Amazon Rainforest’s resilience creates windows of opportunity for passive regeneration

A study conducted at the University of São Paulo highlights the importance of riparian forests in the process.

José Tadeu Arantes | Agência FAPESP – In a period of 32 years between 1988 and 2020, the destruction of the Amazon Rainforest in Brazil reached 457,474 km2, an area greatly exceeding that of Italy and almost the size of Spain. Worse still, the pace of deforestation, which had previously slowed down, has accelerated again in the last four years, especially in 2022.

The good news is that deforested areas amounting to 120,000 km², cleared mainly for use as pasture and later abandoned, have regenerated passively via natural processes.

At the same time as deforestation and degradation of remaining forest areas must urgently be halted, the Amazon displays “windows of resilience” that can be used intelligently to promote regeneration, according to an article published in the journal Science of The Total Environment.

“Many areas are passively regenerating in the Amazon. In our study location, which was Paragominas, Pará, riparian forest areas recovered structural features [stem and canopy density] within 12 years, and basal area within 20 years,” Felipe Rossetti de Paula, first author of the article, told Agência FAPESP. Rossetti de Paula is a postdoctoral researcher at the University of São Paulo's Luiz de Queiroz College of Agriculture (ESALQ-USP) in Brazil.

Many of the regenerating areas are riparian or gallery woodlands fringing water courses, he continued. “The importance of having trees along rivers and streams is that riparian ecosystems are narrow and therefore almost entirely covered by the forest canopy, enabling microorganisms at the bottom of the food chain in these water courses to feed on leaves, fruit and insects that fall into the water and decompose there. These microorganisms in turn are consumed by aquatic invertebrates, which themselves are food for fish,” he said.

This sequence makes aquatic organisms predominantly heterotrophic (dependent on external sources of food). When riparian forest is destroyed and the canopy disappears, organic food sources vanish with it, forcing the organisms in the ecosystem to become autotrophic and produce their own energy and nutrition.

At this point, the role of decomposer fungi in the food chain is replaced by photosynthesizing organisms such as algae, microalgae and aquatic plants, which use sunlight to produce their own food. These organisms are themselves consumed by aquatic invertebrates, and so on. Rising levels of light and temperature in the system can lead to excessive growth of microalgae, increasing water turbidity to an extent that hinders consumption of the water by the local community. Recent studies have also shown that native fish species flourish less in warmer water.

“Riparian forest regeneration restores the canopy, providing the aquatic ecosystem with organic matter and controlling light penetration down into the water. The entire system returns to a heterotrophic state,” Rossetti de Paula said.

Large trees that fall into water courses have important ecological functions, such as providing cavity shelters for fish, and food and anchorage for aquatic invertebrates. Most importantly, they slow down or divert water flow, creating quiet pools that retain organic matter and nutrients.

“Without these pools, the supply of food and nutrients decreases as they tend to be swept along in the current. Pools are also important as habitats for fish species that use the water column to swim, such as tetras,” he said.

When riparian forest is destroyed, trees no longer fall into water courses and these functions of the aquatic ecosystem cease to exist. Even if it regenerates, large trees take far longer to grow and fall than the time needed for recovery of the supply of leaves and control of sunlight (tree diameter increases slowly, whereas the canopy reappears relatively quickly).

“Slender trees from a young forest may fall into the water, but won’t form large or lasting pools. Young trees are small and decompose quickly, or are easily swept away by the current,” he said, stressing the importance of using the “windows of resilience” provided by water courses into which large trees still fall.

“Passive regeneration costs practically nothing to implement compared with conventional restoration, which requires soil preparation and recovery, planting of seedlings, and management of the area to make sure the seedlings don’t die,” he said. “Considering the significant resilience that still exists in the Amazon, the chances of riparian forest recovery are very good.”

Many streams contain large fallen trees that offer hiding places and resources for aquatic organisms, which are an important source of biodiversity even after deforestation. These opportunities should not be wasted.

“If we don’t take advantage of the trees that are still in the channel, they will decompose and be lost, and when riparian regeneration begins, there will be a huge gap until the trees grow in diameter and fall into the stream. During this long hiatus, the stream will be without many functions performed only by tree trunks, leading to local extinctions and biodiversity loss,” he said.

Given the enormous diversity of fish species in the rivers of the Amazon, and the ecosystem services provided by water courses for local communities, they must be protected by leveraging the huge passive regeneration potential of riparian forests and the large tree trunks still in the water to accelerate regeneration at low cost and with many environmental benefits.

Rossetti de Paula noted that in other regions, such as São Paulo State, passive regeneration may not be as efficient as in the Amazon owing to the long history of deforestation and degradation there, meaning that the sources of natural regeneration may have been depleted.

“In some areas, we studied in São Paulo, such as the Corumbataí River basin and the Forest Science Experiment Station in Itatinga, we found riparian forests that were some 32 years old, and yet had much smaller tree diameters than the older regenerated forests we studied in the Amazon,” he said.

Another key point is that many regenerating riparian forest areas are on private property and surrounded by agricultural activities, which act as disturbances that may slow down or even prevent regeneration.

To estimate when regeneration began for the study, the researchers initially used a map of regenerated areas provided by the Amazon Environmental Institute (Imazon), and based on satellite images with a resolution of 30 m for the period 1988-2010. The starting point was later moved back to 1984 using images from Google Earth via Timelapse. “The longer period helped us estimate the age of regeneration and the length of time during which areas were used for grazing before the start of regeneration,” Rossetti de Paula said.

Paragominas, the study location, has several important peculiarities. Since the town was founded in the 1960s, in the wake of construction of a highway between Brasília, the nation's capital, and Belém, the capital of Pará State, the area has seen intense deforestation, mainly for logging and creation of pasture. Many plots soon became unproductive and were abandoned, giving rise to a process of passive regeneration. Recent sustainability initiatives include a Pará State program called Green Municipalities, and these have contributed to regeneration.

“A noteworthy aspect of our study was that it focused on an area some way away from the town center in a vast forest under sustainable management, which also contributed to the passive regeneration process. The proximity of neighboring forests increases the sources of regeneration,” Rossetti de Paula said.

Data collection was performed between 2014 and 2016, during Rossetti de Paula’s PhD research at the University of British Columbia in Canada, with FAPESP’s support, and supervision by Silvio Frosini de Barros Ferraz. In 2018, Rossetti de Paula continued the study with a postdoctoral scholarship from FAPESP. 

The article “Seizing resilience windows to foster passive recovery in the forest-water interface in Amazonian lands” is at: https://www.sciencedirect.com/science/article/abs/pii/S0048969722015182.