Spotted Knapweed Spread and Plant Community Changes in a Lacustrine Dune System

Abstract

Perched sand dunes are ecosystems vulnerable to anthropogenic influence along the Great Lakes, including Grand Sable Dunes in Pictured Rocks National Lakeshore along Lake Superior. Spotted knapweed (Centaurea stoebe) is a nonnative plant species that has colonized these dunes and spread into increasingly more areas of the ecosystem. I conducted plant surveys in areas with and without spotted knapweed in 2003, and then resurveyed those areas in 2018. Frequency of spotted knapweed increased in both areas surveyed. Additionally, dominance shifted with spotted knapweed becoming the most important species in the area originally invaded and third most important in the area originally uninvaded. There were approximately two-thirds of species shared between the two survey years, but many of the added species in 2018 were woody. Species richness increased between years, as did species diversity. Nonmetric multidimensional scaling ordination displayed community shifts as convergent succession. While the plant communities in the two areas were rather dissimilar in 2003, they converged as overlapping communities in 2018. Additionally, the shift between years in the area originally invaded was smaller compared to the shift between years in the area originally uninvaded. Due to the continuous evolution of plant communities within sand dune ecosystems, there is a need to monitor changes and quantify nonnative species spread patterns. Dominance by spotted knapweed may be facilitating dune stabilization and converging communities along a modified successional trajectory.