A tale of ENSO, PDO, and increasing aridity impacts on drought-deciduous shrubs in the Death Valley region.

Abstract

Germination, establishment, phenology, and death among three drought-deciduous shrubs were influenced by ENSO/PDO and precipitation, based on 37years of annual surveys. Encelia farinosa forms near monospecific stands on slopes, whereas E. frutescens and Ambrosia salsola dominate wash habitats. All shrubs exhibited phenological coherence. While germination, establishment, and mortality patterns were similar among wash species, these dynamics contrasted with E. farinosa on slopes. Germination was associated with El Niño years. Slope plant establishment was dependent on precipitation in the subsequent year, but not evidently so in wash species. Major mortality events were episodic, with Encelia mortality just as likely to occur in years with below or above average precipitation. In both Encelia species, mortality was associated with transitions to a cold PDO phase. In E. frutescens this response was more rapid, whereas in E. farinosa mortality lagged 1year, resulting in contrasting slope-wash mortality patterns. 50% of newly established shrubs died within 5, 5, and 18years for E. frutescens, E. farinosa, and A. salsola, respectively. The 90% mortality ages were 26years for E. frutescens, 24years for E. farinosa, and 51years for A. salsola. While maximum life expectancies are unknown, estimated maximum life expectancies were 56, 66, and 86years for E. frutescens, E. farinosa, and A. salsola, respectively. Overall, as the climate has become more arid over the past four decades, the populations in both slope and wash habitats have exhibited similar responses: reduced shrub abundances and reduced total supportable leaf areas.