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Tree plantations have hidden impacts on tropical biodiversity

An escalating trend of carbon-capture tree plantations has been primarily driven by the urgency of the climate crisis. 

In an opinion piece published in the journal Trends in Ecology and Evolution, experts warn that these initiatives might inadvertently endanger tropical biodiversity while offering minimal gains in carbon sequestration. 

Net zero emissions

“The escalating threat of climate change has spurred global commitments to achieve net zero emissions by the middle of this century,” wrote the researchers.

“To reach a balance between reducing emission sources and enhancing greenhouse gas sinks, land-based carbon sequestration is viewed as an important strategy to offset emissions, most prominently via nature-based solutions.”

“This, together with the commodification of carbon and the substantial growth of the voluntary carbon market, has resulted in a boom in the number of commercial tree plantation projects across tropical ecosystems with significant financial flows from private and public sectors towards carbon offsetting projects.”

Ecosystem degradation 

The experts argue that carbon-offset plantations might come with hidden costs. Instead, they say, we should prioritize conserving and restoring intact ecosystems.

“Despite the broad range of ecosystem functions and services provided by tropical ecosystems, society has reduced value of these ecosystems to just one metric – carbon,” wrote the authors, led by Jesús Aguirre-Gutiérrez of the Environmental Change Institute at the University of Oxford. 

“Current and new policy should not promote ecosystem degradation via tree plantations with a narrow view on carbon capture.”

Tropical ecosystems 

Tropical ecosystems, including forests, grasslands, and savannas, are favored for tree plantation because they promote rapid tree growth. 

Yet, contrary to popular belief, tree plantations designed for carbon capture seldom contribute to biodiversity or social-economic welfare, noted the researchers. 

Tropical ecosystems are renowned biodiversity hotspots that provide invaluable ecosystem services, including water quality maintenance, soil health preservation, and pollination.

Carbon-capture plantations 

Unfortunately, many carbon-capture plantations, typically monocultures, predominantly cultivate five tree species: teak, mahogany, cedar, silk oak, and black wattle. Although economically viable, these plantations tend to support lower biodiversity levels. 

For instance, the Brazilian Cerrado savannah experienced approximately 30% reduction in plant and ant diversity following a 40% increase in woody cover. 

Such plantations also inadvertently inflict ecological harm, including stream flow reduction, groundwater depletion, and soil acidification.

Little gain in carbon sequestration 

The experts noted the limited effectiveness of even the most aggressive carbon-capture plantation endeavors in sequestering carbon. 

“The current trend of carbon-focused tree planting is taking us along the path of large-scale biotic and functional homogenization for little carbon gain,” wrote the researchers. “An area equivalent to the total summed area of USA, UK, China, and Russia would have to be forested to sequester one year of emissions.”

Subterranean carbon storage 

The paper also highlights that intact tropical grasslands and savannas already function as significant carbon sinks, predominantly storing carbon underground. 

This subterranean storage, at risk of being lost if these areas are afforested, proves more resilient to environmental disruptions like droughts and fires compared to above-ground carbon stores in tree plantations.

Financial incentives 

Driven by substantial financial incentives, private firms are investing heavily in carbon-capture initiatives, a trend the authors believe is propelled more by economic considerations than ecological concerns. 

While carbon metrics are straightforward to quantify and monetize, an excessive focus on tree-planting for carbon capture could potentially undermine efforts to protect intact ecosystems, leading to unfavorable compromises between carbon, biodiversity, and ecosystem functionality.

The experts say we should prioritize conserving intact ecosystems instead of focusing on commercial tree planting.

“An overarching view on maintaining original ecosystem functioning and maximizing as many ecosystem services as possible should be prioritized above the ongoing economic focus on carbon capture projects.”

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