What impact do global warming, the ongoing rise in the air's carbon dioxide (CO2) content and a number of other contemporary environmental trends have on the atmosphere's methane (CH4) concentration? The implications of this question are huge, in light of the fact that methane is a more powerful greenhouse gas, molecule for molecule, than is carbon dioxide. Hence, we here consider this question as it applies to methane emissions associated with natural vegetation. 

Davidson et al. (2004) report that the climate of the Amazon Basin may become gradually drier due to the intensification of a number of different phenomena, including (1) less recirculation of water between the increasingly-deforested region and the atmosphere, (2) more rainfall inhibition by smoke caused by increased biomass burning, and (3) a warming-induced increase in the frequency and/or intensity of El Niño events that have historically brought severe drought to the eastern Amazon Basin (Nepstad et al., 1999; but see Timmermann et al., 1999 as well). Driven by concern about these potential problems, they devised an experiment to determine the consequences of the drying of the soil of an Amazonian moist tropical forest for the net surface-to-air fluxes of two important greenhouse gases: nitrous oxide (N2O) and methane (CH4).

In the Tapajos National Forest near Santarem, Brazil, the researchers modified a one-hectare plot of land covered by mature evergreen trees so as to dramatically reduce the amount of rain that reached the forest floor (throughfall) while maintaining an otherwise similar one-hectare plot of land as a control for comparison. Prior to making this modification, they measured the gas exchange characteristics of the two plots for a period of 18 months; then, after initiating the throughfall-exclusion treatment, they continued their measurements for an additional three years. This protocol revealed, in their words, that the "drier soil conditions caused by throughfall exclusion inhibited N2O and CH4 production and promoted CH4 consumption." In fact, they say that "the exclusion manipulation lowered annual N2O emissions by >40% and increased rates of consumption of atmospheric CH4 by a factor of >4," which results they attributed to the "direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy."

As for the implications of their work, if global warming did indeed increase the frequency and/or intensity of El Niño events - which real-world data suggest is highly debatable (seeEl Niño - Relationship to Global Warming in our Subject Index) - the results of this study suggest that the anticipated drying of the Amazon Basin would initiate a strong negative feedback to warming via (1) large drying-induced reductions in the evolution of N2O and CH4 from its soils and (2) a huge drying-induced increase in the consumption of CH4 by its soils. Although Davidson et al. envisaged a more extreme second phase response, "in which drought-induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots" (a response that would be expected to increase N2O and CH4 emissions), we note that the projected rise in the air's CO2 content would likely prohibit such extreme events from ever occurring, in light of the tendency for elevated levels of atmospheric CO2 to greatly increase the water use efficiency of essentially all plants (see Water Use Efficiency in our Subject Index, including the subsection Trees), which would enable the Amazon Basin's vegetation to continue to flourish under significantly drier conditions than those of the present.

Strack et al. (2004) also reported that climate models predict increases in evapotranspiration that could lead to drying in a warming world and a subsequent lowering of water tables in high northern latitudes. This prediction literally cries out for an analysis of how lowered water tables will impact peatland emissions of CH4; and in a theoretical study of the subject, Roulet et al. (1992) calculated that for a decline of 14 cm in the water tables of northern Canadian peatlands, due to climate-model-derived increases in temperature (3°C) and precipitation (1mm/day) predicted for a doubling of the air's CO2 content, CH4 emissions would decline by 74-81%. Hence, in an attempt to obtain some experimental data on the subject, at various times over the period 2001-2003 Strack et al. measured CH4 fluxes to the atmosphere at different locations that varied in depth-to-water table within natural portions of a poor fen in central Quebec, Canada, as well as within control portions of the fen that had been drained eight years earlier.

At the conclusion of their study, the Canadian scientists reported that "methane emissions and storage were lower in the drained fen." The greatest reductions (up to 97%) were measured at the higher locations, while at the lower locations there was little change in CH4 flux. Averaged over all locations, they determined that the "growing season CH4 emissions at the drained site were 55% lower than the control site," indicative of the fact that the biosphere appears to be organized to resist warming influences that could push it into a thermal regime that might otherwise prove detrimental to its health.

