Contributing to the success of Bromus tectorum in the Intermountain West may be a mechanism called apparent competition, which occurs when one species increases the pressure of a consumer on a second species. This indirect interaction has been documented only a few times in invasive plant systems, and never in a fungal pathosystem. We examined the effects of the invasive annual Bromus tectorum and predation by the seed pathogen Pyrenophora semeniperda on seedling emergence and survival for two native grasses (Pseudoroegneria spicata and Elymus elymoides), by manipulating B. tectorum densities and P. semeniperda inoculum loads in randomized plots. Identical field studies were conducted in Skull Valley, Utah (xeric site) and Sprague, Washington (mesic site). The addition of inoculum decreased emergence of native grass seedlings at both sites and increased the amount of unemerged native seeds that were killed by P. semeniperda. Higher densities of B. tectorum decreased native grass survival at the mesic site and increased survival at the xeric site probably due to the beneficial effects of B. tectorum litter on soil moisture. At both sites, there were more B. tectorum seeds found in the seed banks in plots with high B. tectorum densities than in low-density plots. This indicates an increase in available prey for P. semeniperda. There was a much lower level of infection in B. tectorum seed bank seeds at the mesic site than at the xeric site. The high level of ungerminated native seeds killed by background levels of P. semeniperda, combined with the increase in available prey for the fungus in high-density B. tectorum plots, shows that apparent competition may play a role, along with direct competition, in the success of B. tectorum. This interaction is important to consider when dealing with control of B. tectorum.

Little information is available on the extent of local adaptation for many native grass species. This is the case for squirreltail (Elymus section Sitanion), despite this group's prevalence and importance in rangeland restoration efforts. I evaluated 32 populations of E. elymoides ssp. brevifolius race C, a phylogenetic subdivision of bottlebrush squirreltail (E. elymoides) centered in the northern Intermountain West, for phenotypic variables and neutral genetic markers to measure their association with geographical origin. Phenotypic traits were measured in common field and greenhouse environments, and genetic diversity was assessed using Amplified Fragment Length Polymorphism. Three factors were extracted from the phenotypic data set using common factor analysis. Factor 1 explained 37.7% of the variation among all of the variables; it had positive factor loadings for phenology (late maturity), biomass, and leaf area index, negative loadings for leaf area and root length, and was negatively correlated with elevation (r = -0.71). Factor 2 explained 14.5% of the variation among all of the variables; it had positive factor loadings for plant height and leaf number per tiller, negative loadings for seed yield and tiller number, and was positively correlated with longitude (r = 0.54) and average annual minimum temperature (r = 0.39). Factor 3 explained 12.8% of the variation among all of the variables; it had highly positive factor loadings for specific root length and specific leaf area, negative loadings for canopy height and mass per tiller. Correlations among phenotypic, environmental, genotypic, and geographic-origin distances were positive (r = 0.723-0.900), which suggests that ecotypic variation is an important feature of this group. This information, in conjunction with previously established Level III ecoregions, was used to delineate four adaptive zones for race C.