Site selection and soil suitability for golden paintbrush restoration on Puget Lowland prairies

Site selection for reintroduction of rare plants is based on a combination of practical, physical, and biological parameters. This study helps to identify soil requirements for the reintroduction of the federally threatened prairie plant, Castilleja levisecta. In 2002, baseline soil data were gathered from nine study sites: three extant C. levisecta populations and six potential reintroduction sites. That March, two experimental reintroduction sites were planted with C. levisecta. Weed competition, survivorship, and vigor of C. levisecta were tracked over the next two years. Results indicate that C. levisecta can tolerate low available nitrogen (N), phosphorous (P), and moisture. In two of the extant sites, low total N (0.04 and 0.08%), high C:N (54.08 and 34.29), form of inorganic N, low organic matter, and coarse soil texture limited plant-available N. All extant sites had low to moderate P availability (8.22, 13.60, and 8.06 mg/g). Soil moisture retention and plant availability were moderate at two sites and low at the third. Compared with extant populations, experimental site A had significantly lower pH and extremely low moisture holding capacity, while soils at experimental site B were more similar to extant populations. Despite over 24 times greater mean weed biomass at site B, survival of C. levisecta was far higher than at site A. At site B, survival dropped from 92% after 2.5 months to 83% after one year and 31% after two years compared to 59%, 6%, and 0% survival over the same time period at site A. Differences in soil characteristics appear to heavily influence C. levisecta survival and vigor. It is recommended that moisture-related soil characteristics be carefully evaluated when selecting reintroduction sites for C. levisecta.