What was done
The authors grew seedlings of three herbaceous emergent aquatic macrophytes (Orontium aquaticum L., Peltandra virginica L. and Juncus effusus L.) plus one coniferous tree species (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 within controlled environment chambers for a period of eight weeks.  The amount of methane (CH4) emitted by the plant foliage was then measured, 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 photosynthetic photon flux density impinging on them.

What was learned
Methane emissions from the four wetland species increased linearly with increases in both stomatal conductance and net CO2 assimilation rate; but changes in stomatal conductance affected foliage methane flux, in the words of the authors, "three times more than equivalent changes in net CO2 assimilation," making stomatal conductance the more significant of the two CH4 emission-controllers.  Furthermore, 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.

What it means
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.  Perhaps that is one of the reasons why the rate-of-rise in the atmosphere's methane concentration has been steadily declining over the last several years to the point that its concentration has now been essentially stabilized (see Methane (Atmospheric Concentrations) in our Subject Index).

References
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.