Abstract
<title>Abstract</title> <p> <italic>Aegilops tauschii</italic> is an invasive grass with seed position effects in wheat ( <italic>Triticum aestivum</italic> ) fields, and its presence results in decreased grain yield. Competition between the invasive <italic>A. tauschii</italic> and wheat was reported in several studies, however, seed position effects in <italic>A. tauschii</italic> haven’t been taken into account. In the present study, seeds from three diaspore positions in compound spike and two/three seed positions in diaspore of <italic>A. tauschii</italic> and seeds of <italic>T. aestivum</italic> were mono- and mixed cultured under different planting ratios. For both species, plant height, number of tillers per plant, number of seeds per plant and mass of seeds per plant were counted and measured after maturation. Also, relative competitive intensity (RCI) and relative performance in interspecific pairs and intraspecific pairs (LnRRˍinter/intra) were determined. Plant height of <italic>A. tauschii</italic> increased, while number of tillers, number of seeds and seed mass per plant decreased as diaspore/seed position on <italic>A. tauschii</italic> increased under both mono- and mixed culture conditions. Plant height of <italic>T. aestivum</italic> decreased, while number of tillers, number of seeds and seed mass per plant increased as diaspore/seed positions of <italic>A. tauschii</italic> increased in mixed culture. Regardless of seed position, competitive ability of <italic>A. tauschii</italic> was higher than that of <italic>T. aestivum</italic> . Intraspecific competition intensity was higher than interspecific competition intensity in <italic>A. tauschii</italic> while it was the opposite in <italic>T. aestivum</italic> . In most cases for <italic>A. tauschii</italic> , growth indicators were highest while RCI was the lowest in the mixed culture ratio of 3:3, indicating that <italic>A. tauschii</italic> had the strongest competitive ability when the number of <italic>A. tauschii</italic> and <italic>T. aestivum</italic> plants was equal. RCI of <italic>A. tauschii</italic> increased as diaspore/seed position increased indicating that competitive ability decreased. Potential ability of <italic>A. tauschii</italic> to colonize new environments increases as diaspore/seed positions increases. Thus, there is a colonization/competition trade-off, which is beneficial for species adaptation in diverse environments. </p>