Canada thistle (Cirsium arvense (L.) Scop.) dynamics in young, postfire forests in Yellowstone National Park, Northwestern Wyoming

Canada thistle (Cirsium arvense (L.) Scop.) is an invasive, non-native plant in many terrestrial systems, often dominating plant communities, particularly in agricultural systems. Its invasion into forest systems is not well understood. Our objectives were to investigate the establishment, persistence, and abundance of C. arvense over a 7-year period following wildfire in a forest system, and to understand environmental factors related to variability in plant community composition. In 2006, we resurveyed 30 sites in young lodgepole pine (Pinus contorta Dougl. ex. Loud. var. latifolia Engelm. ex Wats.) stands that burned during 1988 in Yellowstone National Park, and that had been previously sampled in 1999. C. arvense disappeared from six of nine sites where it was present in 1999, persisting in the remaining three sites; it also established at 4 new sites since 1999. Notably, the relative cover of C. arvense and other non-native plant species decreased during the 7-year period. Important ecosystem measures (plant species diversity and richness, aboveground net primary productivity, and leaf area index) were all higher in sites where C. arvense was present, and its abundance was higher on more fertile substrates and at lower elevations. Plant community composition was strongly influenced by gradients of soil texture and bare ground, rather than the presence or absence of non-native plant species. This project demonstrates the dynamic nature of an invasive species 18 years following a major disturbance, and suggests that intensive management of this species in forested systems is probably unnecessary.