Genetic analysis reveals human-mediated long-distance dispersal among war cemeteries in Trifolium micranthum

Many grassland plant species have limited capacity to disperse their seeds beyond local boundaries by natural means. Meanwhile, various forms of human transportation are observed to provide long-distance dispersal. However, the contribution of human-mediated dispersal to the spatial dynamics of established (meta)populations on the regional scale has only scarcely been addressed. Trifolium micranthum is a very small legume that is rare in western Belgium. It is found (i) inland, as a lawn weed on war cemeteries that are managed by the Commonwealth War Graves Commission, and (ii) in coastal grasslands that are managed for conservation. These stations lie over 30 km apart, though interestingly, a number of satellite war cemeteries are found within the vicinity of the coastal reserves, and these also harbor T. micranthum. We have sampled plants from these populations to study their affinities through genetic analysis (AFLP). Results indicated that the coastal cemetery populations were closely related to the inland cemeteries and not to the coastal reserves. This most likely is the result of exchange of seeds with composted lawn material among regions, whereas mowing machinery and livestock may effectuate frequent dispersal within regions. Assignment tests nonetheless indicated limited genetic admixture to have taken place at the coastal cemeteries. Thus, human management is responsible for the realization of gene flow among these remote gene pools. Human-mediated long-distance dispersal is likely to play a role in the local or regional dynamics of many other native plants, including more common and well-dispersing species.