Piñon-juniper woodlands are the most common forest type in the southwestern U.S., but their future is uncertain due to the increasing prevalence and severity of drought. Historically, this ecosystem has served as a key testbed for understanding the eco-physiological responses of trees to water stress due to the fact that piñon and juniper display markedly different responses to water stress, whereby piñon is relatively drought sensitive and juniper is highly drought tolerant. A severe drought in the early 2000s in the Four Corners region serves as a case study for these species-specific responses, as anomalously low winter precipitation and a hot, dry summer triggered widespread mortality of piñon pine, yet the region’s juniper species experienced relatively little dieback. Another severe drought struck in 2018, but puzzlingly was followed by extensive juniper dieback, leading to questions on why one of the hardiest tree species in the Southwest is experiencing sudden unexplained mortality. To investigate this novel event, we set up a network of plots across the region and conducted stand surveys and eco-physiological measurements to uncover the extent/severity of juniper dieback and its mechanistic causes.
Results/Conclusions
We found juniper dieback to be highly variable but quite severe. In some areas, juniper dieback approached 60% while piñon mortality remained low across the region (< 5%). Stand surveys indicated that juniper dieback was most prevalent in the hot, dry, low elevation areas that were most severely impacted by the 2018 drought. Intermediate size classes were markedly more affected by dieback than either small or large trees, though the age of these trees remains unresolved. No evidence of primary pests/pathogens was found on dying junipers, but large amounts of drought-induced hydraulic damage point to a direct role of the 2018 drought in causing this dieback. Furthermore, efforts to remotely sense this mortality indicate that it is widespread across the region. In sum, we have found extensive juniper dieback that was most likely caused by the direct effects of drought. However, it is important to note that the etiology of this mortality is similar to what we would expect of piñon, yet piñon trees in the region remain healthy. Future efforts will investigate the role of hydrology and cumulative climatic stress in causing juniper to reach this fatal tipping point.