Habitat, dispersal, and distribution of the rare Orange-fruit Horsegentian (<i>Triosteum aurantiacum</i> E.P. Bicknell; Caprifoliaceae) in northern Nova Scotia, Canada

Why some plant species are rare and how rare species persist are foundational questions in community ecology. In 2015 we repeated a 2006 survey of three river valleys in rural Antigonish County, northern Nova Scotia, Canada, which support populations of the rare herb Orange-fruit Horse-gentian (Triosteum aurantiacum E.P. Bicknell) to see how the populations had changed over a decade and to learn more about why the plant remains rare. Our survey confirms previous observations that Orange-fruit Horse-gentian is largely restricted to the understorey of hardwood and mixedwood stands, on bare ground within and near river floodplains, often with White Ash (Fraxinus americana L.). Predictive maps based on geographic information system modelling led to the discovery of new occurrences of the species along the three original rivers and along a fourth river, including a dense cluster in mature hardwood forest, which had not previously been considered habitat. Measurements of photosynthetic capacity using pulse-amplitude modulation (PAM) fluorometry showed significant stress on horse-gentian plants growing in full sunlight or light shade compared with plants beneath closed canopy confirming this plant is shade-adapted. Late-autumn observations of potential consumers of horse-gentian fruit suggest that White-tailed Deer (Odocoileus virginianus) may be the primary long-range disperser of seeds. Hence, this species may remain rare in northern Nova Scotia because its optimal habitat (mature, closed-canopy forest with open understorey and calcium-rich soil) is rare and distributed in disjunct patches (mostly along floodplains) and seed dispersal is limited by the range size of the deer.