The population dynamics of tansy ragwort (Senecio jacobaea) in Northwestern Montana

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Montana State University - Bozeman, College of Agriculture


Following the Little Wolf wildfire of 1994 in northwestern Montana, tansy ragwort (Senecio jacobaea) was classified as a state noxious weed. This project aimed to help prioritize the management of populations of tansy ragwort through an understanding of the biotic and abiotic factors influencing the species. Using field collected data and by constructing a prediction model, we found that the most important variables to predict the presence of tansy ragwort were distance from the main road, slope, cosine of aspect, and several remotely sensed LANDSAT ETM+ bands. Most of the predicted occurrences of tansy ragwort were within the boundary of the wildfire. The life stage and density of tansy ragwort plants in 94 plots in several environments created by the wildfire were used to project the population growth rate after seven to eleven years following the wildfire. Using the relative invasiveness (probability ë > 1.0) of populations to prioritize environments for management, the burned and salvaged logged population had the highest probability of being invasive with probability ë > 1.0 between 0.31 and 0.24. The burned but not logged had a probability ë > 1.0 between 0.20 to 0.10 and the non-burned meadow had a probability ë > 1.0 between 0.01 and 0.00. Additionally, as the native plant populations recovered, the population growth rate and rosette survival declined. Following a field study of the role of slashpiles in the establishment of tansy ragwort we found the highest maximum percent emergence of tansy ragwort seedlings in the low severity burn and non-burned disturbed environment of the slashpiles. The low survival of emerged seedlings and the low projected growth rates for the surviving plants indicated that the dynamics responsible for a large increase in tansy ragwort density did not occur within our slashpiles after two years. With the addition of biological control agents to populations of tansy ragwort, the seed predator fly did not negatively affect the relative invasiveness of tansy ragwort. However, the addition of a foliage herbivore to a limited number of populations reduced the relative invasiveness of these populations.




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