Yucca jaegeriana

Yucca brevifolia var. jaegeriana was described in 1938, and considered a distinct taxon through the late 1970s, until J. L. Reveal treated it no longer distinct from the type. Flora North America (2002a) follows the J. L. Reveal treatment. In 2007, however, it was described as the species level (Lenz 2007a). Morphological, biological (pollination), and genetic studies, support the distinctiveness of this entity and recognition at the specific rank (Royer et al. 2016 and Lenz 2007a). The U.S. Fish and Wildlife Service recognize Yucca jaegeriana and Yucca brevifolia as distinct species (USFWS 2019).

Yucca jaegeriana, Eastern Joshua Tree, is an iconic species and near endemic of the Mojave Desert of the southwestern United States, with occurrences in Arizona, California, Nevada, and Utah. It is threatened by fire, drought, climate change, and numerous threats related to habitat loss including off road vehicle use

Yucca jaegeriana is endemic to the United States and occurs primarily within the Mojave Desert including areas of western Arizona, southeastern California, southern Nevada, and extreme southwestern Utah. To the north, the species range falls within the Great Basin Desert and to the east in Sonoran Desert of Arizona (USFWS 2018). This species overlaps in range with Yucca brevifolia in a small area of Nevada, called the Tikaboo Valley, where the two species hybridize (Royer et al. 2016, Lenz 2007). Range extent was calculated using data from iNaturalist (2021), SEINet (2021), and CCH2 (2021).

Yucca jaegeriana is threatened by fire, drought, climate change, and numerous threats related to habitat loss. One of the more severe threats to the species is an increase in fire frequency, which causes direct mortality and reduced survivorship over time. A greater presence of non-native annual grasses, which produce a fuel load in the environment, have increased the frequency of fire in the deserts where this species occurs. The plant communities and Joshua Tree itself are not especially tolerant of fire. Native plants, when responding with vigorous growth to an unusually wet winter, also contribute to an increased fuel load and are responsible for some fires in the past. Younger, smaller plants are more vulnerable to mortality than older, larger plants. One cause of mortality post-fire is predation by small mammals seeking a food source (USFWS 2018, Cole et al. 2011). With its primary seed disperser, the Shasta Ground Sloth, extinct, the species ability to shift into suitable habitat is limited (Cole et al. 2011).

Climatic conditions threaten the species in the short and long term, in multiple ways, through increased temperatures, drought, and precipitation timing. While this species is tolerant of drought, young plants including seedlings experience greater mortality and lower germination success during drought, which is increasing in severity currently and into the future due to climate change. Studies are needed to understand the impact of this threat and to document declines. The species range is expected to become more arid, specifically drier and hotter, with climate change. The species habitat is predicted to shrink and move into refugia, northward, and to higher elevations (USFWS 2018).

Habitat loss, including degradation of soil structure loss and loss of plant cover, is caused by development, military activities, grazing animals, renewable energy development, and off road vehicle use. About 0.7% of the species range is in areas with residential and urban development. Plants receive some protection by local ordinances (USFWS 2018). About 8.5% of the species range in on military lands, but the species is not believed to be impacted, and potentially benefit by management plans and conservation efforts (USFWS 2018). Around 53% of this species range is allocated for grazing allotments but not all are actively grazed. Grazing animals browse plants, including flowers below 2 meters in height, trample young plants, and contribute to a loss of soil structure that can degrade habitat (USFWS 2018, Lybbert and St. Clair 2017). Off road vehicle use can be severe where it occurs, causing mortality to this species and its nurse plants and damage to the seed bank and degradation to the soil structure (USFWS 2018).

An additional threat to the species is damage and mortality from native herbivores (USFWS 2018).