ORIGINAL PAPER Reproductive strategy of an invasive thistle: effects of adults on seedling survival Robin G. Marushia Æ Jodie S. Holt Received: 13 May 2007 / Accepted: 22 November 2007 / Published online: 8 March 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Invasive species are known for their ability to form monocultures that exclude native species, yet intraspecific interactions among invasives have not been well studied. Cynara cardunculus (L.) is an invasive perennial thistle that establishes high-density popula- tions in coastal California grasslands. We examined the natural distribution of C. cardunculus seedlings in an established population and found that nearly 100% of seedlings grew within 2 m of adults despite an expected distribution peak at 3 m from source plants based on measured dispersal distances. We then investigated the role of mature plants in seedling survival and establish- ment with regard to live vegetation, litter, and seedling distance by planting seedlings at increasing distances around adults and applying removal treatments to the focal adult rosettes. We applied control (no removal), adult rosette removal (live leaves), litter removal (dead leaves), and adult rosette plus litter removal (all aboveground plant material) treatments. Seedlings experienced a higher rate of survival, measured by senescence date, and establishment, measured by return rate the following year, with all adult rosette removal treatments. Inhibition by adult rosettes was reduced with distance to 60–80 cm from the rosette, and there was little effect of adult plants between 80 and 200 cm. These results suggest that adult rosettes may inhibit conspecific seedlings at very close distances but provide a favorable environment for seedlings within nearby interspaces. This pattern may contribute to the creation and maintenance of high-density populations in C. car- dunculus. Land managers seeking to control this species may improve long-term effectiveness by expanding management efforts to include a 2 m radius around adult plants and treating within 5 months of seedling emer- gence to prevent recruitment rather than treating adults alone. Keywords Artichoke thistle Á Seedling distribution Á Inhibition Á Density-dependent mortality Á Cynara cardunculus Á Invasive plant Á Control Á Survival analysis Á Intraspecific interactions Introduction Invasive plants impact native ecosystems directly through mechanisms such as allelopathy and resource capture and by altering ecosystems functions such as nutrient cycling, fire frequency, and water availability (Mack et al. 2000; Pimentel et al. 2000; Vitousek et al. 1996). Although research on plant invasion processes has traditionally focused on negative effects of inva- sive species on native flora, positive interactions, such as facilitation, are becoming an increasing source of attention (Bertness and Callaway 1994; Bruno et al. 2003; Cheng et al. 2006). In particular, some studies of R. G. Marushia Á J. S. Holt (&) Department of Botany and Plant Sciences, University of California Riverside, Riverside, CA 92521, USA e-mail: jodie.holt@ucr.edu 123 Biol Invasions (2008) 10:913–924 DOI 10.1007/s10530-008-9234-1