Assessing invasiveness of exotic weeds outside their current invasive range.
When exotic species invade a region, it becomes important to assess their invasiveness in adjacent uninvaded regions to determine if weed prevention measures are needed. Leafy spurge and knapweed species are absent from the vast majority of eastern Montana, but the region is surrounded by regions heavily invaded by these species. To assess invasiveness of leafy spurge and Russian and spotted knapweed in common eastern Montana grassland sites, I introduced these species to three sites as seeds (120 live seeds plot-1) and seedlings (6 plot-1). I assessed how common grazing regimes influenced invasiveness by imposing cattle, sheep, mixed grazing (i.e., cattle plus sheep), and grazing exclusion treatments for 7 yr. Invader survival did not appear to differ greatly among sheep, cattle, and mixed grazing treatments, but excluding grazing lowered probabilities that plots maintained invaders for the entire study period at two of three sites. At these same sites, grazing exclusion increased growth rates of those invaders that did survive, at least in the case of leafy spurge. Regardless of grazing treatment or site, however, large proportions of plots did not maintain invaders through the end of the study period. At one heavy clay site, only one small leafy spurge plant persisted through the end of the study. In the seventh study year, the plots with the most leafy spurge and Russian knapweed produced 222 and 112 stems, respectively, and the stems remained mostly confined to the 2- by 2-m plots. These findings suggest that, barring intense disturbance, leafy spurge and spotted and Russian knapweed might be incapable of invading some grasslands of eastern Montana, particularly upland sites with high clay content. Any upland sites in the region these species are capable of invading will likely be invaded only very slowly.