THE clean, green image of forests as the lungs of the planet doesn’t sit well with the idea of trees belching out methane. So when researchers reported in January that living plants are releasing the gas in huge amounts, greenhouse gas experts were suitably stunned.
The finding prompted calls for a rethink of Kyoto rules that allow countries to plant forests to soak up carbon dioxide instead of cutting greenhouse gas emissions (91av, 14 January, p 3 and p 13). “It changed the whole way we looked at tree planting. People questioned its value as a useful strategy,” says Miko Kirschbaum, who studies climate change and forest growth at the Australian National University in Canberra.
But such drastic measures may not be necessary after all. For while trees may indeed produce methane, the amount they produce constitutes more of a hiccup than a belch, according to two new studies – one by Kirschbaum, the other led by Francis Kelliher of Landcare Research in Lincoln, New Zealand (Functional Plant Biology, vol 33, p 521 and p 613).
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They say the error in the original study lies in the scale-up from the lab to global production rates of the gas. The original experiments, led by Frank Keppler of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, detected methane being emitted by young, leafy plants. His team also found that sunlight increased methane production three to fivefold. When these emissions were scaled up using an estimate of global plant growth it put plant production of methane at between 62 and 236 million tonnes per year – on a par with the two previously known major sources of methane: farting ruminants and microbial activity in wetlands.
That’s likely to be an overestimation, say Kirschbaum and Kelliher. More than half of total plant biomass is made up of roots and wood, and wood is unlikely to produce methane at the same rate as the more metabolically active leaves, while roots are not exposed to light.
Kirschbaum’s team recalculated global rates using estimates of the total leaf mass in the world and came up with a more modest 36 million tonnes of methane emitted per year. When they plugged methane production by grasslands and forests into greenhouse gas budgets, they found that planting trees was clearly valuable, Kirschbaum says. “The benefit of carbon storage by trees is a hundred times as great as the disadvantage of methane emissions.”
Using similar estimates, Kelliher compared methane production by forests with that by sheep and cows grazing an equivalent area of pasture. Ruminants on grass produce six times as much methane as the same area of forest, he found. When the uptake of methane by bacteria in the soils of forest and pastures was factored in, ruminants produced 16 times as much methane.
Keppler is not convinced by these estimates, however. He suspects that they will be the first of many new approaches, each with their own limitations. “Some people are not happy with our high values. We were surprised as well, but we are still happy with them,” he says.
