Species-specific effects of vascular plants on methane transport in northern peatlands

Recent studies have identified the significant role of plants in controlling methane (CH₄) emission from peatlands by acting as conduits and further demonstrated that such conduit effect is species-specific. In most studies, species-specific plant-mediated CH₄ transport has been estimated indirectly by comparing CH₄ flux from surfaces with different plant communities to that from surfaces where plants responsible for CH₄ transport are cut, known as the clipping technique. However, the estimation based on the clipping technique has shown large uncertainty due to the plant residual effect. Thus, directly investigating the variation in CH₄ transport between different plant species and the factors affecting it is necessary to more precisely assess changes in the CH₄ fluxes of peatland ecosystems in a changing environment.

We measured CH₄ emission directly from shoots of Carex rostrata, Menyanthes trifoliata, Betula nana, and Salix lapponum from the early growing season until the beginning of senescence (June-September 2020 and 2021, three campaigns both years), with three specimens per species and campaign. We also measured CH₄ emission from Equisetum fluviatile and Comarum palustre during high summer in 2021 to further shed light on species-specific characteristics of plant-mediated CH₄ flux. We monitored abiotic factors such as belowground CH₄ concentration, potential CH₄ production and oxidation rate, water table level, and peat temperature.

During high summer in 2021, C. rostrata had the highest CH₄ transport rate per leaf area (6.86 mg m^-2 h^-1). This value was significantly higher than that from M. trifoliata which was the secondarily important CH₄ emitter with the rate of 4.07 mg m^-2 h^-1. E. fluviatile, C. palustre, B. nana, and S. lapponum had limited CH₄ transport rate per leaf area (0.66, 0.02, 0.14, and 0.15 mg m^-2 h^-1, respectively) and thus were negligible CH₄ emitters. CH₄ emission from C. rostrata demonstrated the most pronounced seasonal variation (ranging from 0.02 to 24.78 mg m^-2 h^-1), driven primarily by seasonal vegetation development (phenology) and only secondarily by rhizospheric peat temperature. In contrast, CH₄ emission from M. trifoliata, B. nana, and S. lapponum showed little seasonal variation, and no factors that significantly affected the flux from these species were found. Lastly, the sharp decrease in rhizospheric peat CH₄ concentration during high summer and the simultaneous increase in emission from C. rostrata, the most dominant species in our site, indicated the conduit effect predominated over the CH₄ production and oxidation. The findings highlight the importance of C. rostrata in mediating CH₄, which could exacerbate the climatic impact of the thawing permafrost region where C. rostrata can thrive in wet microsites.