Competition for soil nitrate and invasive weed resistance of three shrub-steppe growth forms

Determining mechanisms responsible for weed resistance and invasion success are two issues that have potential in aiding successful land management decisions. The first experiment evaluates the competitive effects of an invasive annual grass downy brome (Bromus tectorum L.), an invasive biennial forb dyer's woad (Isatis tinctoria L.), and a reclamation shrub prostrate kochia (Kochia prostrata [L.] Schrad.) on nitrate acquisition, shoot and root growth, leaf carbon: nitrogen ratio, and leaf N of a perennial grass crested wheatgrass (Agropyron cristatum [L.] Gaertner x A. desertorum [Fisch. ex Link] Schultes), a native forb western yarrow (Achillea lanulosa Nutt.), and a native shrub big sagebrush (Artemisia tridentata var.wyomingensis [Beetle & A. Young] Welsh). In addition the growth traits of the six species were compared to understand the differences in nitrate acquisition and nitrogen allocation. The grasses acquired more nitrate than neighbors of different growth forms, western yarrow was equally competitive with the invasive forb and prostrate kochia, and all neighbors acquired more nitrate than big sagebrush. The invasive weeds and prostrate kochia had lower leaf C: N ratio (P < 0.01), and greater leaf N concentration (P < 0.01), which is correlated with leaf longevity and nutrient use efficiency, indicating differing strategies to persist in semiarid shrub-steppe ecosystems.; A second experiment evaluated the invasion of downy brome and dyer's woad into single- and four-species plots, and a three-growth form plot composed of perennial grasses, perennial broad-leafed forbs, and shrubs. Species density was altered in single species plots to determine if disturbance increases invasion into each growth form. Seeds of both invasive species were introduced in autumn of 2004 and 2005, and seedling density was quantified in the following two summers. Downy brome invasion was lowest in plots that contained grass species (P < 0.01). Invasion of dyer's woad was consistently lowest in the single-species shrub and three-growth form mix plots (P < 0.01). Both species had relatively low invasion into the three-growth form mix. Disturbance increased seedling density of both invasive species. This study provides some clues to better inform potential restoration efforts in the Great Basin and sagebrush-steppe ecosystems.