In another experimental study, Garnet et al. (2005) grew seedlings of three emergent aquatic macrophytes (Orontium aquaticum L., Peltandra virginica L. and Juncus effusus L.) plus one coniferous tree (Taxodium distichum L.), all of which are native to eastern North America, in a five-to-one mixture of well-fertilized mineral soil and peat moss in pots submerged in water in tubs located within controlled environment chambers for a period of eight weeks. Concomitantly, they measured the amount of CH4 emitted by the plant foliage, along with net CO2 assimilation rate and stomatal conductance, which were made to vary by changing the CO2 concentration of the air surrounding the plants and the density of the photosynthetic photon flux impinging on them.

Methane emissions from the four wetland species increased linearly with increases in both stomatal conductance and net CO2 assimilation rate; but the researchers found that changes in stomatal conductance affected foliage methane flux "three times more than equivalent changes in net CO2 assimilation," making stomatal conductance the more significant of the two CH4 emission-controllers. In addition, they note that evidence of stomatal control of CH4 emission has also been reported for Typha latifolia (Knapp and Yavitt, 1995) and Carex (Morrissey et al., 1993), two other important wetland plants. Hence, since atmospheric CO2 enrichment leads to approximately equivalent - but oppositely directed - changes in foliar net CO2 assimilation (which is increased) and stomatal conductance (which is reduced) in most herbaceous plants (which are the type that comprise most wetlands), it can be appreciated that the ongoing rise in the air's CO2 content should be acting to reduce methane emissions from earth's wetland vegetation, because of the three-times-greater negative CH4 emission impact of the decrease in stomatal conductance compared to the positive CH4 emission impact of the equivalent increase in net CO2 assimilation.

In closing, it would appear that current environmental trends that may impact methane emissions from natural vegetation, including the ongoing rise in the air's CO2 content, primarily tend to reduce this flux; and perhaps that is why the rate-of-rise of the atmosphere's methane concentration has been steadily declining over the last several years to the point that it is now nearly nil (see Methane (Atmospheric Concentrations) in our Subject Index). 


References 

Davidson, E.A., Ishida, F.Y. and Nepstad, D.C. 2004. Effects of an experimental drought on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest. Global Change Biology 10: 718-730.

Garnet, K.N., Megonigal, J.P., Litchfield, C. and Taylor Jr., G.E. 2005. Physiological control of leaf methane emission from wetland plants. Aquatic Botany 81: 141-155.

Knapp, A.K. and Yavitt, J.B. 1995. Gas exchange characteristics of Typha latifolia L. from nine sites across North America. Aquatic Botany 49: 203-215.

Morrissey, L.A., Zobel, D. and Livingston, G.P. 1993. Significance of stomatal control of methane release from Carex-dominated wetlands. Chemosphere 26: 339-356.

Nepstad, D.C., Verissimo, A., Alencar, A., Nobre, C., Lima, E., Lefebvre, P., Schlesinger, P., Potter, C., Moutinho, P., Mendoza, E., Cochrane, M. and Brooks, V. 1999. Large-scale impoverishment of Amazonian forests by logging and fire. Nature 398: 505-508.

Roulet, N., Moore, T., Bubier, J. and Lafleur, P. 1992. Northern fens: Methane flux and climatic change. Tellus Series B 44: 100-105.

Strack, M., Waddington, J.M. and Tuittila, E.-S. 2004. Effect of water table drawdown on northern peatland methane dynamics: Implications for climate change. Global Biogeochemical Cycles 18: 10.1029/2003GB002209.

Timmermann, A., Oberhuber, J., Bacher, A., Esch, M., Latif, M. and Roeckner, E. 1999. Increased El Niño frequency in a climate model forced by future greenhouse warming. Nature 398: 694-696.