Effects of growing conditions on the evidence for selection
Whereas evidence for divergent selection in bud flush was consistent across all gardens, both bud set and SLA showed evidence of divergent selection in two out of three gardens (although for bud set the third garden was very close). Variability in this conclusion was driven by variation in the QST value of the same trait across the three gardens (Fig. 4). This finding suggests that phenotypes shaped by selection pressures across a species’ range can be expressed differently in different growing environments. This variation among gardens led to even larger contrasts in the evidence for selection in the performance traits. For example, we observed high population differentiation in height expressed in the hottest garden (QST = 0.45), but these differences diminished when populations were planted in the moderate and cool gardens. For basal diameter, QST also decreased with decreasing garden temperature, approaching evidence for divergent selection in the hot garden to showing evidence for stabilizing selection in the cold garden. Thus, our detection of selection is dependent on the common garden environment, with some environments enhancing and others dampening population phenotypic differences. This may represent an interaction between the selection pressures shaping natural variation across the species range and novel selection pressures imposed in a common garden experiment or under future climate change. The large population-level trait differences exhibited in the hottest common garden for all traits except SLA is likely driven by the maladaptation of the high elevation populations to the extreme thermal conditions experienced in the hot, Yuma garden. This climate transfer from northern to southern Arizona represents an extreme warming treatment, a scenario that may be imposed on populations under severe heat waves with climate change (Cook et al . 2015). Similarly, Evans et al. (2016) found that the relationship between QST and FST changed through time, with tree height displaying high population differentiation (QST > FST) under the growing conditions in one year but not the next. Long-term common garden experiments can demonstrate how population differences are expressed both across different environments and through time. Given the intensification of extreme events and climate variability going forward (Jentsch et al. 2007; Ganguly et al. 2009; Garfin et al. 2013; Williams et al. 2020), these types of field trials should be expanded to evaluate the correspondence between the degree of existing climate adaptation and the potential for future climate survival, either through phenotypic plasticity, selection on remaining genetic variation, or a combination of the two (Nicotra et al.2010; Josephs 2018).