Germination and establishment of the Joshua tree (Yucca brevifolia) in the northeast Mojave Desert: Implications for management of an iconic species

Background/Question/Methods

In arid environments, demographic changes for long-lived perennials are difficult to measure because favorable conditions for regeneration occur infrequently. Seedling emergence and survival for many desert plants are sensitive to variability in soil moisture and temperature as well as predation. Understanding the dynamics surrounding plant establishment is important for land managers to effectively mitigate the impacts of disturbances and promote re-vegetation, especially for disturbances such as desert wildfires that are affecting large landscapes dominated by Joshua trees in the Mojave Desert. We investigated the influence of microclimate, season, and residence time on seedling emergence and survival, and seed persistence of Joshua trees after simulated secondary dispersal. We placed seeds in shallow caches and manipulated granivore access (caged versus uncaged), cache placement (beneath shrub canopy versus shrub interspace), and season of placement (summer, late fall, and early winter). Emergence and survival were monitored for 2 years, then caches were recovered to examine the influence of residence time on viability as well as removal rates from caches accessible to granivores. Survivorship of seedlings was monitored throughout the study. 

Results/Conclusions

Seedlings emerged in spring and summer when rain increased soil moisture accompanied by warm soil temperatures. Emergence of seeds that were placed in the soil in late winter/spring was enhanced beneath shrub canopy. However, shrub cover had no influence on seedling survival and instead reduced survival for seedlings that emerged in April just before the onset of summer temperatures. Viability of seed remained high in storage (97%) but was reduced after planting from 50 – 68% after one year and declining to less than 3% after 3.5 years. Partial herbivory was low and had little effect on seedling survival. The narrow window of opportunity for Joshua tree regeneration coupled with slow dispersal and infrequent emergence opportunities will require assistance to re-establish Joshua trees to areas of degraded desert.