There are numerous reports of upland forests and savannas that switched for periods of time to CH4 sources , and wet-land forests that switched to CH4 sinks . In most cases the proximate cause for the shift was a change in soil water content, but the ultimate cause varied from seasonal shifts in precipitation and evapotranspiration , to plant community successional stage , to experimentally imposed warming . Because transpiration helps regulate soil water content, these studies suggest that tree physiology influences CH4 fluxes between upland forests and the atmosphere.Tree physiology influences both the production and oxidation of CH4, and can play an important role in determining whether a particular forest is a net source or sink of CH4. In the near absence of studies on plant regulation of CH4 cycling in upland forests, it is instructive to consider studies in wetland systems. Plants are the ultimate source of organic carbon—in the form of root exudates or detritus—that microorganisms metabolize to CH4, and several isotope tracer studies have demonstrated a tight coupling between plant photosynthesis and methanogenesis . A full cycle of CO2 assimilation by plants, release of photosynthate into soils and emission as CH4 requires as little as 2 hours,hydroponic grow table and up to 6% of the assimilated CO2 is emitted as CH4 in wetland ecosystems.
Elevated CO2 concentration stimulates CH4 emissions from wetland soils , an effect that is directly proportional to the stimulation of photosynthesis by elevated [CO2] . Although most studies relating the effects of elevated [CO2] to CH4 emissions from wetland soils have been with herbaceous plants, a single study confirmed a linear relationship between CH4 emissions and photosynthesis in the wetland tree Taxodium distichum Rich.It is reasonable to hypothesize that similar relationships between plant productivity and methane production occur in upland forests. For example, increasing inputs of labile carbon to upland soils may promote CH4 production both by enhancing the electron donor supply to methanogens, and expanding anaerobic microsites via increased microbial O2 demand. Trees exert indirect regulation of CH4 production and oxidation through their influence on soil water content, which determines the proportion of the soil profile that is anaerobic and producing CH4 versus aerobic and oxidizing CH4. An example of tree physiology influencing CH4 cycling in upland forests is provided by the Duke FACE experiment. McLain et al. found that elevated [CO2] increased soil water content, which simultaneously increased CH4 production and decreased CH4 oxidation. The increase in soil water content was caused by reduced transpiration in the elevated [CO2] treatment, and the net effect was a positive feedback on radiative forcing by CH4. A possible mechanism for CH4 emissions from upland vegetation is transport from the saturated zone below the water table through the transpiration stream. In most ecosystems, the deepest 5% of roots occur at depths greater than 1 m and maximum rooting depths can exceed 4 m .
The deepest root systems are found in tropical areas where high concentrations of atmospheric methane have been observed . Specifically, they occur in tropical semiarid to humid savanna, and tropical seasonally dry semideciduous to evergreen forests . Deep roots that access the water table may contribute disproportionately to transpiration fluxes . In such cases, CH4 dissolved in groundwater would presumably be entrained in the transpiration stream in a manner similar to CO2 from root respiration .There are several recent reports suggesting that tropical forests may be larger sources of CH4 than previously believed. The most comprehensive analysis used a satellite-mounted instrument to show that atmospheric CH4 concentrations are far greater than expected from ground-based emissions inventories of tropical rain forests . The deviation between modeled and observed column-averaged atmospheric CH4 concentrations was especially large over the Amazon Basin and was correlated with the distribution of broadleaf evergreen forest. Frankenberg et al. noted that the discrepancies in measured and modeled CH4 concentrations could be explained by underestimates of known emissions sources such as wetlands, biomass burning, termites and cattle. The measurements were taken during the dry season when wetland emissions should be lowest and biomass burning emissions should be highest, suggesting the biomass burning was the more important source. However, localized measurements of atmospheric CH4 concentrations show that there can be significant biogenic CH4 sources in tropical upland forests.
