Invasive alien plants increase CH₄ emissions from a subtropical tidal estuarine wetland

Methane (CH4) is an important greenhouse gas whose emission from the largest source, wetlands is controlled by a number of environmental variables amongst which temperature, water-table, the availability of substrates and the CH4 transport properties of plants are most prominent and well characterised. Coastal wetland ecosystems are vulnerable to invasion by alien plant species which can make a significant local contribution to altering their species composition. However the effect of these changes in species composition on CH4 flux is rarely examined and so is poorly understood. Spartina alterniflora, a perennial grass native to North America, has spread rapidly along the south-east coast of China since its introduction in 1979. From 2002, this rapid invasion has extended to the tidal marshes of the Min River estuary, an area that, prior to invasion was dominated by the native plant Cyperus malaccensis. Here, we compare CH4 flux from the exotic invasive plant S. alterniflora with measurements from the aggressive native species Phragmites australis and the native species C. malaccensis following 3-years of monitoring. CH4 emissions were measured over entire tidal cycles. Soil CH4 production potentials were estimated for stands of each of above plants both in situ and in laboratory incubations. Mean annual CH4 fluxes from S. alterniflora, P. australis and C. malaccensis dominated stands over the three years were 95.7 (±18.7), 38.9 (±3.26) and 10.9 (±5.26) g m-2year-1, respectively. Our results demonstrate that recent invasion of the exotic species S. alterniflora and the increasing presence of the native plant P. australis has significantly increased CH4 emission from marshes that were previously dominated by the native species C. malaccensis. We also conclude that higher above ground biomass, higher CH4 production and more effective plant CH4 transport of S. alterniflora collectively contribute to its higher CH4 emission in the Min River estuary