Within and between species physiological traits and stress resistances of three high-elevation pines

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Chloe Elizabeth Wasteneys and Danielle Marias Ulrich, Department of Ecology, Montana State University, Bozeman, MT

Background/Question/Methods
High-elevation pines inhabit exposed mountain tops, providing habitat for other organisms. However, high-elevation pines are declining rapidly due to a suite of factors exacerbated by climate change including white pine blister rust, bark beetle, and competition with faster growing conifers. The primary restoration strategy is outplanting rust-resistant seedlings. The seedling is the most vulnerable developmental stage and seedling establishment can drive species distributions. Therefore, understanding the physiological mechanisms underlying seedling establishment such as physiological traits and stress resistances is critical for successful restoration and conservation. Here, in greenhouse-grown 5-year old seedlings, we quantified physiological traits and stress resistances in two populations from contrasting climates of three high-elevation pine species: whitebark pine (WBP, Pinus albicaulis), limber pine (LP, Pinus flexilis), and Great Basin bristlecone pine (GBBP, Pinus longaeva). We also measured physiological traits in WBP seedlings of three different ages: 2, 3, and 5 years old. We quantified photosynthetic capacity from photosynthetic-CO2 response curves (maximum photosynthetic capacity (Amax) and RuBisCO efficiency (Vcmax)), high light tolerance from photosynthetic-light response curves (maximum photosynthetic rate in saturating light conditions (Asat) and light saturation point (Qsat)), and drought tolerance from pressure-volume curves (water potential at full turgor and at turgor loss point).

Results/Conclusions
The three species did not significantly differ in any of the physiological traits measured. However, marginally significant differences in traits were observed between LP populations from contrasting climates and between WBP seedling ages. The LP population from a lower elevation site (1,645 m) exhibited greater mean Asat (11.35 µmol m⁻² s⁻¹) than that from a higher elevation (2225 m, 6.63 µmol m⁻² s⁻¹) (two-way ANOVA, p-value=0.055), suggesting that the low elevation population exhibited greater high light resistance. Two-year old WBP exhibited greater mean Qsat (1385 µmol m⁻² s⁻¹) than both 3- and 5-year olds (870 µmol m⁻² s⁻¹ and 1130 µmol m⁻² s⁻¹, respectively) but was only marginally significantly different from the 3-year olds (p-value=0.054). Mean water potential at full turgor of 5-year old WBP (-2.3 MPa) was more negative than both 2- and 3-year olds (-2.1 MPa and -1.7 MPa, respectively) but was only significantly different from the 3-year olds (p-value=0.045. A more negative water potential at full turgor may indicate osmotic adjustment to water stress. The intraspecific and ontogenetic variation in stress resistances suggests a potential mechanism by which these species may persist under future climates.