Differing life histories contribute to difficulties establishing perennial grasses on lands dominated by exotic annual grasses. In a greenhouse study, we investigated to what extent allowing the perennial grass bluebunch wheatgrass to emerge before the exotic annual grass downy brome would increase its competitive ability and whether modifying nitrogen (N) would affect competition. We conducted an addition-series factorial experiment. In three cohort treatments, the two species were seeded concurrently or bluebunch wheatgrass was at the two- or four-leaf stage when downy brome was planted. N treatments were low (ambient) or high (N added to maintain 10 mg kg−1 [0.1286 oz lb−1]). Larger bluebunch wheatgrass avoided suppression by downy brome regardless of N. Under concurrent sowing, doubling downy brome density decreased bluebunch wheatgrass biomass by 22.6% ± 2.38 SE. In contrast, when bluebunch wheatgrass had a four-leaf size advantage, the same increase in downy brome density decreased bluebunch wheatgrass biomass by 4.14% ± 2.31. Larger bluebunch wheatgrass also suppressed downy brome more effectively, but N enrichment decreased the suppressive ability of bluebunch wheatgrass.
Nomenclature: Bluebunch wheatgrass, Pseudoroegneria spicata (Pursh) Á. Löve; downy brome, Bromus tectorum L.
Management Implications: This study demonstrated the importance of both order of emergence and nitrogen (N) availability in the competitive relationship between bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Á. Löve] and downy brome (Bromus tectorum L.). First, we found that larger bluebunch wheatgrass seedlings avoided suppression by downy brome more effectively and that they did so regardless of N availability. However, although larger bluebunch wheatgrass also suppressed downy brome more effectively, N enrichment decreased the ability of bluebunch wheatgrass to suppress downy brome. Our results suggest that order of emergence for desired perennial vs. exotic annual grasses may be an important aspect of revegetation planning and that avoiding conditions that lead to N increases, such as soil disturbance, could allow perennial grasses to better suppress downy brome. To establish an initial size difference that favors seeded species, managers could modify the standard timing of revegetation practices to ensure that perennial species receive a sufficient head start. Another way to promote an initial size difference would be to use transplants or plugs to restore critical sites.