Rosette Recruitment of a Rare Endemic Forb (Gaura neomexicanaSubsp.coloradensis) with Canopy Removal of Associated Species

Abstract

AbstractGaura neomexicanasubsp.coloradensisMunz (Onagraceae), (Colorado butterfly plant), a short‐lived perennial forb, became listed as a threatened species by the U.S. Fish and Wildlife Service in October 2000 because of its small global range and population size. This endemic subspecies consists of only 18 extant populations within southeastern Wyoming, southwestern Nebraska, and northeastern Colorado. Wyoming occurrences are on private land with the exception of three occurrences on F. E. Warren Air Force Base, near Cheyenne.Gaura neomexicanasubsp.coloradensismay be displaced by many competitors, includingCirsium arvense(L.) Scop. (Canada thistle), an invasive, noxious weed. In June 1998, threeG. neomexicanasubsp.coloradensispopulations were examined for rosette establishment in the presence of and after removal of associated species. The four study treatments removed: (1)Cirsium arvense, (2) grass and forb canopy and associated litter, (3) grass and forb canopy, litter, andC. arvense, and (4) no removal of associated plant species (control). MatureG. neomexicanasubsp.coloradensisplants were evaluated for height, number of leaves, and capsule production. The mature plant characteristics monitored in 1998 were not good indicators of subsequentG. neomexicanasubsp.coloradensisrosette densities in the following spring. Recruitment was assessed by counting new rosettes in the fall 1999. Although bothG. neomexicanasubsp.coloradensisand the exoticC. arvensehave comparable habitat, removal ofC. arvensedid not increaseG. neomexicanasubsp. coloradensis vegetative growth, seed capsule production, or rosette density. Removal of other forbs, grass, and litter, however, increasedG. neomexicanasubsp.coloradensisrosette density in the following two years, indicating that canopy removal of associated species can enhance rosette establishment of this rare native. The accumulation of dense vegetative cover and litter associated with the absence of herbivory and fire may contribute to the decline of rare species in rich riparian habitats. Return of herbivory and fire in mesic sites to reduce standing biomass accumulations should be considered in restoring recruitment potential to rare monocarpic species.