Mortality Thresholds of Juvenile Trees to Drought and Heatwaves: Implications for Forest Regeneration Across a Landscape Gradient

Abstract

Tree loss is increasing rapidly due to drought- and heat- related mortality and intensifying fire activity. Consequently, the fate of many forests depends on the ability of juvenile trees to establish following exposure to heightened climate anomalies. Climate extremes, such as droughts and heatwaves, are increasing in frequency and severity, and tree survival in mountainous regions must contend with these landscape-level climate episodes. Recent research focuses on how mortality of individual tree species may be driven by drought and heatwaves, but how mortality from protracted drought and associated warming would vary among multiple species spanning an elevational gradient given concurrent variation in climate, ecohydrology, and physiology remains unclear. We address this question by implementing a growth chamber study, imposing extreme drought with and without a compounding heatwave, for five species that span elevations across a semiarid elevational gradient in the southwestern United States. Overall, the length of a progressive drought required to trigger mortality differed by up to 20 weeks among species, whereas inclusion of a heatwave hastened mortality by about one week. Lower elevation species that grow in warmer ambient conditions died earlier (Pinus ponderosa in 10 weeks, Pinus edulis in 14 weeks) than did higher elevation species from cooler ambient conditions (Picea engelmannii and Pseudotsuga menziesii in 19 weeks, and Pinus flexilis in 30 weeks). When exposed to a heatwave atop drought, mortality advanced significantly only for species from cooler ambient conditions (Pinus flexilis: 2.7 weeks earlier; Pseudotsuga menziesii: 2.0 weeks earlier). Cooler ambient temperatures and associated differences in ecohydrology-related soil evaporation may have provided a buffer against moisture loss during drought, potentially overriding expected differences in drought tolerance due to tree physiology. Our study suggests that droughts will play a leading role in juvenile tree mortality and will most directly impact species at warmer climate thresholds, with heatwaves atop drought potentially exacerbating mortality especially of high elevation species. These responses are relevant for assessing the potential success of both natural and managed reforestation, as differential juvenile survival following episodic extreme events will determine future landscape-scale vegetation trajectories under changing climate.