ï~~ 2011 THE MICHIGAN BOTANIST 139 TANACETUM BIPINNATUM GERMINATION AND COMPETITIVE INTERACTION WITH CENTAUREA STOEBE SEEDLINGS Jordan M. Marshall Department of Biology, Indiana University-Purdue University Fort Wayne, Fort Wayne, IN 46805. USA marshalj @ipfw.edu ABSTRACT Tanacetum bipinnatum (Asteraceae) is limited in distribution in the Great Lakes region to sand dune ecosystems and is threatened in Michigan and endangered in Wisconsin. Germination and establishment interruptions for T bipinnatum may add to the limitations in distribution and be exacerbated by the presence of Centaurea stoebe (Asteraceae), which has invaded many different ecosystems around the Great Lakes region, including dunes. I conducted a germination experiment with T bipinnatum seeds undergoing variable cold storage lengths (0-5 months) or germinating in the presence or absence of light. Also, I conducted a competition experiment with establishing T bipinnatum and C. stoebe individuals. With light present for germination, 1 and 2 month cold storage periods had 10% germination. Germination was not independent of cold storage in dark treatments, with only 5% of seeds germinating in controls without cold storage. In competition with C. stoebe, T bipinnatum had reduced above-ground biomass and root length compared to T bipinnatum grown with a conspecific and individually. Below-ground biomass for T bipinnatum was significantly reduced in competition with C. stoebe compared to when it was grown individually. The only alteration in growth for C. stoebe was in below-ground biomass, which was greater in competition with T bipinnatum. Limited seed germination and competition during establishment may explain why T bipinnatum occurs at lower densities in areas invaded by C. stoebe. Further research is needed to quantify survival of mature T bipinnatum beyond the initial establishment in competition with C. stoebe. KEYWORDS: invasive species, threatened, Lake Huron tansy, spotted knapweed INTRODUCTION The post-dispersal establishment of seedlings is essential to the continuation of a plant population (Harper et al. 1965; Eriksson and Ehrl6n 1992). Interruption of establishment often leads to reductions in densities, modification of community physical structure, or can lead to alteration of successional patterns (Denslow 1980; Pyke and Archer 1991; Luzuriaga and Escudero 2009). Theoretical extreme ends of the continuum of plant population dynamics are those that are limited by the availability of what Harper et al. (1961) defined as "safe sites" in the presence of abundant seed and those that are limited by the availability of seed in the presence of abundant "safe sites" (Duncan et al. 2009). Somewhere along the continuum, plant population dynamics are most likely driven by combinations of site and seed availability. Tanacetum bipinnatum (Lake Huron tansy, Asteraceae, =T huronense) is a wide ranging species and occurs in several Canadian provinces and northern US states from Maine to Alaska, with those occurrences being sparse (Cana
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