Forest disturbances, anthropogenic or otherwise, reduce their resilience and their ability to regenerate themselves. A study recently published in the journal Plant and Soil, researchers illustrate how forest resilience is compromised as soil erosion increases in forests that are frequently disturbed. Photo: C. Ritter

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SOIL EROSION

Eroding the resilience of tropical forests

Understanding the relationships between forest resilience and soil erosion in tropical forests affected by human and natural disturbances

Story highlights

  • Anthropogenic disturbances such as logging and fires have accelerated the pace of soil erosion in tropical forests.
  • Researchers study the role of erosion in compromising the resilience of such forests over intensifying cycles of disturbances.
  • By studying several forests sites in the Amazon, authors are able to conceptually demonstrate how positively reinforcing feedbacks of erosion and disturbances can permanently alter the functioning of these ecosystems.

In the fight against global warming, the Amazon tropical forest remain among the planet’s most effective defence system. But pressures on these forests appear to be rising. Forest disturbances, anthropogenic or otherwise, reduce their resilience and their ability to regenerate themselves.

In a study published in Plant and Soil, centre researcher Arie Staal and colleagues from Brazil and The Netherlands explore the relationships between disturbances such as fires and logging, and forest resilience. Bernardo Flores from University of Campinas, Brazil, was the lead author.

In the study, the researchers illustrate how forest resilience is compromised as soil erosion increases in forests that are frequently disturbed.

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Considering how tropical forests are increasingly exposed to disturbances, understanding how erosion may act as a resilience drain in these systems may help societies to manage the risk of ecological transitions for instance by promoting fast recovery and restoration.

Arie Staal, co-author

A vicious cycle of erosion

Erosion can be defined as the “physical process of topsoil removal” that leads to degradation in soil quality and a reduction in ecosystem productivity. Common erosion agents are water and wind, and different ecosystems have either adapted to or been shaped by these agents. As an example, tropical forests usually receive heavy rainfall, but their dense vegetation cover helps them to resist the eroding effects of water as compared to semi-arid landscapes where vegetation cover is sparse.

Scientists use the term “erodibility” to measure a soil’s vulnerability to erosion. When soils are highly erodible, they lose most of their organic matter and nutrients making them vulnerable to more severe forms of erosion.

Although erosion is a natural process and has shaped the earth’s features over millennia, human activity such as industrial agriculture and deforestation has accelerated its pace with undesirable consequences for the concerned ecosystems.

Forest resilience at risk

That soil erosion impacts agricultural productivity is a globally well researched and understood phenomenon. Instead, Staal and his colleagues focus on the less studied aspect of how erosion affects the resilience of tropical forests.

“We asked if tropical forests exposed to varying forms of disturbance, such as wildfires and logging, also undergo a downward spiral of soil degradation” says Staal.

In light of the severity of the Amazon wildfires, this rather timely article discusses how feedback loops of erosion and disturbances can sometimes tip tropical forests into sparser vegetation landscapes e.g. into a savanna-like state. Using data from 48 forests sites spread across the Amazon forests authors analyse the impact of erosion on two distinct ecosystems within the Amazon, namely upland forests and floodplain forests. All 48 sites had faced disturbances including fire.

Staal adds “In both cases, loss of forest recovery capacity seems to be linked with a reduction in soil fertility. Such gradual loss of soil fertility is expected to reduce forest recovery capacity and this slowing down of forest recovery by itself can be interpreted as an indicator of approaching collapse.”

Tropical forests in different environmental conditions that may determine erosion risk after disturbances. The main erosion-deposition processes are: (a) and (b) sheet and rill erosion from runoff, (c) and (d) flood erosion, (e) high clay deposition, (f) vertical water erosion, flood erosion and wind erosion/deposition

The impact of erosion agents

Tropical forests receiving heavy rainfall are highly vulnerable to flooding that is a strong erosion agent essentially meaning that floodable forests such as those found in parts of the Amazon may be particularly vulnerable to erosion feedback loops.

The article visually represents the effects of erosion across multiple forest types and terrains; thus, providing a handy snapshot of the differential impacts of similar erosion agents (see figure for an example). And by analysing erosion feedback loops across these various topographies it is possible to explain why “certain forests stay in degraded states and how biomes expand and retract”.

But why is this important?

Staal answers, “Considering how tropical forests are increasingly exposed to disturbances, understanding how erosion may act as a resilience drain in these systems may help societies to manage the risk of ecological transitions for instance by promoting fast recovery and restoration.”

As the world tries to grapple with the Amazon wildfires, such and more understandings will be vital.

Methodology

Researchers analysed data from two distinct ecosystems within the Amazon spread across 33 forest sites that had been disturbed multiple times (1–7) by slash-and-burn practices for manioc plantation. For all study sites, data was obtained from all trees >1 cm of diameter. Authors produced chrono-sequences (space-for-time substitution) to analyse temporal changes in tree basal area based on the time after the latest disturbance, and for clay fraction based on the time after the first disturbance event. In this way, they could observe how forest recovery rates changed after each disturbance event, and how this related to soil changes that accumulated since the start of the disturbance regime (see graphs in the article).

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Related info

Flores, B.M., A. Staal, C.C. Jacovac, M. Hirota, M. Holmgren & R.S. Oliveira 2019. Soil erosion as a resilience drain in disturbed tropical forests. Plant and Soil, https://doi.org/10.1007/s11104-019-04097-8

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Arie Staal is a postdoctoral researcher. His research focuses on the resilience of tropical forests in the Anthropocene.

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