On the Spatial Distribution of Cirsium pitcheri and Centaurea maculosa

Abstract

The importance of spatial distribution within a plant community cannot be underestimated. When each neighbor is also a competitor for the same water, light, and nutrients, a plant's location is the difference between life and death. We investigated the spatial distributions of populations of two plant species: Cirsium pitcheri (Pitcher's thistle), a threatened dune species that is endemic to the Great Lakes basin, U.S.A., and Centaurea maculosa (Spotted knapweed), an introduced invasive species that has spread throughout North America. Using a GPS unit to map each plant population in 2007 and 2009 and at two locations on Beaver Island, MI, we looked at the effects of density dependence on size and reproduction using regressions based on the length of the longest leaf and number of flowering heads based on the size of each plants neighborhood. Levels of association and segregation between the two species were analyzed using Monte Carlo randomization tests and nearest neighbor analyses. We found that the relationship between growth and reproductive success was equivocal. The single species nearest neighbor analyses gave conclusive evidence of both species growing in a clumped pattern, but the two species analysis returned a non-significant result. We propose larger sample sizes for a more accurate analysis, as well as manipulative experiments be conducted in an effort to identify any competition between the two species in order to better understand how to protect the threatened Cirsium pitcheri.