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ASU scientists find widespread evidence of carbon declines in tropical forest edges


Carbon in oil palm plantations vs forests

The blue regions are oil palm plantation; the forest regions (yellows and greens) are colored by tree height, which is a proxy for carbon. Credit: Global Airborne Observatory, ASU Center for Global Discovery and Conservation Science.

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March 25, 2020

One of the many consequences of tropical deforestation includes forest fragmentation, a process that involves dividing forests into smaller and smaller pieces, creating new borders between habitats. These borders are exposed to different environmental and biological conditions, called “edge effects,” than compared to more favorable conditions within forest interiors.

Edge effects can include variations in sunlight exposure and precipitation, leading to overall modifications in forest structure over time as some tree species thrive and others die as a result of disturbed growing conditions. With tropical forests playing such a major role in the global carbon budget, it’s important to better understand how edge effects change fragmented forests.

In a new study published today in the Proceedings of the National Academy of Sciences, researchers from Arizona State University’s Center for Global Discovery and Conservation Science and Harvard University utilized high-tech maps produced by the center's Global Airborne Observatory to investigate the impact of edge effects on forest structure and tree canopy characteristics along boundaries between lowland rainforests and oil palm plantations in Malaysian Borneo.

The scientists found widespread evidence of major changes in forest structure along forest edges as well as changes to three important canopy traits related to a tree’s ability to capture sunlight and grow. These changes corresponded to an average 22% decline in aboveground carbon storage along forest edges and extend more than 100 meters into the forest interior.

“Our study suggests a need to mitigate edge-related declines in forest carbon stocks by creating buffer zones between intensively farmed areas and forest ecosystems,” said lead author Elsa Ordway, research fellow at Harvard University and ASU's Center for Global Discovery and Conservation Science. “Although our results indicate that some forests are more vulnerable to edge effects than others, such a strategy could be implemented at scale to reduce the negative impacts of land-clearing on remaining forests.”

“Not all forest-agriculture boundaries are created equal, and most remaining forests change for many years following the original land conversion that takes place nearby, said author Greg Asner, director of the center.

“The importance of this discovery trickles all the way down to how conservation managers work to mitigate biodiversity losses associated with agricultural expansion,” he added.

The results from this study back up previous ones showing decrease in carbon storage and cycling within forest edges. Eric Dinerstein, director of biodiversity and wildlife solutions at Resolve, who wasn’t involved in the new study noted, "This study adds yet another dimension to the list of environmental threats posed by forest fragmentation: declining carbon levels. Additional findings of this study pointing to long-term changes in carbon storage are likely applicable to wherever agriculture is dicing up intact mature forest stands.”

Since tropical forests make up the largest terrestrial share of the global carbon budget and nearly 20% are located within 100 meters of a non-forest edge, a decrease in local carbon storage for these important ecosystems has global implications.

“We have 10 years to maintain our terrestrial carbon sinks if the world has any hope of staying below 1.5 degree Celsius global average temperature rise. The research presented here shows that a moratorium on forest conversion and emissions must occur even sooner than 2035," Dinerstein added.

The paper can be accessed at: https://www.pnas.org/content/early/2020/03/24/1914420117/tab-article-info

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