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 explanation is very likely.
Although phenology and growth traits showed significant regressions with
climate of origin (Fig. 3), only the growth traits reversed the sign of
that relationship across gardens. Specifically, phenology trends were
mostly 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 was a positive relationship
between trunk diameter 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
(Fig. 3b). Whether this higher performance of local populations is
enabled by their leaf phenology and morphology traits measured here vs.
additional plant functional traits is an important area for further
study.