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To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2009.01520.x Byline: Andrea S. Thorpe (1), Giles C. Thelen (1), Alecu Diaconu (2), Ragan M. Callaway (1) Keywords: allelopathy; ([+ or -])-catechin; Centaurea maculosa; novel weapons hypothesis; invasive species; biogeographical comparison; spotted knapweed Abstract: Summary Exotic invasive plants can have strong effects on native communities. Centaurea maculosa, a forb that is native to Eurasia, has created near-monocultures in many parts of its invaded range in western North America and produces the root exudate ([+ or -])-catechin. Controlled laboratory experiments suggest that the phytotoxic effects of ([+ or -])-catechin may be stronger on some North American species than on some European species. We conducted experiments in the field in two different years in the native (Romania) and invaded (MT, USA) ranges of C. maculosa, testing the effects of ([+ or -])-catechin on species that co-occur with C. maculosa in both ranges. ([+ or -])-Catechin reduced the growth of native plant species in Montana in both years, although there was some variability between species in the effect of ([+ or -])-catechin on leaf growth in 2005. There was no effect of ([+ or -])-catechin on plants in Romania. This first in situ test of the novel weapons hypothesis supports the notion that novel biochemical constituents of some invasive species may contribute to their success. Synthesis. In addition to providing information useful for understanding invasions, our results indicate that some species in the native range of C. maculosa may be adapted to its particular biochemical traits, raising the possibility that interactions among plant species may be affected by a common evolutionary history. Author Affiliation: (1)Division of Biological Sciences, University of Montana, Missoula, MT 59812, USA (2)Biological Control Laboratory, Institute of Biological Research, Bd Carol I 20-A, 6600 Iasi, Romania Article History: Received: 5 November 2008; accepted: 18 April 2009Handling Editor: James Cahill Article note: (*) Correspondence author. E-mail: andrea@appliedeco.org