Data were collected between 2014 and 2016, during Rossetti de Paula’s doctorate at The University of British Columbia, Canada, with support from FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo), under the guidance of Silvio Frosini de Barros Ferraz. In 2018, Rossetti de Paula continued her study with a FAPESP postdoctoral fellowship.
