The ecology and distribution patterns of a rare serpentine endemic, Packera layneae
Many rare and threatened California plant species are endemic to ultramafic serpentine soils. Understanding what specifically makes these species rare is of high value to conservation and management efforts statewide. While there is no one definition of what makes a plant rare, there are many characteristics that are thought to contribute to a species’ rarity. My study focuses on environmental and reproductive attributes of a rare serpentine endemic species to evaluate their contribution to the plant’s rarity. Packera layneae, or Layne’s Butterweed, is a federally threatened and state listed rare species occurring in small, fragmented populations on gabbro and serpentine soils in the northern California Sierra Nevada. I evaluated environmental factors that potentially define its restrictive habitat, including elevation, light availability, duff depth, and slope. I found the studied populations to occur between 2140 and 2920 feet in elevation, and to have established in a variety of habitats where the evaluated environmental factors were moderate, including presence beneath intermediate canopy cover (40-60% of available light), widely variable duff depth (0.25 to nearly 6cm), and generally lower slopes (5 to 16%). I also conducted an experiment to evaluate the role of dispersal limitation as a contributing factor to this species’ limited distribution, and found that this species disperses the majority of its seeds very close to the parent plant. I also conducted an experiment to determine the specific germination requirements for P. layneae, and found that its seeds will germinate in a variety of soil types, but only following a period of light exclusion. Using the modeling program Maxent, a species distribution model was constructed using presence locations in conjunction with environmental variables in an attempt to yield a predictive map of potential habitat occupied by this species. Unfortunately, the very small number of known populations available for input into the model, in combination with high environmental variability across presence locations, resulted in a model with reduced predictive function. Determining why this species is so patchily distributed and understanding its unique habitat and physiological requirements will be vital to its conservation and long-term success. This work will also contribute to our understanding of plant rarity and help guide management efforts.