Future management of the threatened Cirsium pitcheri: effects of interspecific competition and percent cover on both individual plant and population survival.

Abstract

Pitcher's thistle (Cirsium pitcheri), a threatened plant endemic to the shores of the Great Lakes, occupies habitats ranging from active beaches and blowouts, through partially stabilized sand dunes, to stabilized sandy grass areas (Chen and Maun 1998). The unique and rapid changes in dune habitat caused by sand erosion and accretion are particularly conducive to the success of Pitcher's thistle. The persistence of thistle populations requires large dune system landscapes with an abundance of open sand area in which to disperse seeds and continuous geological processes allowing colonization of newly created habitats. This metapopulations dynamic of C. pitcheri is essential to understand when attempting to determine what factors are important in protection and recovery (Pavlovic 2000). Both sand movement and seed dispersal are greatly affected by community structure of populations (McEachern 1992). In the present study, we focus on two elements of community structure: interspecific competition, measured as nearest neighbor distance, and percent plant cover and investigate there relationship to both individual plant and population success. We predicted that both populations and plants will be most successful with less than 50% plant cover. We also predicted that individual plants will be most successful with the presence of neighbors, to help stabilize the sand, but without excessively close neighbors to seed production and growth. We expected to see more individual success with intermediate neighbor distances. We observed five different thistle populations and collected data on height, age, # of buds, and neighbor distance for ten representative plants. We assessed total stem count and percent plant cover for each population pertaining to both individual plant success (juvenile:height/age; adult: # of buds) and population success (total stem count) to test our hypotheses. Our data show no correlation between percent plant cover nor interspecific competition and individual plant and population success of C. pitcheri. Our results suggest that other factors of community structure such as climate, soil, nutrients or wind may be more important to the success of Pitcher's thistle. These results may yield a greater understanding of thistle habitat and could potentially facilitate the protection and recovery of this threatened plant.