Invasion of farmland-grassland ecosystems by the exotic sweet clovers, Melilotus officinalis and M. albus

In the agro-pastoral ecotone of northern China, two exotic sweet clovers have spread widely and have significant impacts on local animal husbandry, vegetation and the visual landscape. In order to evaluate the invasion mechanism of these exotic plants in a farmland-grassland ecosystem, a line-transect survey was conducted and field sampling was undertook in August of 2011. The results showed that land use patterns markedly influenced the distribution and dispersal of exotic sweet clovers in a farmland-grassland ecosystem and sweet clovers grew well in hay-making grassland but growth was restricted in farmland and free-grazing grassland. Environmental parameters may be also the important factors in the invasion process. Distance affected the distribution of exotic sweet clovers. When comparing farmland and grassland, the density of the plants declined significantly with increasing distance from the interface between farmland and grassland. Prevailing winds may also be a trigger factor that facilitated the distribution and dispersal of the two exotic sweet clovers. There was a higher density of sweet clovers in a downwind direction than in an upwind direction in the grassland ecosystem. Furthermore, competition with native plants may affect the establishment and development of the two exotic sweet clovers. The dominant species, Chinese leymus (Leymus chinensis), had a negative correlation with the two exotic sweet clovers for above-ground biomass. The native dicotyledonous plant, Saussurea runcinata, had a positive effect on the biomass of the two exotic sweet clovers. The agro-pastoral ecotone of northern China, which is made up of farmland and grassland, was ideal for a study on the invasion of alien plants and the prevention and control of these exotic plants.