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).