Spotted knapweed growth, reproduction, and seed banks: A synthesis of the constraints imposed by resource limitation, competition, and biological control

Background/Question/Methods

Spotted knapweed (Centaurea stoebe) occurs in over three million ha in rangeland across North America. We developed graphical models to conceptualize the invasion process and to identify the conditions required to explain the current, contradictory findings on biological control, and then employed a series of manipulations to evaluate Centaurea responses to herbivores in a series of greenhouse and field experiments.  We monitored attack rates of seed and root- feeding weevils and plant responses to these insects for eight years. In several multi-year experiments, we tested how variation in resource availability, competition, landscape position, soil disturbances, and the intensity of herbivore damage influenced growth and physiology of spotted knapweed at various life stages.

Results/Conclusions

Centaurea exhibited large biomass responses to N addition, but the presence of grasses suppressed the ability to exploit this N. Invasion was greatly diminished when realistic levels of plant competition and biological control limited seed production. Root damage by weevils decreased flower production across all resource environments. Plant competition reduced adult growth, and concurrent herbivory by root and seed weevils caused additive negative effects to fitness.  In the field, N limitation and competing vegetation greatly reduced C. stoebe growth. Seed weevils most intensively reduced seed production in low N soils. Root weevils reduced flower production and aboveground biomass across all levels of soil N and competition. In a separate experiment, the seed weevil exerted larger negative effects in terms of plant physiology and potential reproductive output than the root-feeder.

Our results support the use of biological control as a successful method to reduce C. stoebe infestations and their associated impacts. Reductions in growth and seed production due to multiple biological control insects, in the presence of limited soil resources as imposed by competitive vegetation, are predicted to restrain expansion of the spotted knapweed populations and reduce ecological and economic impacts of the weed in currently infested areas.