Small freshwater ponds that dot the Arctic tundra have been filling in with vegetation in recent decades. Photo: Christian Andresen

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By Rebecca Heisman
22 September 2016

(Eos) – Climate change has caused a boom in aquatic plant biomass on the Arctic tundra in recent decades. Those plants, in turn, are releasing increasing amounts of methane into the atmosphere, according to a paper published last week in the journal Global Change Biology.

Christian Andresen of Los Alamos National Laboratory in Los Alamos, N.M. and his colleagues suspected that longer growing seasons and increasing permafrost thaw were altering the ecology of small freshwater ponds that pepper the tundra landscape—and perhaps the amount of methane they give off. Without a time machine, however, it’s hard to go back and see just how ecosystems and their greenhouse gas dynamics have been affected on a local scale. Andresen and his colleagues didn’t have a time machine, but they had the next best thing.

A multinational project in the late 1960s and early 1970s called the International Biological Program (IPB) collected detailed data on major ecological communities around the world, including the Alaskan tundra. IPB researchers weren’t working with climate change in mind, but the data they gathered have provided Andresen and his colleagues with a unique window onto the state of the tundra ecosystem decades ago.

The IPB data didn’t include information on methane, so Andresen and his colleagues developed a mathematical model to determine how such factors as vegetation biomass and thaw depth correlate with the amount of methane released today. By plugging the ecological data collected in the 1970s into the model, they could estimate the methane flux at that time.

Their results show that the biomass of grasses and sedges growing in tundra ponds has increased by 20% to 25% since the IPB era, primarily because of longer growing seasons and the increased availability of nutrients released from the thawing permafrost. During the same time period, Andresen and his colleagues estimate, the methane flux from these wetlands has increased by about 60%. Wetland plants that cover just 5% of the tundra’s surface may account for as much as two thirds of the region’s methane flux, the researchers estimate.

Warmer conditions in the Arctic don’t lead to increased methane flux on their own. The new study confirmed that aquatic grasses and sedges act like straws, drawing methane up from the anoxic muck at the bottom of a pond and releasing it through their leaves before it can be converted to carbon dioxide.

“If methane’s only path to the atmosphere is through the soil, then the likelihood of a methane molecule being oxidized before it makes it to the atmosphere is very high,” said Joe von Fischer of Colorado State University in Fort Collins, an expert on ecosystem function and methane flux who was not involved in the study. “The presence of these plants as conduits for methane is fascinating.” [more]

Aquatic Plants May Accelerate Arctic Methane Emissions

ABSTRACT: Plant-mediated CH4 flux is an important pathway for land–atmosphere CH4 emissions, but the magnitude, timing, and environmental controls, spanning scales of space and time, remain poorly understood in arctic tundra wetlands, particularly under the long-term effects of climate change. CH4 fluxes were measured in situ during peak growing season for the dominant aquatic emergent plants in the Alaskan arctic coastal plain, Carex aquatilis and Arctophila fulva, to assess the magnitude and species-specific controls on CH4 flux. Plant biomass was a strong predictor of A. fulva CH4 flux while water depth and thaw depth were copredictors for C. aquatilis CH4 flux. We used plant and environmental data from 1971 to 1972 from the historic International Biological Program (IBP) research site near Barrow, Alaska, which we resampled in 2010–2013, to quantify changes in plant biomass and thaw depth, and used these to estimate species-specific decadal-scale changes in CH4 fluxes. A ~60% increase in CH4 flux was estimated from the observed plant biomass and thaw depth increases in tundra ponds over the past 40 years. Despite covering only ~5% of the landscape, we estimate that aquatic C. aquatilis and A. fulva account for two-thirds of the total regional CH4 flux of the Barrow Peninsula. The regionally observed increases in plant biomass and active layer thickening over the past 40 years not only have major implications for energy and water balance, but also have significantly altered land–atmosphere CH4 emissions for this region, potentially acting as a positive feedback to climate warming.

Rising plant-mediated methane emissions from Arctic wetlands



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