April 1, 2015

Abronia umbellata var. breviflora on the Oregon coast: Reintroduction and population monitoring

Denise E.L. Giles-Johnson and Thomas N. Kaye | 2014

Pink sand-verbena (Abronia umbellata var. breviflora) is listed by the Oregon Department of Agriculture as endangered, and a Species of Concern by the U.S. Fish and Wildlife Service. Since the

Pink sand-verbena (Abronia umbellata var. breviflora) is listed by the Oregon Department of Agriculture as endangered, and a Species of Concern by the U.S. Fish and Wildlife Service. Since the late 1970’s, the number of natural populations in Oregon has dwindled from around ten down to about five. The primary threats to the species include competition from European beachgrass (Ammophila arenaria) and habitat disturbance by off road vehicles.
This report summarizes pink sand-verbena research along the Oregon coast since 1997, including seeding and transplantation experiments and population monitoring at several beach and dune habitats. In 2012-2014 plant measurements were taken on randomly selected plants at select Forest Service and BLM sites. Substrate samples were collected at each site monitored in 2012.

In 2014, our actions and observations included:

  •  1. USDA Forest Service, Siuslaw National Forest sites
    • a. Tahkenitch: 40,00 seeds were distributed at Tahkenitch in 2014, resulting in a total of 110 plants (43 reproductive). This was the fourth seeding at the site since 2005. In 2014 seeding was focused in the southern 1/3 of the site as this was the area that had been most successful in previous years.
    • b. Siltcoos Creek: 10,000 seeds were distributed in the portion of habitat still present at the site and 10 plants (8 reproductive) were found in the fall of 2014. Although a large portion of the occupied habitat was washed away in winter storms of 2011/12, the area still hosts a relatively high density of plants per unit area and should continue to receive seeds in the remaining suitable habitat. Continued seeding efforts to establish a seedbank at the site will be crucial for the maintenance of this population.
    • c. Overlook: Despite significant decreases in this population in 2014, this population is now the second largest in Oregon and we strongly recommend continued beachgrass control and seed addition. In 2013 and 2014, both North and South Overlook had some of the lowest numbers of individuals since augmentation (introduction) began at the site.
      • i. North: 60,000 seeds were distributed, resulting in 281 plants (149 reproductive)
      • ii. South: 60,000 seeds were distributed resulting in 109 plants (66 reproductive).
  • 2. USDI Bureau of Land Management, Coos Bay District sites
    • a. New River: In 2014, 100,000 seeds were distributed by BLM staff, resulting in 366 plants (160 reproductive). This represents a significant increase from previousyears. As in 2012 and 2013, most plants were found in areas where breaks in the foredune had allowed overwash to occur– highlighting the affinity of pink sand-verbena for areas with repeated disturbance (and possibly nutrient input). Continued seeding at the site should augment the existing seed bank; management in the form of continued European beachgrass control will be vital in maintaining suitable habitat for this and other dune species.
    • b. Coos Bay North Spit: 100,000 seeds were distributed by BLM staff. Monitoring of the area was divided into three sub-sections and the entire population was subsampled. In 2013, there was only an estimated 74,805 plants down from more than 300,000 in 2012. This is in large part due to decreases in the density of plants in the southern portion of the habitat that did not receive beachgrass removal treatment. In 2014, after beachgrass removal resumed, the population increased, with over 400,000 estimated plants. This highlights the precarious position of this species which is reliant on continued management actions to maintain suitable habitat.

We observed that 2014 was generally a poor year for Abronia umbellata ssp. breviflora across the coast: many sites monitored had fewer plants than previous years, (and in most cases were among the lowest ever observed), plant measurements of reproductive individuals also generally show smaller plants with fewer flowers/infructescenses. Only Tahkenitch, New River and the Coos Bay North Spit showed increases in plants from 2013.
As in previous years, competition from European beachgrass continues to be a major factor in the success of pink sand-verbena populations. This was particularly evident at Overlook, Tahkenitch, and the Coos Bay North Spit where in 2013 the boundary between the disced and un-disced area also clearly marked a significant decrease in the number and vigor of flowering individuals.
The population of pink sand verbena on the Coos Bay North Spit- is the largest known population in Oregon and currently the only large enough to provide significant quantities of seed for restoration of the species. Continued management of the population is key to the success of this disturbance loving species. Additionally, in recent years, there appear to be unidentified factors (potentially including soil chemistry or microclimate variations) that are contributing to yearly fluctuations in population size.
Natural disturbance at the site has resulted in the erosion of more than 80% of the area occupied by ABUM in the 2010-11 growing season. This process illustrates the transitory nature of the species’ preferred habitat which includes dunes particularly along the ever shifting and (sediment) nutrient dense mouths of Northwest streams. Despite the significant loss of habitat several plants were present in 2014 and continued seeding is recommended for the site to maintain a seedbank as the habitat re-establishes.