Mechanisms of Methane Transport Through Trees

Although the dynamics of methane (CH4) emission from croplands and wetlands have been fairly well investigated, the contribution of trees to global CH4 emission and the mechanisms of tree transport are relatively unknown. CH4 emissions from the common wetland tree species Populus trichocarpa (black cottonwood) native to the Pacific Northwest were measured under hydroponic conditions in order to separate plant transport mechanisms from the influence of soil processes. Roots were exposed to CH4 enriched water and canopy emissions of CH4 were measured using a canopy enclosure. Measurements of CH4 flux from 28 trials on eight trees indicate that emissions are normally constant over the half-hour sampling period. The average canopy CH4 flux for the 18 trials with constant emission was 3.0 ± 2.6 μg CH4 min−1. Flux magnitudes from stem experiments scaled to the area of the main tree stem are comparable to whole-canopy flux values, indicating that the majority of CH4 emitted from the tree leaves through the stem. Stem experiments also show that CH4 flux decreases in magnitude with height of the tree. Samples for stable carbon isotope composition were taken during the canopy experiments. Compared to the isotopic composition of root water CH4, canopy CH4 was depleted in C on average by 8.6 ± 3.3h; this indicates that CH4 moving through the tree is not following a purely bulk flow pathway (where no depletion would occur), but is instead subject to at least one fractionating mechanism. Isotopic measurements of emitted CH4 reveal information i about transport mechanisms, but transport mechanisms do not necessarily correlate with any particular physical pathway. Using the same procedure as the canopy enclosure trials, temperature was varied from 17.1 C to 25.2 C over several weeks. The flux at the coolest temperature is significantly different from the higher flux at the warmest temperature (p-value less than 0.02). The calculated Q10 for CH4 flux was 2.4 ± 2.0, which is in the range of typical Q10 values for biological processes. Analysis of C values of emitted CH4 in the temperature experiments shows increasing depletion with cooler temperatures and lower flux. This indicates that not only does the magnitude of flux vary with temperature, but the actual dominant transport mechanism changes as well. When the amount of CH4 leaving the root chamber was compared to the amount measured as flux in the canopy experiments, a significant amount of CH4 was ”missing.” Several experiments were conducted to account for this missing amount; evidence suggests CH4 is stored, at least temporarily, within the tree itself by an unknown mechanism. Transport efficiency was calculated and found to be relatively slow; that is, trees do not transport CH4 very efficiently. This has important ramifications for CH4 emission from trees, since increases in soil production of CH4 may not result in significantly higher CH4 flux from trees. This work lays a foundation for understanding CH4 transport mechanisms, however these mechanisms will need to be more thoroughly defined in order to make accurate predictions about CH4 emission from trees and to describe how flux will change under altered climate conditions.