Methane concentration profiles in three upland forests of the Brazilian Amazon showed a CH4 source within the lower 10 m of the forest canopy , and nighttime pooling of CH4 at 2 m above the soil surface was observed in a mixture of forest and savanna in Venezuela . In both cases, when extrapolated to large areas, the estimated CH4 emission rates were potentially significant on a global scale . Scharffe et al. concluded that soil emissions were a relatively small contribution to CH4 sources at the Venezuelan site and suggested that termite mounds and waterlogged pools were unmeasured CH4 emission hotspots. Crutzen et al. reinterpreted these data as evidence of an aerobic plant CH4 source. Regardless of whether the source of the CH4 in these systems was vegetation or a combination of several known sources, none of which can be distinguished by these studies, it is clear that CH4 exchange between tropical upland ecosystems and the atmosphere has not been adequately characterized.Frankenberg et al. recognized that the discrepancies in measured and modeled CH4 concentrations could be explained by a “… hitherto unknown methane source that might be directly related to the broad leaf evergreen forest.” Just 7 months later, Keppler et al. published the first observations supporting one possible unknown CH4 source—direct emissions from aerobic vegetation. They reported that CH4 was emitted from every plant tissues tested, including detached leaves from 30 species, leaf litter and intact plants. The data of Keppler et al. suggested that sunlight, temperature and physiological activity were key variables regulating aerobic CH4 emissions. The sunlit rates for intact plants were significantly higher than those for detached leaves , dark emission rates for intact plants and detached leaves were significantly lower than sunlit leaves ,growing strawberries hydroponically and the temperature coefficient was about 2 over the range 30–70 °C. The process appeared to be non-enzymatic because emissions increased monotonically up to 70 °C and CH4 was emitted from commercially available apple pectin. More recently, Dueck et al. used an isotope-labeling technique in an attempt to verify emissions of CH4 from aerobic plant tissues. This approach indicated rates that were not significantly different from zero, and at best, an order of magnitude lower than those of Keppler et al. . Increasing the amount of plant biomass in the experimental chambers improved the detection limit of their technique and suggested that little or no CH4 is emitted by plant tissues. These data suggest that the fluxes reported by Keppler et al. were an artifact of their methods. The experiments performed by Dueck et al. were more controlled and physiologically relevant than those by Keppler et al. , but it is unclear whether the hydroponic system they used effectively excluded CH4 oxidizing bacteria, which are aerobic and capable of consuming CH4 produced by plant tissues. The negative rates of CH4 production reported by Dueck et al. were reasonably interpreted as experimental error, but they could also be interpreted as net consumption of CH4 and it is unclear whether the leak tests they performed were long enough to allow for this possibility. Given the absence of in situ measurements of aerobic plant CH4 emissions, it is instructive to compare the Keppler et al. rates to other volatile organic carbon compounds such as methanol, which are relatively well understood. There are many different VOCs, but the total flux from foliage is dominated by a few compounds such as isoprene and methanol. The initial studies of methanol emissions from plants reported rates from mature leaves that typically ranged from about 0.8 to 44 µg g–1 h–1 , which is at least an order of magnitude higher than the CH4 emission rates observed by Keppler et al. under similar conditions of light and of temperature. Methanol emission rates from young leaves are even higher than rates observed for mature leaves .
Lower methanol emission rates have since been reported for most plants, but average methanol emission rates for mature sunlit leaves are at least 1.5 µg g–1 h–1, which is four times the CH4 emission rate reported by Keppler et al. . These figures suggest that the global contribution of CH4 from aerobic plant biomass, if it occurs at all, are considerably less than global emissions of methanol, which are estimated to be between 100 and 260 Tg year –1 .Keppler et al. offered a provocative global extrapolation of their intact plant CH4 emission rates that suggested up to 243 Tg year –1 of CH4 was emitted from this new source. This figure was derived by scaling leaf-mass-based emission rates to the globe with day length, growing season length and total net primary productivity as driving variables, all stratified by the major biomes. Alternative extrapolations of the same data were subsequently published that accounted for differences in foliage turnover rates between biomes, significantly lowering the global strength of a putative aerobic plant source . To further constrain the potential magnitude of global CH4 emissions from upland plants, we used a foliar VOC emissions model—MEGAN or Model of Emissions of Gases and Aerosols from Nature—to incorporate certain canopy and physical processes that were not considered by Kirschbaum et al. and Parsons et al. . In particular, we used the temperature responses reported by Keppler et al. and accounted for the effects of self-shading within the plant canopy. We used MEGAN with the assumption that the mechanism of CH4 production, if it exists at all, shares some features of the biochemical pathways that produce other VOCs such as methanol. MEGAN includes a detailed canopy environment model that calculates solar radiation and leaf temperature of sun and shade leaves for each of five canopy depths. Driving variables include wind speed, humidity, soil water content, above-canopy direct and diffuse solar radiation, and ambient temperature. MEGAN includes emission factors for light-dependent and light-independent components of emissions, and irradiances that vary because of self-shading in the plant canopy. Light-dependent and light-independent emissions of CH4 were estimated based on the emission factors recommended by Keppler et al. . Although Keppler et al. did not report light response curves, we assumed that emissions increase nearly linearly with irradiance to a saturation point. This is the behavior we observe for other biogenic VOC and is thus a reasonable starting point for the CH4 extrapolation. The emission algorithm for dark emissions was based on the temperature response shown in Figure 1 of Keppler et al. . A range of global annual CH4 emission estimates was generated using different combinations of the alternative land cover and weather databases described by Guenther et al. . Our parameterization of light and temperature in the MEGAN model is similar to the global model of aerobic CH4 emissions developed by Butenh off and Khalil . The global distribution of CH4 emissions from foliage simulated with MEGAN is shown in Figure 1. Tropical forests are a major source region, which agrees with the predictions of Keppler et al. and the observations of Frankenberg et al. . The annual global CH4 emission from living vegetation estimated with MEGAN ranged from 34–56 Tg year –1, depending on the land cover and weather data used to drive the model. This figure is nearly one order of magnitude lower than the highest estimates provided by Keppler et al. and is consistent with the magnitude of alternative extrapolations provided by Kirschbaum et al. and Parsons et al. , and the global model developed by Butenhoff and Khalil . Our estimates would be about an order of magnitude lower if we had used the mean rate reported by Dueck et al. of 21 ng g–1 h–1.Observations of unexpectedly high atmospheric CH4 concentrations in forested landscapes have revealed a gap in our understanding of trace gas emissions. The wide variety of plausible explanations offered for these observations encompass specialties ranging from soil microbiology to plant physiology to atmospheric chemistry. These disparate research communities should continue to study the problem in order to inform modeling and public policy related to climate change. It is doubtful that these observations can be explained by the aerobic plant CH4 source proposed by Keppler et al. because independent extrapolations and rate measurements suggest emission rates from plant tissues are far lower than initially believed .