Testing Direct Seeding for an Arid Endemic
As home gardeners know, sowing seeds is much cheaper than buying plant plugs. In 2019, Rare Care started an experiment to test if direct seeding of White Bluffs bladderpod (Physaria douglasii ssp. tuplashensis) could be an efficient method to establish new populations.
Establishing a new population of White Bluffs bladderpod would help meet the Recovery objectives for this threatened species. It is a single-site endemic found only on a narrow, 17-km stretch of the White Bluffs on the east side of the Columbia River. The distinctive 10-15m wide strip of “caliche-like” soils, comprised of calcium-carbonate-rich clay and fine gravel, possesses a unique growing environment that most plants cannot tolerate. Even invasive cheatgrass that blankets disturbed areas across eastern Washington grows more sparsely on this soil.
White Bluffs bladderpod is at risk of extinction due to landslides, invasive species, and climate change. Irrigation runoff and seepage from farming on the plateau above the bluffs has resulted in landslides that destroy this plant’s habitat. Nonnative species growing in the area surrounding the White Bluffs degrade bladderpod habitat and promote fires that could adversely affect the species. Additionally, counts of this short-lived species population fluctuated annually between 2,500 and 58,500 from 2014 to 2020, showing it relies on new recruitment from seeds. Hotter, drier climate conditions could prevent drought-sensitive young seedlings from establishing.
These threats illustrate the inherent risk of occupying only one site. To address this risk, the U.S. Fish and Wildlife Service planted bladderpod plugs to a new site with similar soils north of the current range in order to establish a second population. When plugs planted at the new site survive, they successfully produce new seedlings. However, plugs survive at low rates the year after planting. As the bladderpod produces abundant seeds and is relatively easy to grow in a nursery; seed-increase beds are feasible if we can successfully establish plants by direct seeding.
To test direct seeding as an effective strategy for introductions, we established seeded and unseeded plots at the edge of the native population and at the new site. We then compared the number of seedlings in plots at each site. The introduction site has similarly exposed caliche soils, which the White Bluffs bladderpod occupies at the native site. However, this site is not perfect: it has more invasive species and it faces south, exposing the bladderpod to the sun’s heat throughout the day, unlike the native site which faces west.
At both sites, we tested if raking or landscaping fabric could help prevent the harsh wind at the site from blowing seeds away. We installed plots between 2019 and 2021 and visited them each spring to count seedlings. Our results showed that seeding and raking increased the number of seedlings in the plots. Therefore, seeding to establish bladderpod at a new site should be accompanied by raking. Raking also increased the number of seedlings in unseeded native site plots because the disturbance stimulated bladderpod seeds dormant in the soil seed bank to germinate. Raking at the native site could help augment the native population. Similarly, sown seeds stayed dormant for multiple years at the introduction site, suggesting that seeding to establish bladderpod at new sites could occur every few years (rather than each year).
Locating additional sites that have caliche soils is challenging. Although we only see the bladderpod on this type of soil, it is unknown if these are the only soils in which it can establish. For example, a more common relative, Douglas’s bladderpod (P. d. ssp. douglasii) occurs nearby on non-caliche soils. For our next project, we will investigate whether White Bluffs bladderpod is a soil specialist, or if other factors restrict it to caliche soils.