Centaurea stoebe is an emerging invader in northeast US, and is a major invasive plant in the northern Midwest and western USA. Although it’s been present in New York State for over 100 years, its apparent recent population increases and spread provide a rare opportunity to study this transition to invasiveness in the early stages of invasion. Most of the population of spotted knapweed in the Northeast occur along roadsides from which it spreads to other parts of the environment. In this study, I used Integral projection models (IPMs), to evaluate the effect of roadside disturbance and establishment density on the population dynamics of spotted knapweed in Northeast US. In 2014, I established a field experiment to understand how these factors interact to influence spotted knapweed population growth in the Northeast US. The experiment had 2 disturbance treatments (present and absent), 2 density treatments (low and high), and used seeds from 8 plant families collected from the Long Island area. Using plant size as a state variable, I constructed IPMs for a combination of experimental treatments and plant families and examined the response of lambda and vital rates to these experimental conditions.
Results/Conclusions
In general, the relationship between the plant size and the population growth rate was dependent on the disturbance treatment, with plants disturbed plots having a higher population growth rate that those in undisturbed plots. There was no significant difference in the population growth rate between high and low density after 3 years of establishing the experiment. There was also no significant difference between plant families for the treatments. When combined, the highest growth rates were obtained from the combination of disturbance present/low density and disturbance present/high density. Elasticity analysis suggested that for all of the models, the survival and growth of the smallest plants in the population contributed the greatest amount to the population growth rate.
These results suggest that the populations of spotted knapweed in the Northeast US are likely to grow and spread faster in areas where they are subjected to disturbance. The minimal influence of density at this stage of the experiment may be due to the size of the plant during the experimental period. It is likely that with time, as the plants grow, intraspecific density will have a greater influence on population growth rate.