Threats to the western Joshua tree
A number of threats point towards a reduction in range for this iconic species.
Rising temperatures
A 2012 study estimated that a temperature increase of 3°C – with no change in precipitation – would reduce suitable habitat for Joshua trees by 90% (Barrows and Murphy-Mariscal). This temperature increase corresponds with a projected high emissions scenario that is well above current levels.
Even under a best case scenario in which emissions are reduced to zero by 2080, the mean annual temperature within Joshua Tree National Park is projected to increase by 1.5°C (Gonzalez, 2019).
A temperature increase of only 1°C is estimated to reduce suitable Joshua tree habitat by one-third (Barrows and Murphy-Mariscal, 2012).
Emissions
A 2019 study looked at several possible outcomes for Joshua tree habitat. In the best-case scenario, major efforts to reduce heat-trapping gases in the atmosphere would save 19% of the tree habitat towards the end of the century. In the worst case, with no reduction in carbon emissions, the park would retain a mere 0.02% of its suitable Joshua tree habitat, leading to “an almost complete elimination of Joshua trees from the park”. (L. Sweet, Congruence between future distribution models and empirical data for an iconic species at Joshua Tree National Park, 2019).
Wildfire
Climate change presents an additional threat to the Joshua tree in the form of wildfire. Atmospheric carbon dioxide accelerates growth of invasive grasses like mustard and brome. In undisturbed Mojave ecosystems, bare soil separates vegetation with low incidence of severe wildfire. Field research in Joshua Tree National Park found that nitrogen deposition from vehicles has increased grass growth at two of four sites, significantly increasing wildfire risk.
One of the most evident risks of wildfire threat to the Joshua tree took place in Mojave National Preserve in the world’s largest Joshua tree forest. In August 2020, 1.3 million eastern Joshua trees (Yucca brevifolia var. jaegeriana) burned in the 43,000-acre Cima Dome wildfire.
The combination of increased temperatures and variable precipitation attributable to climate change, spread of invasive species, and increased human activity favor opportunities for wildfire and increased Joshua tree mortality. Young trees under one meter in height are particularly vulnerable to extreme fire temperatures as their formative plant tissue is close to the ground.
Development
Local jurisdictions in the western Joshua tree range have minimal protective measures in place and often provide exemption for single family homes. Threats can be even greater on private lands, which account for 40% of western Joshua tree habitat. Only a small fraction of private land retains any kind of protections for this vulnerable species from development. With a growing list of threats stacked against shrinking population projections, virtually all suitable habitat may be lost in the coming decades without strengthened protections for the western Joshua tree.