Experimental scheme. Ark = Arkansas, BC = British Columbia, Mar = Maryland, Mon = Montana, Ver = Vermont, Ach = Achillea, Hel = Helianthus, Poa = Poa, and Vul = Vulpia.

Single abbreviations (the left panel; e.g., Ark or Ach) represent one plant grown alone (i.e. no competition), and two abbreviations together (the right panel; e.g., Ark+Ach) represent two plants grown together (i.e. competition). There were 14 and eight replicates for Centaurea stoebe populations (i.e. Ark, BC, Mar, Mon, and Ver) and North American species (i.e. Ach, Hel, Poa, and Vul) grown alone (i.e. no competition), respectively; there were seven replicates for the pair-wise competition.