Demographic analysis of Erigeron decumbens var. decumbens, an endangered plant species of the Willamette Valley, Oregon

Erigeron decumbens var. decumbens, known only from the Willamette Valley of western Oregon, is listed as Endangered by the U.S. Fish and Wildlife Service and by the state of Oregon.  In 1993 a long-term population monitoring program for Erigeron decumbens var. decumbens was initiated at Baskett Slough National Wildlife Refuge (Polk County).  The objectives were to assess long-term population trends, particularly patterns of reproduction, recruitment, and mortality.

This report presents the results of the demographic monitoring from 1993-1999.  Considered together the following demographic parameters indicate that the measured population at Baskett Butte (Baskett Slough National Wildlife Refuge) is declining.

  1. The population size decreased every year but one with the 1999 population only 48% of the 1993 population.
  2. Increasing mortality rates exceeded recruitment rates every year but one.
  3. The percentage of the population that flowered generally decreased every year.
  4. The average number of heads per flowering plant decreased each year beginning in 1995 to 3.4 heads per flowering plant in 1999.
  5. The pattern of average basal area per plant varied but the 1999 average basal area is only 63% of that measured in 1993.


To stabilize the population, adaptive management strategies need to be developed in conjunction with experimental approaches that will determine the factors responsible for the population decline.  A second approach to aid in the recovery of Erigeron decumbens var. decumbens is to increase the population by transplanting cuttings or plants grown from seed under greenhouse conditions or by sowing seeds in the field under more controlled conditions.  It is strongly recommended that future efforts to stabilize the population at Baskett Slough National Wildlife Refuge be coordinated with other agencies in the Valley who are also responsible for management of Erigeron decumbens var. decumbens populations.