The European Geosciences Union is one of the largest scientific conferences in the world and between the 17th and 22nd April this year saw 13,650 scientists from 109 countries descend upon Vienna for a week of cutting edge science. This year for the first time there was a session on ‘Forests and the Methane Cycle’ chaired by MethaneNet’s director, Vincent Gauci, reflecting the rapid growth in interest in the field.
The first talk was by the solicited speaker Patrick Megonigal of the Smithsonian Environmental Research Centre, discussing research done by his lab and collaborators in China into upland forests and their contribution to the global methane sink. At their sites in Maryland, USA, they found that warm and wet soils resulted in high stem methane fluxes which offset 5% of the annual ecosystem sink at one site and 3% at a more westerly site. Stem fluxes also showed diurnal patterns, with fluxes increasing during the daytime and decreasing at night which shows the importance of considering plant metabolism and physiology as a control. The Chinese upland site had higher methane fluxes than their US sites but did receive higher precipitation and warmer temperatures meaning the stem fluxes had a far more significant effect at an ecosystem scale. The key message from Megonigal is that the global methane sink has been overestimated by 3-60%.
Vincent Gauci, of the Open University, then presented the results of a major sampling campaign in the Brazilian Amazon on behalf of Sunitha Pangala who was unable to attend. The campaign in flooded forests along the edges of the Amazon sampled over 2,400 trees at three heights across 13 sites and found substantial tree stem fluxes at all of them. The stem fluxes dominated at each site, accounting for 70-80% of ecosystem fluxes, with young stems being the largest single source of methane.
The focus then switched from living trees to what happens after they die. Kris Covey, a PhD student at the Yale School of Forestry & Environmental Studies has been studying cores of deadwood from hundreds of sites across the USA to look at two important greenhouse gases: methane and nitrous oxide. Deadwood methane production declines with age however still produces above ambient levels, highlighting that detritus and deadwood needs to be factored into the ecosystem models. Nitrous oxide was uniformly consumed within deadwood which again could be important at ecosystem and regional scales.
Kateřina Macháčová of the Global Change Research Institute CAS in Brno, Czech Republic, presented a summary of her group’s work into methane fluxes in boreal forests. They compared three common species: Scots pine, Norway spruce and silver birch. Birch stem fluxes were the greatest of the species studied and likely due to differing physiology to the coniferous species. She also reported seasonal variation in fluxes with two distinct pulses of methane release: the first in February (likely due to snow melt) and the second during the growing season of May to October.
Keeping with the boreal theme, Mari Philatie from the University of Helsinki, presented the results of an experiment attempting to constrain the size of fluxes from component parts of the forest (forest floor, lower trunk, mid trunk and shoots/leaves). Using LiDAR to examine microtopographic features they were able to correlate topography and soil water content to the methane fluxes measured. They also noted that fluxes from tree shoots did not show seasonal variation, nor were correlated with other physical features measured. As with Machacova’s study they also found that birch at their site produced more methane than spruce.
Taken together, the research presented during this exciting session shows the growing interest in the role that trees and their stems and shoots play in the global cycling of greenhouse gases. Hopefully the session will act as a springboard for future research directions and collaborations.
Written by Bertie Welch. Bertie is a PhD student at the Open University, researching emissions of methane and nitrous oxide from temperate and tropical forests. He tweets as @WelchEcology and has blogged for MethaneNet previously.