Disturbance is a key factor for providing invasive species a foothold into a new environment. However, not all sites are equally vulnerable to invasion. Seedling germination and establishment studies have been used to examine the success of a wide variety of species in native and non-native environments. Manipulative field studies provide an opportunity to examine invasion under realistic environmental conditions. The purpose of this study was to determine the effect of disturbance and site on spotted knapweed seedling establishment in Colorado. Sixty .1m2 plots were established across three sites that differed in slope, aspect, and vegetative composition. At each site 10 control plots with native vegetation intact and 10 distrubed plots were randomly distributed across the landscape. Plots were seeded at a rate consistent with high seed density observed in the field. The number of knapweed seedlings in each plot was recorded monthly May – October and new germination was marked. An ANOVA was used to determine the effect of aspect, plant competition, and their interaction on spotted knapweed seedling germination and establishment.
Results/Conclusions
Results of the ANOVA show a significant effect of site on seedling germination and first year establishment. However, there was not a significant effect of plant competition or an interaction effect of competition and disturbance for either germination or first year establishment. There was a wide range in germination and establishment, with site averages from 13.1%-32.5% of seeds germinating in the first year. Of the seeds sown in the plots, 0.95 ± 0.33%, 4.35 ± 0.75%, or 12.8 ± 1.6% established in the first year, depending upon site. These results demonstrate that site characteristics play an important role in vulnerability to invasion and may help explain the wide variation in knapweed seedling establishment rates in the literature.