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For comparison of seedling growth competitive responses in a controlled environment, monocultures (intraspecific) and 2 species mixtures (interspecific) of mountain rye (Secale montanum), crested wheatgrass (Agropyron cristatum × desertorum 'Hycrest'), and downy brome (Bromus tectorum) were established. Seedling dry root and shoot weights, shoot area, and maximum root length were compared at 1, 2, 3, and 6 weeks of growth in shoot roots boxes under a growth chamber environment (16 hr @ 14 degres C, 1,000 micro-E m-2 sec-1; 8 hr @ 10 degrees C, dark). Soil moisture depletion was monitored gravimetrically. Dry root and shoot weight, shoot area, and root length of mountain rye was greater than that of both downy brome and Hycrest crested wheatgrass at every sampling period over the 6-week study when grown in two-species mixtures. No difference was obtained for these seedling growth characters between downy brome and Hycrest mixtures, except for a 6.4 cm vs. 4.8 cm maximum root length at 1 week of growth. Similarly, in monoculture, mountain rye generally produced greater seedling growth than the other 2 species, although exceptions occurred for root weight, shoot area, and root length by 6 weeks of growth. Mountain rye depleted soil moisture in the growth boxes more rapidly and to a lower potential than the other 2 species. The results of this study indicate mountain rye provide vigorous competition as a seedlling. This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020