Scientists have long known that forest fragments are not the same ecologically as intact forest landscapes. When forests are slashed into fragments, winds dry out the edges leading to dying trees and rising temperatures. Biodiversity often drops, while local extinctions rise and big animals vanish. Now, a new study finds another worrisome impact of forest fragmentation: carbon emissions.

Looking at forest fragments in Brazil’s Atlantic Forest, new research in the journal Nature Communications finds that carbon loss along the forest edges was more significant than believed. The researchers used remote sensing and computer modeling to measure how much carbon was released from a forest fragment’s edge, defined as a 100 meter strip along the periphery.

“Tree mortality increases, so that they can’t store as much carbon as healthy trees in the centre of the forest, the core area,” explained Sandro Pütz, the lead author of the study, with the Hemholtz Centre for Environmental Research (UFZ).

The make-up of forests along the edges of a fragment becomes significantly different. Here, faster-growing, weedier species dominate, which store less carbon. The big, old trees that store the most carbon often don’t last in the new conditions, nor does their progeny survive.





A forest fragment rising from a cornfield in Brazil. Photo by: Rhett A. Butler.

“Most vulnerable are trees specialized for life in the dark forest understory and that need animals such as birds or bats to disperse their seeds and pollen,” Henrique Nascimento, of Brazil’s National Institute for Amazonian Research in Manaus, told mongabay.com in 2006. “When you fragment rainforest, hot winds from the surrounding pastures kill many trees, which just can’t handle the stress.”

Given this, scientists have long expected that forest fragmentation and the resulting so-called “edge effects” would lead to higher carbon emissions for these forests, but they hadn’t put a number on it until now.

“Within 10 years the Brazilian Atlantic Forest has lost 69 [million tonnes of carbon], and the Amazon 599 [million tonnes of carbon] due to fragmentation alone,” the researchers write in their paper.

The Atlantic Forest, also known as the Mata Atlântica, is arguably the world’s most degraded tropical rainforest: only about seven percent of the ecosystem survives. Worse still, 90 percent of this biodiversity-rich forest survives in fragments smaller than 100 hectares. Instead of swiss cheese, this forest is more like the holes cut out of the more intact cheese.

In all, the researchers estimate that forest fragmentation alone emits 200 million tonnes of carbon (736 million tonnes CO2) to the atmosphere every year. This is a staggering number: experts have previously estimated that deforestation accounts for around 800 million tonnes of carbon (2.9 billion tonnes CO2) annually.



Forest fragments in the Atlantic Forest surrounded by sugarcane. Photo by: Usina Trapiche.

“It is a forgotten process in the global carbon circulation of the vegetation,” said co-author Andreas Huth also with UFZ. “However, this effect should urgently be taken into account.”

Currently, calculations by the Intergovernmental Panel on Climate Change do not take into account forest fragmentation when estimating carbon losses due to deforestation and land-use.

The researchers also calculated that in order for a fragment to be large enough to avoid these extra carbon losses from edge effects, it would have to be at least 10,000 hectares, shape-dependent. This is about the size of the Bronx or the island of Jersey.

Citations:

Sandro Pütz, Jürgen Groeneveld, Klaus Henle, Christoph Knogge, Alexandre Camargo Martensen, Markus Metz, Jean Paul Metzger, Milton Cezar Ribeiro, Dantas de Paula, M. & Andreas Huth. Long-term carbon loss in fragmented Neotropical forests. Nature Communications 5:5037 doi: 10.1038/ncomms6037 (2014). http://dx.doi.org/10.1038/ncomms6037

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