Trait analysis
We analyzed five traits for phenotypic differentiation: fall bud set, spring bud flush, specific leaf area (SLA), height, and trunk basal diameter. Height and SLA were assessed the first year of growth in the summer of 2015 for every genotype in the common gardens. Specific leaf area was measured using the average of three fully expanded leaves that were free or nearly free of herbivore damage, collected from the southern aspect of each tree in June and July. Leaves were scanned and leaf area was measured using ImageJ software (Schneider et al . 2012). After the area scans, leaves were dried in an oven at 70°C for at least 72 hours, following protocol recommendations in Perez-Harguindegayet al .’s (2013) updated trait handbook. Trunk basal diameter was measured at the end of the second year of growth (mid-January in the Yuma and Agua Fria gardens and in early March in Canyonlands) as the diameter at root collar, ~10cm from the soil, on every tree in the gardens. Diameter at root collar was used instead of diameter at breast height due to the short stature of the stems.
Phenology of bud set and bud flush were measured in the fall of 2015 and the spring of 2016, respectively. Bud set was recorded as the initiation of bud formation, where internode elongation had ceased and the newly emerged, bundled leaves were clustered at the same level on the stem, offset from the shoot axis (Frewen et al. 2000). Bud set was measured at 6-10 day intervals from September through December of 2015 on three replicates of all 12 genotypes per population in each garden. We scored trees based on the bud stage exhibited by 50% or more of the apical meristems. This measurement is a good approximation of whole plant progression towards dormancy as there was little within-plant variation in apical bud development. Spring bud flush was recorded as the first sign of full leaf emergence on the tree. Bud flush was measured biweekly from February through the end of April in the Yuma and Agua Fria gardens, and through the end of May in the Canyonlands garden, where colder temperatures persist later into the spring.