Post-Fire Recruitment of Western Joshua Tree (Yucca brevifolia) Study

The California Department of Fish and Wildlife is issuing a California Endangered Species Act and Western Joshua Tree Conservation Act scientific, educational, or management permit pursuant to Fish and Game Code sections 2081, subdivision (a), and 1927.2, subdivision (h) (Permit No. 2081(a)-25-017-RP) for a project to investigate practical approaches to post-fire restoration of western Joshua trees in the Mojave Desert, a species limited by low germination and high seedling mortality. A pilot experiment will be implemented on MDLT property to compare differences in germination between two habitats (burned vs. unburned) with two different microclimates (open vs. underneath shrubs). The experiment will build on research indicating the importance of microclimates on the recruitment of this species. A thermal time germination model that was developed from growth chamber data will be used to help determine the best field sowing windows, matching soil conditions to the temperatures needed for germination. The pilot will test whether model-informed sowing combined with microsite selection and water addition can substantially improve germination and early survival in post-fire habitats. Up to 60 plots measuring 50 centimeters by 50 centimeters will be divided evenly between burned and unburned habitats. Forty western Joshua tree seeds will be sowed in each plot. Plots will be assigned to one of three treatments within each habitat (burned vs. unburned): (1) under shrub canopy; (2) open microsites; or (3) open microsites with water addition. A total of 40 plots will be caged to deter herbivory. Sensors will be deployed at a depth of <5 centimeters in a total of six plots to investigate whether microclimates differ in temperature and soil moisture. Plots receiving the water addition treatment will be irrigated with a backpack sprayer up to once a week for up to a 6-month period to simulate natural precipitation events during the early germination window. Soil volumetric water content, available soil moisture, soil temperature, precipitation, and ambient air temperature data will be collected. Plots will be monitored once a week for germination and survivorship. Data collected from the experiment will be used to validate the hydrothermal germination model to predict optimal germination conditions in burned and unburned habitats.