Invasive Species Removal Promotes Habitat Restoration but Does Not Immediately Improve the Condition of a Threatened Plant Subspecies

Abstract

Impacts of invasive species on rare species are relevant to conservation. We studied the response of Leedy's roseroot Rhodiola integrifolia subsp. leedyi, a subspecies listed as threatened under the Endangered Species Act, to removal of the invasive species Japanese knotweed Fallopia japonica. Japanese knotweed has invaded the largest known population of Leedy's roseroot, affecting about 10% of all Leedy's roseroot in the world. Japanese knotweed shaded Leedy's roseroot, but the two did not share belowground resources because of their position on cliffs. To study this interaction and, ultimately, to restore Leedy's roseroot habitat to an open, high-light condition, we removed Japanese knotweed in a three-treatment block design. We measured Leedy's roseroot abundance, growth, and reproduction in treatment blocks and in uninvaded areas before and after treatment. Compared with uninvaded areas, Japanese knotweed invasion negatively affected Leedy's roseroot abundance, growth, and reproduction. Light interception by Japanese knotweed degraded the habitat for Leedy's roseroot. Herbicide removal of Japanese knotweed resulted in increased light and temperature compared with untreated invaded plots but did not affect Leedy's roseroot abundance, growth, or reproduction over the 2 y of our study. These results show that invasive species removal is conducive to restoring Leedy's roseroot habitat, but recovery in the subspecies may lag behind restoration of the habitat, suggesting that additional action or time may be required to restore preinvasion performance of Leedy's roseroot. Results of this study may inform restoration efforts for other systems and contribute to the literature on interspecific interactions.