Technical Report
 

Demography and Reproduction of Erigeron decumbens var. decumbens, 1993-1994 Field Studies

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  • Erigeron decumbens var. decumbens, the Willamette Daisy, is endemic to the Willamette Valley of Oregon. This taxon is a Category 1 Candidate for listing under the federal Endangered Species Act (U.S. Fish and Wildlife Service 1990) and is listed as Endangered under Oregon state law. Herbarium records indicate that E. decumbens var. decumbens was once widespread in prairies throughout the Valley. In 1934, however, records ceased and E. decumbens var. decumbens was considered to be extinct until two populations were discovered in 1980, at Willow Creek (Lane County) and Finley National Wildlife Refuge (Benton County; Clark et al. 1993). Several populations have been extirpated within the last decade, and there are currently only 18 known populations of E. decumbens remaining in the Valley. Most populations are small, consisting of fewer than 50 plants, with the largest concentration of sites occurring in wetland prairies west Eugene area. Erigeron decumbens var. decumbens occurs in both wetland and upland prairie habitats. Willamette Valley wetland prairies are characterized by seasonally flooded hydric soils, and are often dominated by Deschampsia cespitosa. The upland prairies occur on well-drained soils and formerly supported a mix of native perennial bunchgrasses, such as Festuca rubra and Elymus glaucus. Before the mid-19th century, both prairie types were probably maintained by seasonal fires set by the native Kalapuya people (Toepel 1991, Boag 1992) to maintain favored forage plants and to aid in hunting. The prairies of the Willamette Valley are considered to be one of the rarest ecosystems in western Oregon, and much E. decumbens habitat has been destroyed by agricultural/urban development and encroachment by woody species and exotic plants. In 1993, we were requested by the Oregon Department of Agriculture to update the status of Erigeron decumbens var. decumbens and to initiate a long-term population monitoring program for this taxon. Clark et al. (1993) and Ingersoll et al. (1993) found major gaps in our knowledge of the biology and ecology of Erigeron decumbens, and identified areas of study needed for its protection and management. In 1993, we established permanent demographic monitoring plots, collected first-year data on population and reproductive characteristics, and established a protocol for future monitoring (Ingersoll et al. 1993). The objectives of this monitoring program are to assess long-term population trends of E. decumbens, particularly patterns of reproduction, recruitment, and mortality. In 1994, we continued demographic monitoring and conducted studies of seed and vegetative regeneration. In this report, we present and discuss the results of two years of demographic monitoring of E. decumbens. Regeneration studies are discussed in a companion report (Clark et al. 1995).
  • Keywords: Population studies, Willamette River Valley (Or.), Native species, Threatened species, Conservation, Demographic analysis, Willamette daisy, Endangered species, Erigeron decumbens var. decumbens
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  • Companion Report: Regeneration biology of Erigeron decumbens var. decumbens, an endangered plant of the Willamette Valley
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  • Prepared for Oregon Department of Agriculture, Plant Conservation Biology Program, 635 Capitol Street NE, Salem, Oregon

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