Assessing the potential for assisted colonization in four serpentine endemic plants: the importance of geology, soils and biotic interactions

Background/Question/Methods

Plant species on serpentine soils contribute greatly to biodiversity and presumably have extremely limited abilities to survive climatic warming through latitudinal or elevational dispersal. Thus, they may be high-priority candidates for assisted colonization, a strategy for mitigating species loss by moving species to higher latitudes or elevations. Topography can create large differences in microclimate over short distances. Topography thus offers the potential for species to survive climate change through short-distance movements within rugged landscapes. We ask, is it reasonable to expect cooler microhabitats (north facing slopes) to act as refugia for species threatened by climate change, or will differences in geology, soil properties, or biotic interactions make these microhabitats unsuitable for assisted colonization? Using the rugged terrain of the Oregon Siskiyou Mountains we relocated four serpentine endemics with relatively restricted ranges that differ in plant functional traits (Arabis aculeolata, Horkelia sericata, Senicio hesperius, and Phacelia corymbosa) to 11 locations with paired north and south facing slopes along an elevation gradient. We characterized the bedrock of each site using thin-section microscopy and X-ray diffraction, described soil morphology at each site, and analyzed soil nutrient status. Lastly, we conducted litter and neighbor removals to assess the importance of biotic interactions.

Results/Conclusions

We found that the bedrock at six of our sites is dominated by partially serpentinized peridotite and five sites are dominated by massive or scaley serpentinite. Physical and chemical properties of soils (such as water content, organic matter, pH, P, and Ca:Mg) differed with bedrock, among sites and among slope positions within a site. Germination of our four target species was influenced by different factors. Horkelia germination was lowest of all species and was strongly influenced only by biotic interactions. Germination of Senecio was highest of all species, was higher on partially serpentinized peridotite and north facing slopes, and was influenced by elevation, light, and soil pH and Ca:Mg. Phacelia germination was higher on north facing slopes and was strongly influenced by biotic interactions, and soil phosphorus content, pH and Ca:Mg. Arabis germination was highly variable. The differences we found in the underlying lithology, soil properties and biotic interactions indicate important patterns for plant-soil interactions and for assisted colonization. Our results suggest that multiple factors will influence the success of assisted colonization and that before undertaking any such relocation there is a clear need for nuanced, empirically-informed assessments of the risks and benefits of assisted colonization for plants.