Ventenata [Ventenata dubia (Leers) Coss.], an invasive winter annual grass, negatively impacts grassland community composition and function in the Pacific Northwest. Ventenata dubia established in Palouse prairie (PP) and canyon grasslands (CG) of northern Idaho/eastern Washington in the mid-1980s to early 1990s. Understanding and comparing patterns of invasion can elucidate future trends as its range expands. We performed surveys in PP (2012 and 2013) and CG (2018) to assess V. dubia abundance. Specifically, we correlated species richness, Shannon diversity, rank abundance, and indicator species with no, low (<12.5%), and high (>12.5%) V. dubia cover. We used nonmetric multidimensional scaling analysis (NMDS) to visualize species similarities and associations with abiotic variables. In both ecoregions, V. dubia was very common, appearing in nearly 60% of 450 plots. When present, V. dubia cover averaged 26% (±2.3 SE) in PP and 19% (±1.8 SE) in CG. Indigenous plant species richness and diversity were lowest in plots with high V. dubia cover. In CG, this relationship held for nonindigenous species; in PP, nonindigenous plant richness and diversity were higher with high V. dubia cover. Ventenata dubia and other winter annual grasses (Bromus spp., medusahead [Taeniatherum caput-medusae (L.) Nevski]) were moderately associated according to the NMDS analysis. Indicator species analysis showed V. dubia was positively associated with nonindigenous winter annual grasses and negatively associated with indigenous low shrub species. Abiotic factors that explained V. dubia abundance included shallow soils and a south to west aspect. Overall, these findings indicate V. dubia can successfully invade both dry and relatively wet plant communities and is more abundant than other invasive annual grasses. We suggest these findings foreshadow what will happen in sagebrush steppe and Great Plains grasslands, regions where V. dubia recently became established.

Predictions of root architecture have been tested using only a narrow set of environmental conditions and species. There is little information on root architecture of plants growing on semi-arid grasslands where soil nutrient heterogeneity and defoliation might impose particular restrictions on root growth. We conducted a split-root study to evaluate the effect of nonuniform nutrient supply and clipping on root architecture and biomass allocation of three range grasses found in the Intermountain West of the USA: ‘Whitmar’, a cultivar of Pseudoroegneria spicata, Bromus tectorum (cheatgrass), and ‘Hycrest’, a hybrid between Agropyron desertorum and A. cristatum. Root systems of individual seedlings were equally divided between the two divisions of a partitioned 10-l pot, and each of the divisions was independently supplied with ‘high or ‘low’ nutrient solution. Plants were either clipped or not. Characteristics of root architecture (topology and link length), root size (root length and number of root tips), and biomass distribution (specific root length and root mass[ratio ]root volume ratio) were evaluated. Root topology (branching distribution) changed with nutrient treatments and defoliation in all three species. Length of links (a segment of root between two branches, or between a node and a root tip) responded to nutrient treatment only in defoliated plants of ‘Whitmar’, in which relative growth rate was associated with changes in architectural traits (external–external and internal–internal link length), while biomass accumulation and relative growth rate in cheatgrass and ‘Hycrest’ maintained allometric relationships with root length. Our results suggest that large plasticity in root architecture, together with inherently low relative growth rate are adaptations that allow ‘Whitmar’ to grow well in heterogeneous and poor soil-nutrient conditions. Cheatgrass and ‘Hycrest’ exhibited characteristics suggesting capability for rapid exploitation of nutrient patches.