Comparing Juvenile Physiology and Morphology of Two High-Elevation Pines, Pinus albicaulis and P. balfouriana

Abstract Whitebark pine ( Pinus albicaulis , PIAL) and foxtail pine ( P. balfouriana , PIBA) are slow-growing, high-elevation, five-needled pines. Recently, PIAL has experienced significant mortality while PIBA has experienced minimal decline. PIBA exists in two disjunct southern (PIBA S ) and northern (PIBA N ) populations. Our study compared juvenile physiological and morphological traits between the two species (PIAL, PIBA) and foxtail populations (PIBA N , PIBA S ) to investigate mechanisms underlying responses to environmental stressors in their high-elevation environments. We grew four-year-old PIAL and PIBA trees in a common greenhouse environment. We measured traits describing their morphology, biomass, stomatal and xylem conduit size and density, budburst phenology, gas exchange, whole plant volatile organic compounds (VOCs), phloem volatile resin (PVR) compounds, and non-structural carbohydrates (NSC). We found that PIAL and PIBA displayed different suites of traits that similarly promote resistance to environmental stressors ( e.g., high wind, drought, herbivores). The two PIBA populations did not differ in most traits except for VOCs where PIBA S emitted higher concentrations of specific and total whole plant VOCs than PIBA N , suggesting that PIBA S may differ in susceptibility to stress from PIBA N . For many traits and especially evident in whole plant VOC concentration and composition, PIAL and PIBA N were most similar while PIAL and PIBA S differed the most, consistent with growing season total precipitation of the juveniles’ climates of origin. Our observed trait differences between species and populations offer insight into mechanisms by which PIAL and PIBA juveniles survive in their environments, improving our ability to understand drivers of current and future species distributions.