Rapid and surprising dieback of Utah juniper in the southwestern USA due to acute drought stress

For nearly two decades, the American Southwest has been in the grips of a long-term 'megadrought', punctuated by a number of short and severe 'global change type droughts' (i.e., negative precipitation anomalies cooccurring with high temperature). These events have caused widespread mortality of the drought-sensitive pinon pine (Pinus edulis) while co-dominant Juniperus spp. have historically been much more drought tolerant and thus have rarely died. However, a severe drought occurred in 2018 that rapidly (<1 year) caused canopy dieback of Juniperus osteosperma across a vast area of southeastern Utah. In order to uncover the etiology behind this surprising dieback event, we set up a series of survey plots that captured gradients in microclimate, topography, and dieback severity. We also quantified xylem hydraulic damage and assessed the presence of various biotic agents in declining junipers to identify the primary causes of this dieback. We observed that juniper canopy dieback was most severe (>60% canopy dieback) at hot, dry, low elevation sites, and was associated with drought-induced hydraulic damage. There was no evidence that biotic agents could be the primary drivers of this dieback, implicating the acute effects of drought as the main causal agent. The speed and scale of this drought-induced juniper dieback seems to be historically unprecedented in the region and foreshadows an uncertain future for pinon-juniper woodlands as the region continues to get warmer and drier.

Automated summary

The American Southwest has been facing a long-term 'megadrought' for nearly two decades, marked by severe 'global change type droughts' in recent years. The drought-sensitive pinon pine has suffered widespread mortality, while the historically drought-tolerant Juniperus spp. also experienced significant dieback in 2018 due to drought-induced hydraulic damage. Biotic agents were not found to be the primary drivers of this dieback, highlighting the acute effects of drought in the region.