The Joshua tree (Yucca brevifolia, Yucca jaegeriana) is a large, evergreen monocot distributed patchily across the southwestern United States. The plant occurs on alluvial fans, plains, and bajadas primarily in the Mojave Desert, but populations are also found in the Great Basin and Sonoran Deserts. Named by the Mormon pioneers for the branching, supplicating arms reaching towards the sky, the species has become an emblematic symbol of the Mojave desert for residents and visitors alike. Joshua trees inhabit cooler, moister microclimates within the larger desert macroclimate, and have been identified as being potentially vulnerable to future climatic regimes characterized by warmer and drier conditions. The species was recently petitioned for listing under the US Endangered Species Act. In response, the US Fish and Wildlife Service conducted a comprehensive assessment examining the current population status, threats, future viability, and resilience of the species to environmental change. In order to better understand species response to projected future climate, we used an ecological niche modeling program (MaxEnt) to examine future distribution under different climate scenarios. We incorporated previously unrecorded occurrences, locally downscaled climate data, and newly derived climate predictors designed to mimic ecological processes, which differentiated our analysis from previous modeling efforts.
Results/Conclusions
In addition to ongoing and projected future climate change, the assessment identified changing wildfire regimes and habitat loss as the greatest threats. Evaluation of these stressors based upon identified risk scenarios indicated that future viability for a small percentage of populations (less than 10%), and a slightly larger percentage of populations (less than 30%), may be threatened by wildfires and habitat loss, respectively. We utilized climate data from ClimateNA V5.10 for MaxEnt models, as this climate data set has been recommended for species inhabiting heterogeneous, mountainous areas such as the Joshua tree, which occurs at elevations ranging from 600 – 2200 meters. Our results indicated that the species climatic niche was governed by predictors related to precipitation in the cool season and monsoonal months (July-August), and minimum winter temperature. The geographical location of suitable habitat projected for the future shifted across the landscape under different climate scenarios. Areas of future suitable climatic niche were evaluated in relation to land ownership and identified as potential refugia or assisted migration sites, which can inform conservation and management actions.