Using forests to mitigate greenhouse gases

According to a study by Ensis, the joint venture between Australia's CSIRO and New Zealand's SCION, the use of new forests as carbon sinks remains a valuable mechanism for mitigating greenhouse gas emissions.

A recent article by Frank Keppler and colleagues in the international science journal, Nature, reported for the first time that plants can directly emit methane "“ a greenhouse gas.

The article fuelled suggestions by other commentators that new forests might increase greenhouse warming through methane emissions rather than decrease it by sequestering carbon dioxide.

Ensis scientists, led by Dr Phil Polglase, have tested this assumption, using methodology provided in Keppler's article, by comparing estimates of methane emissions for radiata pine forests in Australia with the amounts of carbon stored to determine the overall effect.

"Unpublished calculations have shown that the average amount of methane emitted would offset less than 5 per cent of the amount of carbon dioxide stored by trees in terms of its effect in contributing to global warming," Dr Polglase says.

Dr Polglase says the key question regarding afforestation lies in determining the amount of change when planting new forests on agricultural land.

"Several issues need to be addressed. For example, soils take up and oxidise methane," Dr Polglase says. "Also, Keppler suggests that all plants emit methane, so it may even be possible for some agricultural land practices to emit more methane than forests for any given land area.

"Methane emissions by trees might have a small negating impact, but the effect could be much less or even positive, depending on the net emissions of methane from trees and soil relative to the preceding agricultural land."

Dr Polglase says Keppler's finding is highly significant as methane is an important greenhouse gas and was thought to come only from anaerobic (oxygen limited) conditions such as from bogs, wetlands, landfills, rice paddies, and from livestock, termites, and from vegetation fires.