Local adaptation to climate
Whereas leaf phenology and morphology traits (bud flush, bud set, and
SLA) had the highest degree of differentiation in our study, likely due
to climate-related divergent selection, tree growth traits (height and
trunk basal diameter) were less differentiated. This result suggests
that the neutral processes of gene flow and drift may override weak
selection on growth traits (McKay & Latta 2002), or that growth is
strongly constrained by the relative success dictated by the rest of a
plant’s phenotype (Saint-Laurent et al. 2007; Leinonen et
al . 2013). In this case, the latter is very likely. Although phenology
and growth traits showed significant correlations with climate of origin
(Fig. 3), only the growth traits reversed the sign of that correlation
across gardens. Specifically, phenology trends were constant across
gardens, with warmer source populations setting bud later and flushing
earlier regardless of growing environment (except for bud flush in
Canyonlands). However, height and trunk diameter declined as transfer
distance increased (in terms of both hotter and colder climates) for
populations relative to their home sites. This indicates local
adaptation, where the highest productivity is observed in populations
whose source climate best matches that of the garden climate. In the
hottest garden (Yuma), there were positive trends between growth traits
and warmer provenance climates. In the coldest garden (Canyonlands), the
reverse was true, where trees from cooler provenances grew significantly
taller than those from the warmer sites. Whether this higher performance
of local populations is enabled by their particular values of the leaf
phenology and morphology traits measured here vs. additional plant
functional traits is an important area for further study.