QST - FST: Comparison of quantitative trait differentiation to neutral genetic expectation
In support of our second hypothesis, we found evidence of divergent selection (QST > FST) driving phenotype differences in most traits (Table 2, Fig. 4). Overall, the average QST value across all traits and all gardens (0.48) was above the FST confidence interval (0.144-0.205), consistent with directional selection shaping trait differences and local adaptation across these populations. Phenology traits exhibited some of the highest levels of population differentiation, with mean QST values for bud flush between 0.53-0.90 and bud set between 0.42-0.79. In all but one case (bud set in Agua Fria), the confidence intervals for these QST values lie above the confidence interval for FST. Divergent selection on specific leaf area was also apparent in the mid and cold gardens of Agua Fria and Canyonlands, but not detectable in the hot Yuma garden. In contrast to this relative consistency in results for leaf traits, traits related to tree performance showed more variation in QST - FST comparisons across gardens. Tree height showed significant divergent selection (QST > FST) when measured at the hottest common garden in Yuma, Arizona, but was not statistically different from FST in the two cooler gardens of Agua Fria and Canyonlands. We found evidence of stabilizing selection for basal trunk diameter in the cold Canyonlands garden, where the QST value (0.03) fell below the FST confidence interval. However, this trait was indistinguishable from the neutral expectation of FST in the warm and mid gardens.