Showy stickseed exploits environments with low competition

Hackelia venusta seedlings
Rare Care
Hackelia venusta hardens off outdoors before outplanting. Photo by Gail Roberts.

Rare Care, along with faculty and graduate students at the University of Washington’s School of Environmental and Forest Sciences, wrapped up a multi-year study on the federally-endangered showy stickseed (Hackelia venusta). The study was funded by the US Fish and Wildlife Service to support recovery efforts for the species by developing a better understanding of its habitat requirements and by improving propagation techniques.

One focus of the study was to characterize the physical and chemical properties of the soils at the site of the population. The soils appear to be quite different than other soils in the area. The species grows on a loose granitic sand on slopes exceeding 30 degrees. Vegetation on these soils is quite sparse, and the surface soils are actively sliding downhill. This raised the question as to whether the soils have a unique chemical composition that facilitates showy stickseed’s survival.

Soil studies were completed by Betsy Vance for her Master’s thesis. She found that the soils had low organic content, nitrogen levels typical of Pacific Northwest forests, and high extractable phosphorus levels. Notably, she determined that nothing in the soil chemistry profile was uniquely different from typical PNW soils that would explain showy stickseeds preference for the site. The high phosphorus levels likely result from the wildfires that have occurred at the site and are also found in other forest soils in Washington.

Hackelia venusta
Rare Care
Photo by Ellen Kuhlmann

She also looked for the presence of arbuscular mycorrhizal fungi (AMF), an association that would allow showy stickseed better access to soil nutrients. DNA was extracted from several root samples and from soil in the immediate vicinity of the plant roots to determine whether showy stickseed was colonized by AMF. The DNA testing results were inconclusive: AMF was present in the roots of a showy stickseed plant at a nearby introduction site but not in the roots of a plant collected at the native population.

The study also evaluated above-ground environment, including the composition of the overstory and understory, and availability of light. We found that showy stickseed occupied sites where only 20 to 37 percent of the ground was covered by herbaceous vegetation, with most of the ground either bare or covered with non-vascular species or woody debris. Its preferred microsites with low cover of shrub, grass, and non-vascular plants, but relatively higher forb cover. In other words, showy stickseed is occupying sites that other forbs also seem to prefer within this dynamic environment. It seems that its low competitive ability is what restricts it to these sites.

From these results, we surmise that the limited distribution of showy stickseed is due primarily to its ability to exploit the coarse, unstable soils at the sites that result in a low competitive environment. Interestingly, many members of the Hackelia family are found on unstable soils and tend to divide their taproots as a strategy to survive in dynamic environments. Because of this, many species of Hackelia are patchily distributed on the landscape. In the case of showy stickseed, it is so rare because the combination of terrain, soil, aspect and vegetation cover that provides it suitable habitat appears to be limited to the immediate vicinity of the existing population.

Rare Care also continued work on improving propagation protocols for showy stickseed. Previous experiments yielded germination rates of 10 to 20%, a rate that is too low to develop a viable strategy for augmenting the population. Under this study, we were able to achieve germination rates of 85%, a vast improvement that allows us to produce adequate material for outplantings. We also evaluated several combinations of growth media and containers and we will be outplanting the plants produced from these experiments in November.

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