Seedling growth and competition of a late-seral, native perennial grass and 2 early-seral, native forbs in the presence of 2 densities of the invasive annual grass Bromus tectorum L. (Poaceae)

Abstract

The invasive, annual cheatgrass (Bromus tectorum L. [Poaceae]) has invaded millions of hectares of the North American sagebrush steppe biome. Because of reduced forage of more desired species, cheatgrass-infested sites have significantly lower rangeland productivity resulting in significant lost revenue. Further, cheatgrass-invaded sites have far lower species richness and diminished ecosystem function. The scale of disturbance and the size of restoration projects in the region make seeding, often with little or no seedbed preparation, the only feasible restoration treatment. Seeding early-seral natives with characteristics advantageous for site colonization and stabilization has been seen as a possible means to transition between cheatgrass dominance and desired native plant diversity. We conducted greenhouse trials to compare competitive ability of 2 native, early-seral forbs, curlycup gumweed (Grindelia squarrosa (Pursh) Dunal [Asteraceae]) and hoary tansyaster (Machaeranthera canescens (Pursh) A. Gray [Asteraceae]), and also 1 late-seral perennial grass, bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) Á. Löve [Poaceae]), when planted among low and high densities of cheatgrass during the critical seedling stage of growth. We also tested whether curlycup gumweed, a novel species under investigation for germplasm release and use in restoration seedings, had any negative effects on bluebunch wheatgrass. Over 12 wk, cheatgrass grew faster and produced more biomass than the natives. In addition, low and high densities of cheatgrass significantly reduced plant volume and aboveground biomass of all 3 natives. Of the natives, only bluebunch wheatgrass caused any notable reduction in cheatgrass growth. Curlycup gumweed did not affect bluebunch volume or biomass to any extent beyond the response it would have with a same-species competitor. Our results indicate that the 2 early-seral forbs tested were not effective at outcompeting cheatgrass, and their value in capturing cheatgrass-infested sites may be limited. Finally, we found no inhibitory effect from curlycup gumweed on bluebunch wheatgrass in the absence of other restoration practices during the first 12 wk of growth.