Great Basin bristlecone pine mortality: Causal factors and management implications

Abstract

High-elevation five-needle pines are foundational species and iconic components of subalpine forests across western North America. Because they often grow at environmental extremes, high-elevation pines are vulnerable to changing climate conditions. In addition to the direct effects of recent climatic changes, these species are increasingly threatened by biotic disturbances that thrive in the warming and drying conditions now occurring at higher elevations. Among the high-elevation pines, Great Basin bristlecone pine (Pinus longaeva) is revered for its extreme longevity and has been considered an icon of stability during periods of change. Life history strategies of Great Basin bristlecone pine that contribute to its longevity include physiological traits that enhance survival in harsh and dry habitats, and defensive traits that make it less vulnerable than other high-elevation pines to tree-killing bark beetles. Recent increases in growing degree days with no associated increase in precipitation is causing temperature-amplified tree drought stress, while warming temperatures positively influence bark beetle population growth. We report on preliminary investigations into recent and unexpected Great Basin bristlecone pine mortality at two sites, including the potential roles of weather-induced stress and bark beetles. At both sites climatic water deficit (CWD), a cumulative measure of moisture stress, and mean annual temperature increased during the 2010 decade and CWD was the highest in 2020 relative to any time during the past 40 years. Although Great Basin bristlecone pine mortality has not previously been attributed to bark beetles, we observed recent (i.e., 2013 to 2020) bark beetle-attacked trees at both sites, coincident with the timing of increasing temperature and CWD. Few adult beetles were produced, however, and our results support previous research that Great Basin bristlecone pine is a population sink for bark beetles. Because bark beetles are likely not self-sustaining in Great Basin bristlecone pine, bark beetle-caused mortality of this iconic species will most likely occur when it grows mixed with or near other pine species that support bark beetle population growth. We found Ips confusus and Dendroctonus ponderosae attacking Great Basin bristlecone pine in areas where their host trees, P. monophylla and P. flexilis, were also growing. These results suggest that the presence of these infested conifers likely contributed to Great Basin bristlecone pine mortality. We highlight several factors that may be used for prioritizing future research and monitoring to facilitate development of management strategies for protecting this iconic species.