DIRECT AND INDIRECT EFFECTS OF ENVIRONMENTAL VARIABILITY ON GROWTH AND SURVIVORSHIP OF PRE-REPRODUCTIVE JOSHUA TREES, YUCCA BREVIFOLIA ENGELM. (AGAVACEAE)

Premise of study: Accurate demographic information about long-lived plant species is important for understanding responses to large-scale disturbances, including climate change. It is challenging to obtain these data from desert perennial plants because seedling establishment is exceptionally rare, and estimates of survival are lacking for their vulnerable early stages. Desert wildfires, urbanization, and climate change influence the persistence of the long-lived Yucca brevifolia. Quantitative demographic attributes are crucial for understanding how populations will respond to disturbances and where populations will recede or advance under future climate scenarios. Methods: We measured survival in a cohort of 53 pre-reproductive Y. brevifolia at Yucca Flat, Nevada, USA, for 22 yr and recorded their growth, nurse-plant relationships, and herbivory. Key results: Herbivory by black-tailed jackrabbits (Lepus californicus) caused severe losses of plants during the first and second years (45% and 31%, respectively). Surviving plants experienced <2.5% annual mortality. Survival for the population was 19% over 22 yr. Plants <25 cm in height had lower life expectancy. Average growth rate (+/- SD) for plants that survived to the last census was 3.12 +/- 1.96 cm yr(-1), and growth rates were positively associated with precipitation. Thirty-year-old Y. brevifolia had not yet reproduced. Conclusions: A rare establishment event for Y. brevifolia during 1983-1984, triggered by above-average summer rainfall, provided a unique opportunity to track early survival and growth. Infrequent but acute episodes of herbivory during drought influenced demography for decades. Variability in survival among young Y. brevifolia indicates that size-dependent demographic variables will improve forecasts for this long-lived desert species under predicted regional climate change.