Predicting site vulnerability to nonnative plant establishment remains a difficult goal. Seedling survival is an important component of population dynamics and can affect the success of control strategies. Field manipulations allow potential causal mechanisms of site vulnerability to be evaluated under realistic environmental conditions. We conducted field studies to determine the effects of plant competition and differing precipitation regimes on spotted knapweed seedling survival. We also examined the effect of herbivory on rosette survival and growth. Seeds were sown into plots with vegetation intact or removed at three sites. Seeds were also sown into plots where plant competition and precipitation were manipulated in a factorial design at a single site. Field studies demonstrated that site accounted for much of the variation in emergence rate, while herbivory and plant competition affected seedling survival rates. We observed a wide range in emergence rates, with site averages ranging from 13.1 to 42.5%. Survival the following year ranged from 0.5 to 9.4% of sown seeds. Rosette survival was significantly higher when herbivores were excluded from plots. Below average precipitation reduced seedling survival; however, even with supplemental water, dry-down of exposed sites resulted in low seedling survival. Of the 8,000 seeds added to plots in one study, by autumn, only eight plants resulted, seven of which survived in watered plots with intact vegetation. Collectively, these results show that seedling survival is a critical phase in spotted knapweed population dynamics and can vary among habitats on the basis of plant competition and precipitation. Furthermore, herbivory affects all stages of the lifecycle from the seedling onward. The observed differences help explain the reported variability in seedling survival in the literature and inform efforts to control spotted knapweed using plant competition and biological controls.
Nomenclature: Spotted knapweed, Centaurea stoebe L. ssp. micranthos (Gugler) Hayek.
Management Implications: Spotted knapweed is an invasive, perennial rangeland plant in North America. It is mostly unpalatable to livestock and has the potential to produce thousands of seeds yearly from dense monocultures that limit local plant diversity. Various biological control agents have been introduced to control this plant. They reduce plant reproduction both directly through seed predation and indirectly through reducing flower production and plant life span. This study was conducted in a spotted knapweed infestation on the Front Range of Colorado near Boulder, CO. We examined how established vegetation and herbivory affected the fate of the seeds that are produced. We added spotted knapweed seeds to field plots where vegetation and precipitation levels were manipulated. We also monitored plots in established spotted knapweed patches to examine the effect of herbivory on small rosettes. Spotted knapweed seedling survival was lower when plant competition was present but varied substantially among the sites. The effect of plant competition on seedling survival was subject to a moisture gradient; the presence of non-knapweed plants had a positive effect on seedling survival when precipitation levels were high. Herbivory negatively affected both seedling and rosette survival. The presence of established vegetation and insect herbivory decrease spotted knapweed survival, and biological control may be sufficient to control spotted knapweed at sites that have a high insect populations and low seedling emergence and survival.
