Diminished Seed Banks of Two Endemic Scrub Herbs in Roadside Habitat Belie Their Population Booms: Are Robust Roadside Populations a Conservation Mirage?

Abstract

Persistent soil seed banks are a mechanism by which plant populations, especially those of pioneer species, replenish themselves after disturbance. Germination, and therefore aboveground replenishment, is linked to the abundance and vertical distribution of viable seeds in the soil, but it is unclear how different types of disturbance influence seed-bank structure. This distinction is critical for conservation, as anthropogenic sources increasingly alter the types of disturbance that ecosystems experience. We characterized the vertical structure of the seed banks of Paronychia chartacea (Papery Whitlow-wort) and Hypericum cumulicola (Highlands Scrub St. John's Wort), 2 federally endangered herbs endemic to Florida scrub, in habitats that experience fire (scrub habitat) and vehicle disturbance (roadsides). Within populations of each species in each habitat, we collected soil from 4 soil depths. We used microscopy and a germination study to measure total and viable seed density, then related these values to habitat, soil depth, and aboveground vegetation density. The vertical structure of viable seeds in the seed bank did not significantly differ between the 2 habitats. The top 4 centimeters of soil contained the majority of viable seeds for both species in both habitats, and viable seeds were denser in scrub habitat compared to roadsides. Roadsides contained relatively depauperate seed banks compared to natural, interior scrub. Abundance of viable seeds did not relate to aboveground vegetation density, possibly reflecting higher germination or recruitment on roadsides. This finding calls into question the long-term stability of apparently healthy roadside populations of endemic herbs, and necessitates further research into how seed banks influence the dynamics of these vulnerable species.