Competition between Elymus elymoides and Taeniatherum caput-medusae

Two field experiments were conducted from 1993-1994 through 1995-1996 growi ng seasons in Harney County, OR, to determine the relative competitive abil ities of Elymus elymoides (squirreltail) a native perennial range grass, an d Taeniatherum caput-medusae (medusahead), an exotic annual grass weed. The 1993-1994 growing season was very dry, 1994-1995 was dry, and 1995-1996 wa s wetter than average. One experiment tested seedlings vs. seedlings in eac h of three seasons. The second experiment tested seedlings plus second- and third-year established E. elymoides plants vs. T. caput-medusae over 2 yr. Biomass, seed production, and soil moisture utilization 15, 30, 45, and 60 cm deep by the two species were measured. A randomized block design with f actorial arrangement was used, with 25 2.25-m(2) plots per block. Initial s eeding densities of each species were 0, 10, 74, 550, and 4,074 seeds m(-2) in all combinations of density. In the seedling vs. seedling experiment, i ntraspecific competition by T. caput-medusae on itself was always significa nt (P less than or equal to 0.10) for bath biomass and seed production. Int erspecific competition gy E. elymoides seedlings on T. caput-medusae biomas s and seed production was not significant (P greater than or equal to 0.10) in 2 of 3 yr and was always less than intraspecific competition by T. capu t-medusae. Only 0.4% of E. elymoides seed germinated, and no seed was produ ced in the very dry first year, but 84% of remaining seed was viable for th e next year. which had better moisture conditions for germination and estab lishment, Interspecific competition affected (P less than or equal to 0.10) E. elymoides seedling biomass and seed production throughout the study. In traspecific competition affected (P less than or equal to 0.10) seedling E. elymoides seed production in the dry year but not in the wetter than avera ge year. In the mature E. elymoides experiment, intraspecific competition b y T. caput-medusae on weight and seed production per plant was greater than interspecific competition from E. elymoides. Seedling/mature E. elymoides reduced T. caput-medusae weight per plant in the dry year but the effect wa s not biologically significant. Larger, mature E. elymoides plants produced 600 to 3,000 seeds per plans during the wet year, neither intra- nor inter specific competition was a factor. Taeniatherum caput-medusae was better ab le to access deeper soil moisture and was more aggressive at extracting soi l moisture than were E. elymoides seedlings in the wee year. Cold soils and low oxygen due to wet soils may have restricted E. elymoides seedling root activity. Mature E. elymoides plants did not appear restricted by cold soi ls or low oxygen. Established second- and third-year E. elymoides plants we re able to compete for soil moisture down to 45 cm. The generally greater i nterspecific competitive effects of T. caput-medusae on E. elymoides than v ice versa suggested that it will be difficult to establish an E. elymoides stand in an existing T. caput-medusae community without first suppressing T . caput-medusae. Individual E. elymoides plants did establish and were prod uctive with and without T. caput-medusae competition.