Lethal combination for seedlings: extreme heat drives mortality of drought-exposed high-elevation pine seedlings.

Abstract

Hotter drought- and biotically-driven tree mortality are expected to increase with climate change in much of the western United States, and species persistence will depend upon ongoing establishment under novel conditions or migration to track ecological niche requirements. High-elevation tree species may be particularly vulnerable to increasing water stress as snowpack declines, increasing the potential for adult mortality and simultaneous regeneration failures. Seedling survival will be determined by ecophysiological limitations in response to changing water availability and temperature. We exposed seedlings from populations of Pinus longaeva, Pinus flexilis, and Pinus albicaulis to severe drought and concurrent temperature stress in common gardens testing timing of drought onset under two different temperature regimes. We monitored seedling functional traits, physiological function, and survival. The combined stressors of water limitation and extreme heat led to conservative water use strategies and declines in physiological function, with these joint stressors ultimately exceeding species' tolerances and leading to complete episodic mortality across all species. Growing conditions were the primary determinant of seedling trait expression, with seedlings exhibiting more drought-resistant traits such as lower specific leaf area in the hottest, driest treatment conditions. Water stress-induced stomatal closure was also widely apparent. Under adequate soil moisture, seedlings endured prolonged exposure to high air and surface temperatures, suggesting broad margins for survival. The critical interaction between soil moisture and temperature suggests that rising temperatures will exacerbate growing season moisture stress. Our results highlight the importance of local conditions over population- and species-level influences in shaping strategies for stress tolerance and resistance to desiccation at this early life stage. By quantifying some of the physiological consequences of drought and heat that lead to seedling mortality, we can better understand the future effects of global change on the composition and distribution of high-elevation conifer forests.