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Journal Club: How to measure the global potential for biomass vegetation

A new estimate suggests that, were it not for humans, the earth could double its vegetation biomass.  Image: Shutterstock/Sultonyohe

A new estimate suggests that, were it not for humans, the earth could double its vegetation biomass.
Image: Shutterstock/Sultonyohe

Plants play a big role in removing carbon from the atmosphere. Almost a quarter of carbon from car exhaust and industry gets absorbed by vegetation. That’s a significant amount.

But a recent study suggests that amount has the potential to be much larger. A team of European researchers pulled together myriad data sources to create a series of maps that suggest the earth’s potential vegetation biomass—that is, just how much greenery our planet could support and where, were it not for the impact of humankind. In principle, a landscape untouched by humans is at its full potential whether a rich rainforest or desert; a forest thinned by logging, for example, has less than its potential vegetation biomass—a clear-cut forest has even less.

While deforestation is a well-known cause of vegetation loss, the team’s findings, published in Nature, show that widespread land management practices such as logging or grazing-depleted grassland create nearly as large a gap between potential and actual amounts of vegetation on the global scale.

Estimating global vegetation biomass has proven challenging. Some researchers use the heights and diameters of a series of trees to estimate the biomass of a forest—a time-intensive approach that provides good estimates for an individual forest but is difficult to translate to larger areas. Others rely on satellite sensing technologies that capture sweeping swaths of green but provide coarser estimates. A coffee plantation with shade trees looks much like a forest from space, notes environmental physicist Sassan Saatchi of NASA’s Jet Propulsion Laboratory.

Land systems scientist and lead author Karl-Heinz Erb of Alpen-Adria University in Austria and colleagues took a different approach. “What are the best maps out there from different schools of thinking?” he questioned. “Let’s put them together and analyze how robust the signal of human impact is.”

The researchers identified over ten previously published datasets, including data from Saatchi, who was not involved in the study. Dataset sources included land-use data, on-the-ground plant censuses, and remote satellite sensing. The team then created six global maps of vegetation biomass by drawing on different combinations of datasets. They borrowed a seventh map from the literature.

By averaging the total amounts of vegetation suggested by each map, the team estimated that the earth’s vegetation stores about 450 petagrams of carbon. (Humans currently release about 9 petagrams of carbon into the atmosphere each year.)

The team then created five maps estimating the potential global vegetation biomass by drawing on additional datasets. These included remote sensing data that could identify remnants of natural vegetation and ecological databases of the typical carbon contents of different vegetation types. The team also used an existing map, which estimates the carbon stock lost when converting natural vegetation to cropland across the globe. By taking the median of the differences between each of 42 possible pairs from the two sets of maps, the team found that, were it not for humans, the earth could double its vegetation.

Their findings also suggest that land cover change, such as converting a forest to farmland, is responsible for over 50 percent of the difference between actual and potential vegetation. Meanwhile, land management, such as thinning a forest through logging or grazing, is nearly as damaging, accounting for 42-47% of the difference.

This quantification of the impact of land management is a really important advance, says environmental scientist Edward Mitchard of the University of Edinburgh, who was not involved in the study. “There is a lot of talk in the news media about deforestation,” he says. “But forest degradation, where you take out some trees, probably affects an area that is between 2 and 10 times larger every year.”

“This paper could help policymakers to really pay attention to that level of degradation and to design techniques to monitor,” says Saatchi. Indeed, the United Nations’ REDD+ program, includes limiting forest degradation among its carbon emission reduction goals. Saatchi does wonder, however, whether the paper’s potential vegetation biomass figure may be an overestimate, possibly biased by researchers favoring sampling “beautiful majestic forests” over naturally sparse ones.

If vegetation could reach the global potential estimated here, it could store the equivalent of 50 years of carbon emissions. This, of course, is an impossible goal. “We are 7.5 billion people,” says Erb. “We need cropland to feed ourselves, forests to build houses.”

Still, by identifying the large impact of land management practices, Erb believes his study suggests that encouraging vegetation for increased carbon absorption is an option. “There is quite a potential for refilling the forests,” he says.

Categories: Agriculture | Applied Biological Sciences | Climate science | Journal Club and tagged | |
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