Models of Future Suitable Habitat for Joshua Trees (Yucca brevifolia, Yucca jaegeriana) in the Mojave and Sonoran Deserts Based on High Resolution Distribution Data

Joshua trees (Yucca brevifolia, Y. jaegeriana) are iconic, foundational species of the Mojave and Sonoran Deserts in North America. Due to their ecosystem importance, long generation times, and low resilience to disturbance, these hybridizing sister species are increasingly the focus of conservation efforts. Predicting Joshua tree responses to future climate, along with the extent of suitable future habitat and/or climate change refugia, is critical to ongoing management planning.
 
We used a recent high-resolution, field-validated distributional database of nearly complete presence and absence records (Esque et al. 2023), along with a life-history based model of dispersal capacity (Engler et al. 2012), to project Joshua tree distributions into future time periods and climate scenarios based on species distribution models (SDMs). SDMs predict habitat probabilities (ranging from 0 to 1) for a species of interest by modeling association between known occurrences and environmental covariates and are a primary means of predicting species-level responses to future climate or land management scenarios. In this approach, SDMs are fit to current day or ‘baseline period’ climate and subsequently projected into the same or broader areas using climate variables defined in future time periods and / or emissions scenarios, such as those from the Coupled Model Intercomparison Project phase 6 (CMIP6) representative concentration pathways corresponding to the 6th Intergovernment Panel on Climate Change (IPCC) Assessment Report (IPCC 2021). From the habitat probabilities resulting from future-projected SDMs, reflecting all potential suitable future habitat, climate change refugia can be further defined as areas of predicted future suitable habitat that intersect with currently occupied habitat ('in situ refugia' assuming no dispersal to new habitats; sensu Ashcroft 2010) or areas of suitable future habitat that are likely to be accessible through ordinary means of dispersal (ex situ refugia; sensu Ashcroft 2010). The latter requires pairing SDMs with a model of potential dispersal incorporating aspects of the species biology and life history; for plants, this may include generation times and propagule dispersal mechanism (Engler et al. 2012; Bateman et al. 2013). The combination of in situ and ex situ refugia may be considered the total accessible refugia for a species. 
 
This project develops new SDM predictions for Joshua tree’s future habitat suitability and in situ vs. ex situ climate change refugia based on high resolution range mapping, updated future climate assessments, and modeled dispersal capacity.
 
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