Semiarid ecosystems such as grasslands are characterized by high temporal variability in abiotic factors, which has led to suggestions that management actions may be more effective in some years than others. Here we examine this hypothesis in the context of grassland restoration, which faces two major obstacles: the contingency of native grass establishment on unpredictable precipitation, and competition from introduced species. We established replicated restoration experiments over three years at two sites in the northern Great Plains in order to examine the extent to which the success of several restoration strategies varied between sites and among years. We worked in 50‐yr‐old stands of crested wheatgrass (Agropyron cristatum), an introduced perennial grass that has been planted on >10 × 106 ha in western North America. Establishment of native grasses was highly contingent on local conditions, varying fourfold among years and threefold between sites. Survivorship also varied greatly and increased significantly with summer precipitation. No consistent differences were found between drilling and broadcasting in their effects on establishment, but survivorship was nearly threefold higher in broadcast plots. Plots without seed added, or with native hay added, had almost no seedlings of native grasses. In contrast, broadcasting the residue remaining after cleaning seeds from native hay produced the highest seedling densities of any treatment. Competition from A. cristatum was significantly and consistently reduced through annual application of a generalist herbicide (glyphosate), which increased native grass establishment and survivorship and the richness and total cover of native species. Herbicide decreased standing crop and increased soil moisture and available nitrogen. A. cristatum was controlled without suppressing native vegetation, both by spraying in early spring, which selectively killed the cool‐season A. cristatum, and by application with a wick, which selectively killed the taller A. cristatum. A. cristatum persisted over four years, however, in spite of annual herbicide application. A. cristatum cover in control plots increased significantly with summer precipitation. In summary, broadcasting and drilling differed little in their effects on establishment, but broadcasting increased survivorship and will allow the emergence of plant‐induced heterogeneity. Competition from introduced species can be reduced but not eliminated by continuing herbicide application. Lastly, the positive relationships between precipitation and both A. cristatum and native seedling survivorship suggest that management should focus on controlling A. cristatum during dry years and on introducing native species during wet years. Corresponding Editor: I. C. Burke.

. 2002. Cropping sequence affects wheat productivity on the semiarid northern Great Plains. Can. J. Plant Sci. 82: 307-318. Extension of the commonly used spring wheat (Triticum aestivum L.)-fallow rotation to include broadleaf crops requires information on their effects on a following wheat crop. We grew a spring wheat test crop on the stubbles of wheat and seven broadleaf crops: desi chickpea (Cicer arietinum L.), dry bean (Phaseolus vulgaris L.), dry pea (Pisum sativum L.), lentil (Lens culinaris L.), mustard (Brassica juncea L.), safflower (Carthamus tinctorius L.), and sunflower (Helianthus annuus L.). This study was conducted near Swift Current, SK, from 1993 to 1997, and Congress, SK, from 1995 to 1997. After harvest, soil water differed among crop stubbles and by sampling depth. To the 60-cm depth, only soil under dry bean stubble held more water (8 mm), while soil under lentil, desi chickpea, sunflower and safflower stubbles held less water (6, 8, 9 and 17 mm, respectively) than wheat stubble (P < 0.05). From 60 to 120 cm, soil under dry pea and dry bean held more water (7 and 10 mm, respectively), and under sunflower and safflower stubbles less (7 and 14 mm, respectively), than under wheat stubble (P < 0.05). Lentil, dry bean and dry pea stubbles averaged 5, 6 and 9 kg ha -1 greater soil N in the 0-to 120-cm soil depth than wheat stubble (P < 0.05). The average yield of wheat grown on the four pulse crop stubbles was 21% greater than yields on wheat stubble, but did not differ from the oilseed stubbles (P < 0.01). Compared to wheat stubble, wheat grown on broadleaf crop stubbles had higher grain protein concentrations, increasing by 8 and 5%, for pulses and oilseeds, respectively (P < 0.01). Nitrogen removal in the wheat test crop grain yield averaged 15 kg ha -1 for pulse stubbles compared with wheat stubble. Soil N contribution by pulse stubbles was an important factor contributing to wheat growth under a dryland cropping system on the northern Great Plains. Après la récolte, la teneur en eau du sol variait avec les résidus de culture et la profondeur de l'échantillonnage. À 60 cm de profondeur, seul le sol sous chaume de haricot contenait plus d'eau (8 mm) que celui sous chaume de blé, tandis que le sol sous chaume de lentille, de pois chiche desi, de tournesol et de carthame en renfermait moins (6, 8, 9 et 17 mm, respectivement) (P < 0,05). Entre 60 et 120 cm de profondeur, le sol sous chaume de pois et de haricot contenait plus d'eau (7 et 10 mm, respectivement) que celui sous chaume de blé tandis que celui sous chaume de tournesol et de carthame en renfermait moins (7 et 14 mm respectivement) (P < 0,05). En moyenne, le chaume de lentille, de haricot et de pois apporte 5, 6 et 9 kg de N de plus par hectare au sol que le chaume de blé, à une profondeur de 0 à 120 cm (P < 0,05). Le blé cultivé sur le chaume des quatre légumineuses a donné un rendement moyen de 21 % supérieur à celui du blé cultivé sur le chaume de blé, mais son rendement était identique à celui du blé cultivé sur chaume d'oléagineu...

Restoration practice suggests that neighbor control is essential in semiarid grasslands, but ecological theory predicts that neighbor effects are relatively small in young fields. We investigated the effectiveness of neighbor control (mowing and herbicide) for establishing native grasses in a recently abandoned field in southwestern Saskatchewan, Canada. We also examined its interactions with common restoration techniques, such as mulching (straw and sawdust) and contrasting sowing methods (drilling, and broadcasting cleaned seeds, cleaning remainders, and native hay). The experiment was repeated over three years to examine the effect of weather. Neighbor control had no effect on establishment and rarely any effect on first‐year survival. This contrasted with significant effects of neighbor control on community and ecosystem‐level variables (species richness, water, N). The lack of neighbor effects is concordant with theory which predicts low competition intensity from ruderal annuals. Establishment in seeded plots varied two‐fold among years in drilled plots and 50‐fold in broadcast plots, and it was lowest during a cool, dry summer. Thus, variables beyond human control are a major factor determining restoration success. Overall, broadcasting was as effective as drilling. The highest long‐term establishment of native species was produced by broadcasting cleaning remainders. Almost no seedlings emerged from plots supplied with native hay. Straw mulch increased soil moisture and available N, and sawdust decreased N, but neither had any long‐term effect on native grasses. Our results suggest that restorations of semiarid old fields should focus less on neighbor control and more on strategies for exploiting suitable years for germination, either by monitoring soil moisture or through repeated seeding.