Abiotic and Biotic Constraints to Blue Oak Restoration in the Sierra Foothills

Jeremy James, California Polytechnic State University San Luis Obispo, Department of Natural Resources Management & Environmental Sciences

Nikolas S. Schweitzer, University of California Sierra Foothills Research and Extension Center Staff Research Associate

Iris Bookholtz, California Polytechnic State University San Luis Obispo Department Student

Restoring oak woodland at scale has proven to be difficult and is becoming increasingly challenging as California experiences more frequent and severe drought.  The broad objective of this study was to examine the relative importance of biotic and abiotic factors limiting blue oak (Quercus douglasii) regeneration from planted acorns and seedlings in the Sierra foothills.  We planted 16 hectares of previously cleared blue oak woodland fall 2020 at approximately 80 planting sites per hectare.  We randomly assigned each planting site to be planted as an acorn or a seedling and receive a combination of treatments aimed at alleviating potential abiotic and abiotic stress.  These treatment combinations included herbicide application or mowing to reduce the density of annual grasses, wire mesh to reduce herbivory, plastic cones to reduce to herbivory and provide a more favorable microenvironment, as well as irrigation to maintain greater soil moisture.   In addition, we examined the potential to reduce annual grass impacts on steep slopes by using timed grazing over large pastures. Preliminary results following the first growing season suggested that acorns had significantly higher survival than transplanted seedling in part due to higher rodent activity around seedlings.  Irrigation had a positive benefit particularly when irrigation was extended to the summer dry season.  We also found evidence that plastic cones provided additional protection over wire mesh, suggesting an abiotic constraint that was potentially stronger than biotic constraints associated with herbivory.  Lastly, scaling annual grass control treatment using grazing was largely unsuccessful due to incidental animal impacts on other treatment and restoration infrastructure.  Collectively this study highlights the abiotic and biotic constraints that occur at scale in efforts to restore blue oak woodland while also highlighting how abiotic conditions (microclimate and soil water availability) can interact with biotic factors (predation by rodents) to greatly limit opportunities for blue recruitment in large scale restoration.