Ecology and invasive properties of musk thistle (\u3ci\u3eCarduus nutans\u3c/i\u3e) in the Central Prairies of Nebraska

Musk thistle (Carduus nutans) is an herbaceous monocarpic herb introduced to the U. S. from Eurasia. The invasion of musk thistle can reduce forage area, soil stability, and reduce recreation and open areas for humans and wildlife. Resistance of warm season and cool season perennial grass communities to musk thistle invasion is important for land managers to consider, especially where disturbance has made an area particularly susceptible. Our results show that disturbances, such as overgrazing can open up niches in canopies of warm season grass communities and facilitate invasion but not in cool season grass communities. The mechanism of invasion by musk thistle may depend on an overlap in the timing of resource use patterns by the invader and perennial grass communities. Our results showed that an undisturbed (e.g., non-grazed, normal precipitation) stand of warm season perennial grasses can suppress the establishment of musk thistle by restricting the amount of light that reaches the soil surface. A disturbance of extreme drought creates more niches in warm season perennial grass communities (e.g., reduced growth), but newly germinating musk thistle plants cannot compete for the reduced amount of soil moisture. High disturbance (e. g., grazing) allowed extensive amounts of light to penetrate into overgrazed warm season perennial grass communities, which facilitated the successful invasion by musk thistle. In cool season perennial grass communities, light is less critical, regardless of soil moisture. The unsuccessful invasion of musk thistle into cool season perennial grass communities is most likely due to grass root phenology and distribution (88 m m-2). Root growth and development and distribution patterns should be taken into account for effective perennial grass restoration in areas with high risk from invasive plant species. The ecology of musk thistle seed suggests germination is strongly influenced by temperature, light, moisture, salinity, dormancy and habitat types and should be factored into an integrated invasive plant management plan that targets early growth of musk thistle seedlings. Advisor: Stephen L. Youn