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than more diverse plant communities. As seen in other studies, when non - seeded species invade <br />a site and increase diversity, production values decrease (Bowen, 2003; Schladweiler et al., <br />2003). Productivity was also significantly higher (F <0.01) in 1978 compared to 2003 for <br />Russian wildrye plots. Grazing impacts could have also influenced decreased productivity in <br />2003 compared to 1978. Spring wheat plots, which were reseeded with smooth brome in 1982, <br />will be referred to as smooth brome plots for the duration of the paper because of the different <br />characteristics of smooth brome compared to spring wheat. Smooth brome plot productivity was <br />highest on sandy -clay loam subsoils (B), crested wheatgrass plot production was highest on <br />sandy loam subsoils (C), Russian wildrye plot production was highest on sandy -clay loam <br />subsoil B, and there was not a significant difference in alfalfa plot productivity by subsoil type. <br />Plant relationships with subsoil type examined in 2003 were similar to relationships examined by <br />Merrill et al. (1998), indicating no significant change through time. <br />Table 2. Mean monthly temperature in degrees Celsius at the Beulah, ND NOAA station. <br />1976 1977 1978 1979 1980 1981 2003 <br />°C <br />Apr. 7.7 9.1 5.8 0.5 8.7 8.2 9.3 <br />May 12.9 17.4 14.4 9.7 14.9 12.0 13.0 <br />June 18.6 18.9 16.9 17.9 18.2 15.3 16.8 <br />July 20.8 21.2 20.3 21.3 22.3 20.8 22.3 <br />Apr.-July 15.0 16.7 14.3 12.3 16.0 14.1 15.3 <br />Productivity was significantly lower (F <0.01) in native mixture and alfalfa plots at the SSW <br />in 1978 compared to 2003. Invasion of smooth brome into other plots on the SSW caused <br />increased yields of less productive plots. Smooth brome is a very aggressive species with high <br />aboveground biomass and invaded other plots once it was seeded after completion of the Power <br />et al. (1981) study. <br />Table 3. Past vs. present mean production for all soil depths on the ZSW and SSW. Dash <br />indicates no data was available for that year. All values are significantly different <br />from each other in each vegetation category at the 0.01 probability level. <br />Crested Russian Crested Native <br />wheatgrass wildrye wheatgrass mixture Alfalfa <br />Year (ZSW) (ZSW) (SSW) (SSW) (SSW) <br />Mean <br />Production <br />(kg ha -1) <br />1976 - 3206 828 1073 <br />1977 520 - 1084 1041 577 <br />1978 3103 2172 4527 1432 786 <br />1979 1419 1149 1819 845 2630 <br />1980 448 245 <br />1981 1432 854 - <br />2003 2110 1698 3504 3043 3614 <br />1238 <br />