EGU 2016 summary – forests and methane

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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.

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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.

Methane emissions from upland boreal forest soils

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Large uncertainties still exist in the magnitude of the sources and sinks that contribute to the global methane budget.  Boreal landscapes typically contain a mixture of forests, wetlands and peatlands, and the total catchment flux of methane will be determined by the source/sink behaviour of each ecosystem.  Upland forests are typically considered to be net sinks of methane, although periods of methane release have been observed after heavy rainfall.  Contrary to this, peatlands and wetlands are well known to emit large quantities of methane.  However, there is a still a lack of knowledge concerning how inter-annual variations in climate can affect the total catchment methane budget.

To address this knowledge gap, a group of researchers led by the Finnish Meteorological Institute conducted a twenty-eight month study in the Pallaslompolo catchment in northern Finland.  Methane fluxes were measured from a minerotrophic fen and a nearby upland forest.  The forest soil was observed to be a small methane sink (-0.18 to -2.3 mg CH4 m-2 d-1) from September 2010 to August 2011.  Following three months of heavy rainfall, the forest then became a large methane source until January 2012 (max = 92 mg m-2 d-1).  After this spike, the forest returned to acting as a methane sink for the remainder of the study (which ran until January 2013).

When upscaling flux measurements to the entire catchment, the forest consumed the equivalent of 10% of the methane emitted by the fen in the dry year.  During the wet year when the large forest methane spike was observed, the forest was a source equivalent to 57% of fen emissions. During August and September 2011, monthly methane fluxes from the forest were twice as large as the emissions from the fen.  The authors hypothesise that high forest methane emissions occur during wet conditions, but also require other factors which include: 1) high soil temperature; 2) an input of soil carbon from roots/litter; 3) high methanogenic activity.  This explains why high forest fluxes where not observed during spring snowmelt when soil moisture values peaked.

A signal of the high forest emission was also detected in the atmospheric methane concentration measured nearby. In the long-term data, the mean September concentration anomaly was well explained by the water level of the nearby lake, suggesting that the water level could act as a better proxy for soil methanogenic potential than a point measurement of soil moisture.

This paper shows that methane fluxes in boreal forests can show extreme variation between years according to differences in precipitation.  It demonstrates the importance of taking frequent, year-round flux measurements. Lead author, Annalea Lohila said: “We were surprised to see how differently the forest and wetland ecosystems responded to excess wetness, and how important the upland forests are for methane balances after upscaling to the landscape level.”

Lohila, A., Aalto, T., Aurela, M., et al. 2016. Large contribution of boreal upland forest soils to a catchment-scale CH4 balance in a wet year. Geophysical Research Letters, DOI: 10.1002/2016GL067718.

Forests and the Methane Cycle Workshop

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During the first week of June a workshop took place covering the topic of “Forests and Methane Cycling.”  The meeting was jointly organised by MethaneNet and Mari Pihlatie of the University of Helsinki, and was held on the idyllic banks of Lake Kuivajärvi at Hyytiälä Forestry Field Station in Finland.  The aim of the event was to synthesise current knowledge on forest methane cycling from various ecoregions, and at different spatial and temporal scales.

A general introduction set the scene for the workshop, but also raised important questions concerning methods for upscaling fluxes, and the appropriateness of different metrics in reporting data.  After this it was on to individual presentations, beginning with a boreal focus.  Speakers discussed both micrometeorological and chamber approaches to measuring methane in northern coniferous forests, which allow complex methane dynamics to be elucidated.  For example, at Hyytiälä there is evidence that suggests the forest floor is a methane sink, whilst the trees act as a small source; similar results from trees have been reported elsewhere.  This introduced the question: “are trees the missing source of methane in boreal forests?”

For the afternoon session, the science switched from the boreal zone to the tropics.  It was noted that carbon cycling in tropical wetlands is still poorly understood.  Furthermore, the possibility was presented that unquantified methane emissions from tree stems might be the reason for the regional discrepancy between bottom-up and top-down estimates of atmospheric methane sources.  The mediating role of tree physiology on greenhouse gas emissions was also broached.

Day 2 of the workshop was a more interactive affair, with conversations about knowledge gaps and the future direction of forest methane research.  There was time for two field trips, however.  The first was to SMEAR II; an atmospheric research facility where measurements of greenhouse gases from soil and trees are ongoing using chamber methods.  Greenhouse gases are also measured using micrometeorological methods on a 124 m tower.  Our second trip was to the scenic Siikaneva peatland, where greenhouse gases are measured using chamber and eddy covariance methods.

