Evaluating Leaf Trait Variation in High Elevation Bristlecone Pine (Pinus longaeva) Under Increasing Water Stress: Insights…

Increasing aridification caused by climate change is altering growth patterns in trees. There is revived attention on how foliar traits respond to climate and the relationship of these traits to ring width. Bristlecone pine (Pinus longaeva, DK Bailey), a long-lived conifer found at high elevations in the cool and dry intermountain west of America, is used in paleoclimate reconstructions by measurement of their annually resolvable tree rings. The species also has annually datable needles retained on their branches for an average of 45 years making it the ideal subject for research on foliar trait and growth relationships under contemporary changes in hydroclimate. To explore this relationship, we sampled six individuals: three from the alpine treeline ecotone and three from the subalpine forest, on Mt. Washington in Nevada. From each tree, we measured ring width using tree cores and measured length and stomatal density of needles from a branch. Analysis included generalized-least squares regression followed by an ANOVA. For individual trees, needle length and ring width had variable growth trajectories over time compared with an inconsequential response of stomatal density to time. By site, stomatal density was higher and ring width was wider at the alpine treeline. The greater stomatal density of ATE trees explains the wider rings at this location by increasing the number of stomates per needle as mean needle length did not differ by site. Ring width and needle length were positively correlated and each trait reported higher first-order autocorrelation. Stomatal density, on the other hand, had a weak negative relationship to ring width and low first-order autocorrelation implying a minimally plastic response to climate conditions. The varied response of these traits by individual and site shows that the response to climate is not uniform within sites or between traits likely due the topoclimatic conditions experienced by each individual. By expanding beyond ring width, interannual variation in needle physiology allows us to understand the response of the bristlecone pine to recent climate change and the relationship of these traits to one another.