Physiological traits and stress tolerances of three high-elevation pine species

Abstract

The increasing frequency and intensity of drought and heat events are increasing forest mortality rates worldwide. To improve restoration and management effort effectiveness for at- risk tree species and to accurately predict how these species will respond to future climates, the physiological mechanisms of seedling establishment and survival need to be fully understood. Using a greenhouse common garden approach, we compared the physiological traits and stress tolerances of three high-elevation five-needle pine species, Pinus albicaulis (PIAL), P. flexilis (PIFL), and P. longaeva (PILO). We measured a suite of physiological response curves to determine photosynthetic capacity, high-light tolerance, drought tolerance, and heat tolerance traits. We compared these traits among the three species (Q1), among three different ages of PIAL seedlings (2-, 3-, and 5-years-old; Q2), and among seven families of 2-year-old PIAL originating from different locations (Q3). Among species (Q1), only one heat tolerance trait (Tcrit, the temperature at which minimal fluorescence begins to increase) significantly differed and increased from PILO to PIFL to PIAL, suggesting that PIAL had greater heat tolerance than PIFL and PILO. Among PIAL ages (Q2), two drought tolerance traits, saturated water content (SWC) and leaf mass per area (LMA), and two heat tolerance traits, Tcrit and T50_EL (temperature that results in a 50% increase in electrolyte leakage), significantly differed among ages. SWC decreased, LMA and Tcrit increased, and T50_EL partially increased with increasing age, suggesting that older PIAL seedlings may be more drought and heat tolerant than younger PIAL seedlings. Among PIAL families (Q3), no traits significantly differed. However, we observed four significant correlations between our measured traits and growing season mean temperature and vapor pressure deficit of the seven PIAL families. Families from relatively cooler, wetter locations tended to have greater high-light tolerance and greater heat tolerance (based on T50_EL) while families from relatively warmer, drier locations tended to have greater heat tolerances (based on Tcrit and T50 measured with chlorophyll fluorescence). Together, these findings improve our understanding of physiological mechanisms underlying seedling establishment and our ability to predict how these species may be affected by future climates.