PS 36-188 - A tale of two pines: Comparing seedling physiological traits between whitebark pine and limber pine

Background/Question/Methods

Rapidly changing climates challenge our ability to predict climate change effects on forest distributions and function. In the western US and Canada, warm, dry conditions and forest pests have resulted in widespread forest mortality and migration of subalpine tree species, shifting species’ geographic distributions. A tree species’ ability to migrate (naturally or assisted) depends on successful seedling establishment. Examining mechanisms of seedling establishment will improve our understanding of and ability to predict drivers of species distributions under future climates. Two high-elevation five-needled pine species in decline in the western US and Canada are whitebark pine (Pinus albicaulis, PIAL) and limber pine (Pinus flexilis, PIFL). Both species have similar adult morphology, growth habit, and dispersal mechanism, and are declining due to the same threats (white pine blister rust, mountain pine beetle), yet the two species have contrasting geographic distributions. To isolate the physiological mechanisms that may underlie seedling establishment and their contrasting distributions, we compared physiological traits (stress tolerance, stomatal, morphological, budburst phenology, photosynthetic) of greenhouse-grown 5-year-old seedlings of PIAL and PIFL. We asked: 1) How do seedling physiological traits differ between PIAL and PIFL? And 2) How do these traits link to their contrasting geographic distributions?

Results/Conclusions

We found that PIFL exhibited significantly greater high light tolerance (higher saturating light level (Qsat), higher fascicle density, lower ratio of sunlit to total leaf area (STAR), two sample t-test, p< 0.05) than PIAL. PIFL had significantly longer and wider needles and greater leaf mass per area (LMA) than PIAL (p< 0.05). PIFL exhibited significantly greater stomatal density and stomatal size than PIAL (p< 0.05). PIFL and PIAL did not significantly differ in traits describing budburst phenology, photosynthetic capacity (as measured with photosynthetic-CO₂ (A-C_i) response curves), tolerance to drought (as measured with pressure-volume (PV) curves), and tolerance to heat and cold (as measured with temperature-chlorophyll fluorescence and -electrolyte leakage response curves). We are also analyzing foliar carbon isotope ratios (d¹³C), nitrogen (N) content, and non-structural carbohydrates (NSCs) of the two species. Together, PIFL’s greater high light tolerance, larger and thicker needles, and more, larger stomata suggest that PIFL may inhabit more high-light environments, may increase its whole plant C assimilation, and may have a greater capacity take up nutrients, respectively, than PIAL. Because these traits promote growth and establishment on high light, windy, exposed sites, PIFL may be a relatively stronger pioneer species than PIAL.