Patterns and processes of invasion of the exotic plant Marrubium vulgare (horehound) in a mixed grass prairie

Abstract

Invasive plants pose a global threat by changing natural communities and ecosystems in ways that may be irreversible. Marrubium vulgare L. (horehound), native to Eurasia, is an invasive exotic plant found throughout the United States. Little is known about M. vulgare in the U.S. and this study aimed to understand the basic biology, habitat, and population dynamics of M. vulgare in its invasive habitat as well as the role of disturbance in the invasions. Wind Cave National Park (WCNP), an area comprising ponderosa pine and mixed grass prairie vegetation types, was used as the site for this study. In a GPS and GIS mapping study, M. vulgare was found to be significantly associated with prairie dog towns, an Aristida purpurea - Dyssodia papposa (threeawn and fetid marigold) vegetation community (a vegetation type found exclusively on prairie dog towns), and certain types of loamy soils. M. vulgare was rarely found off of prairie dog towns and then only in other disturbed areas such as bison trails or tree falls. Population studies showed M. vulgare populations were mostly stable or slightly decreasing in density (though not significantly) from 2010 to 2011; however, a few of the populations did increase dramatically in density from 2010 to 2011. M. vulgare populations existed in high densities (78.3-322.9 ramets/m2) and in low densities (0.1-2.2 genets/m2) throughout WCNP. Disturbances including prairie dog burrows, bison trails, prairie dog and bison presence, mowing, and percent cover of bare ground were measured in relation to population dynamics. While M. vulgare populations were disturbance dependent, there was no clear relationship between disturbances and population dynamics. Examination of M. vulgare life history traits found that while M. vulgare had relatively low biomass allocation to reproduction as compared to native perennial prairie species, M. vulgare produced an extremely high number of small seeds (estimated 1487/plant). Seedling establishment rates were also high. While not quantified, M. vulgare appears to be effectively dispersed through bison epizoochory. These results help to inform management of M. vulgare populations: minimizing disturbance, decreasing propagule supply, and minimizing dispersal may help to reduce M. vulgare invasions.