Adaptive traits across families and common gardens: Results from Pinus strobiformis common garden experiments

Kristen M. Waring, Ethan Bucholz and Jessica A. Hartsell, School of Forestry, Northern Arizona University, Flagstaff, AZ, Amy V. Whipple, Ehren Moler and Jared Swenson, Biological Sciences, Northern Arizona University, Flagstaff, AZ

Background/Question/Methods
Maintaining healthy forests is an increasingly complex challenge, and climate change is often implicated in decreasing forest resilience. Southwestern white pine (Pinus strobiformis) is native to the southwestern US and Mexico where rapid warming and drying is underway. Our research uses seed sourced from across the distribution of SWWP to investigate relationships between seedling growth and survival, warming temperatures during seed development and seedling growth, and drought. In a series of common gardens across an elevational gradient (hot, warm and cold gardens) in northern Arizona, we planted seedlings starting in 2015, and subjected 50% of those seedlings to growing season drought conditions starting in 2017. We recorded mortality, growth, and phenology, and measured physiology on a subsample of seedlings (leaf mass area, hydroscape area, carbon isotope ratios). We investigated relationships between these variables with climate of the source populations during analyses.
Results/Conclusions
Transfer distance (Source Environment minus Garden Environment) impacted overall survival and annual variation in temperature and precipitation impacted phenological development in seedlings at all three gardens. Controlled seed warming significantly influenced seedling root length, shoot length, and diameter at root collar and showed significant effect sizes using Cohen’s Local f2. Seedlings from the coldest seed source region exhibited the lowest cumulative mortality across all gardens. The largest difference among seed source regions in seasonal mortality was due to a high proportion of winter-related mortality exhibited by seedlings from the warmest seed source region in both the hot and cold gardens. Mortality was negatively affected by bud damage at all three sites and spring phenology had site specific relationships with survival. Trees at the warm garden that developed buds more rapidly had greater survival, whereas at the cold garden trees that grew rapidly had higher mortality. Evidence for stomatal closure was found at the hot garden for all populations, where δ13C was significantly higher than the warm or cold gardens, along with a lower percent nitrogen content. Instantaneous water use efficiency, was lower at the hot garden and likely caused by a higher vapor pressure deficit and reduced photosynthetic rate by the seedlings. We found evidence for partial isohydry, indicating a priority of maintaining tissue water over transpiration. Further, we calculated hydroscape area at 0.83MPA2, lower than that of other southwestern conifers. This suggests low drought resistance. Our results also support that this species opts to avoid drought through increased stomatal closure, suggesting longer-term droughts may negatively impact survival.