The workshop organizer Dr Mari Pihlatie of the University of Helsinki said “this workshop was an eye opener to see how big uncertainties we still have in understanding the role of trees in methane dynamics in forest ecosystems. Also, the meeting gave an excellent opportunity to initialize collaboration between research groups and disciplines to work towards a comprehensive understanding of the methane cycling in forest ecosystems.” Dr Vincent Gauci of MethaneNet added: “the workshop highlighted that we are at a relatively early stage of developing fundamental knowledge of how forested wetlands and upland ecosystems participate in the methane cycle.  The future of this field of research will have important implications for characterisation and modelling of ecosystem sources of this gas under a range of global change scenarios.”

Blog: Trace Gas Dynamics in Tropical Forests

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By Bertie Welch. 18th December 2013.

After carbon dioxide, methane is the second most important greenhouse gas, yet how much is produced and transferred to the atmosphere by tropical rainforests is not fully understood, because much of the research in the tropics focuses on rice paddies and wetlands, as these are significant sources of anthropogenic methane. Rice et al. (2010) suggest that globally, hardwood trees could account for emissions of ~60 Tg Ch4 yr-1.

These potentially significant methane emissions are a result of trees allowing the methane produced in deeper, anaerobic soils to bypass the methanotrophs and nitrifying bacteria in the aerobic upper soils (Milich, 1999). We know from temperate forest and mesocosm studies that trees can act as conduits for soil methane into the atmosphere.  Trees can adapt to cope with flooding-induced soil anoxia in a variety of ways: hypertrophied lenticels, adventitious roots and enlarged aerenchyma (Havens, 1994; Kozlowksi, 1997).  Studies of temperate trees such as alders have shown that they use all the above adaptations when in waterlogged soils. This results in increased methane fluxes from the trees as there is increased surface area for emission and easier transmission from soil to atmosphere (Rusch and Rennenberg, 1998; Gauci et al., 2010).

Pangala et al. (2013) showed that in a tropical peat forest in Borneo, tree stems were responsible for 60-80% of total ecosystem methane fluxes, demonstrating that trees can be a significant source of emissions. This new study in lowland evergreen tropical rainforest in Panama aims to expand on this work and investigate the extent of tree stem emissions in an area of relatively free draining soils on a fortnightly basis for 5 months, covering the dry to wet season transition starting March 2014.

There is a second aspect to the study – we don’t know how trace greenhouse gas biosphere-atmosphere exchange will be affected by an atmosphere that is being enriched with CO2. It is thought that rising atmospheric CO2 concentrations will increase primary productivity in rainforests resulting in greater amounts of litterfall. The fieldwork will be done at the Smithsonian Tropical Research Institute, Panama, on plots that are part of an ongoing litter manipulation experiment meant to simulate elevated atmospheric CO2. There are 15 plots in total: 5 control, 5 with litter removed and 5 with litter added. Sayer et al. (2011) discovered soil respiration to be significantly higher in the litter addition plots compared to the control and litter removal plots. We hypothesise that due to increased litter input (and therefore more source carbon for methanogenesis) methane emissions will be greatest in the addition plots.

Gauci, V., Gowing, D.J.G., Hornibrook, E.R.C., Davis, J.M. & Dise, N.B. (2010) Woody stem methane emission in mature wetland alder trees. Atmospheric Environment, 44, 2157-2160.

Havens, K.J. (1994) The formation of hypertrophied lenticels, adventitious water roots, and an oxidized rhizosphere by Acer rubrum seedlings over time along a hydrologic gradient. Virginia Institute of Marine Science, Gloucester Point, Va.

Kozlowski, T.T. (1997) Responses of woody plants to flooding and salinity. Tree Physiology, Monograph No. 1, 29.

Milich, L. (1999) The role of methane in global warming: where might mitigation strategies be focused. Global Environmental Change, 9, 179-201.

Pangala, S.R., Moore, S., Hornibrook, E.R.C. & Gauci, V. (2013) Trees are major conduits for methane egress from tropical forested wetlands. New Phytologist, 197, 524-531.

Rice, A.L., Butenhoff, C.L., Shearer, M.J., Teama, D., Rosenstiel, T.N. & Khalil, M.A.K. (2010) Emissions of anaerobically produced methane by trees. Geophysical Research Letters, 37, L03807.

Rusch, H. & Rennenberg, H. (1998) Black Alder (Alnus glutinosa (L.) Gaertn.) trees mediate methane and nitrous oxide emission from the soil to the atmosphere. Plant and Soil, 201, 1-7.

Sayer, E.J., Heard, M.S., Grant, H.K., Marthews, T.R. & Tanner, E.V.J. (2011) Soil carbon release enhanced by increased tropical forest litterfall. Nature Climate Change, 1, 304-307.

Image: Forest of Taman Negara National Park by Vladimir Yu. Arkhipov, Arkhivov under Creative Commons CC BY-SA 3.0.