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Living trees are unlikely source of methane

April 21, 2017, by Tim Radford

Trees Maryland

Cows have competition … tree trunks in an upland forest in Maryland, US, have been found to emit methane. Image: Cocoa Dream via Flickr

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Scientists have long been aware of a forest’s ability to absorb carbon, but a new US study has discovered trees that emit methane.

LONDON, 21 April, 2017 – In the great climate change challenge, forests play a key role in absorbing atmospheric carbon. But trees – at least in some cases – are also a source of greenhouse gases, in particular, methane.

Methane is one of the lesser greenhouse gases that drive global warming, but it is also one of the most potent, at least 25 times stronger that carbon dioxide. And a new study by US scientists shows that tree trunks in at least one sample of an upland forest actually emit methane into the atmosphere.

The scientists report in Ecosystems journal that they measured the traffic of carbon dioxide and methane in a stretch of woodland in the state of Maryland in the growing season, between April and December. They were looking for the precise “sink or source” roles of the growing trees, the soil between the trees and the coarse woody debris in various stages of decay on the forest floor.

Trees – and all vegetation – have been monitored for carbon dioxide exchanges for decades, along with respiration of both CO2 and methane in the soil. The dead wood that is part of any natural forest has, normally, been left out of the equation.

And, the researchers confirm, the picture changes with temperature.

“We believe our work can help fill in some
gaps in methane budgets and environmental
processes in global ecosystem models”

Methane in soils seem to follow a temperature gradient where higher temperatures are related to higher uptake of methane but that’s not necessarily the case for coarse woody debris or for tree trunks,” says Rodrigo Vargas of the Department of Plant and Soil Sciences at the University of Delaware, who led the study.

We believe our work can help fill in some gaps in methane budgets and environmental processes in global ecosystem models.”

This research does not suggest that forests make climate change worse; it simply confirms that life’s traffic with the atmosphere is a two-way process. It also substantiates what all biologists have always known: that even the most innocent plant is a solar-powered chemistry set in continuous operation throughout the growing season.

Polluting trees

President Reagan once opined that trees caused more pollution than automobiles, a statement that was never true. But trees both absorb and secrete atmospheric gases, and research such as this is intended to discover more about the intricate link between living things and the air we breathe, and, ultimately, the climates that permit life on Earth to survive. The latest study is just another little piece in the mosaic of understanding, and it describes one small patch of woodland in one larger ecosystem in one state of one country.

Its significance is thrown into sharp relief by a new survey from Botanic Gardens Conservation International, which has made a new country-by-country survey of the world of trees.

Scientists report in the Journal of Sustainable Forestry that there are now known to be 60,065 tree species – a tree is a woody growth with a stem at least 5cm in diameter at breast height – on every continent except Antarctica. Around half of these are found only in one country. Brazil, for instance, is home to 8,175 species and, of those, 4,333 are endemic; that is, found only in Brazil. The tropical regions of Central and South America are host to 23,000 species. The most impoverished region, in arboreal terms, is the north of North America, with 1,400 species.

Research such as this is fundamental. Forests play a vital role in the climate machinery, so climate scientists need to know how they moderate air temperatures, what kind of age structure matters most in a forest, how many trees there might be on the planet and which are most at risk.

Threat of extinction

Around 9,600 species are threatened with extinction, say the BGCI scientists. Around 300 species are represented by fewer than 50 individuals. And the biggest family of trees is within the enormous plant group called Leguminosae.

To complete the survey, scientists spent two years combing 500 sources of data to amass the 375,500 records in their GlobalTreeSearch database.

Although it seems extraordinary that it has taken us until 2017 to publish the first global, authoritative list of tree species, it is worth remembering that GlobalTreeSearch represents a huge scientific effort encompassing the discovery, collection and describing of tens of thousands of plant species,” says Paul Smith, secretary general of BGCI. “This is ‘big science’, involving the work of thousands of botanists over a period of centuries.” Climate News Network

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