Climate as an agent of selection
The strength of the correlations between traits and provenance climate
varied across gardens (Table 3, Fig. 3), supporting our third hypothesis
of strong associations between phenotype and climate for some traits in
some environments. A single axis (PC1) explained 95.8% of the variation
in provenance climate and was influenced primarily by temperature and
growing season-related climate variables (Supplemental Table 2).
Populations sourced from areas with higher temperatures, lower
precipitation, lower elevation, and longer growing seasons had higher
PC1 scores. Bud set exhibited the strongest relationship with provenance
climate across the gardens (R2 = 0.67 to 0.77), while
bud flush showed significant correlations in the two warmer gardens
(R2 = 0.49 in Yuma and 0.66 in Agua Fria), but not in
the cold garden (R2 = -0.01) (Table 3, Fig. 3a). In
Canyonlands, population variation was constrained as all trees flushed
at approximately the same time, late in the spring. Specific leaf area
did not show significant trait-climate correlations in any garden,
although we see overall SLA values increasing from the hot to the cold
garden site (Fig. 3a).
Tree growth traits were more likely to show garden-dependent
relationships between population origin and performance (Table 3, Fig.
3b). Tree height showed no climate relationships when planted in the
hottest garden, however the correlation become stronger in the mid to
cold gardens. When planted at the coldest garden, trees sourced from
colder, wetter climates, including the three populations from the
Colorado Plateau, were taller than populations from hotter, drier
environments. Similarly, the DRC relationship in the hottest garden
showed trees sourced from warm, hot environments had larger trunk
diameters compared to trees from colder climates (Fig. 3b). Together,
tree height and basal trunk diameter act as indicators that overall tree
performance is consistent with local adaptation, with hot, southern
populations growing larger in the hottest Arizona garden, and northern,
cold adapted populations growing larger in the coldest Utah garden.
Phenotypic plasticity was significantly correlated with population
source climate for all traits except SLA (Table 3, Fig. 3). Populations
sourced from hot, dry climates exhibited increased plasticity in
leaf-level phenology traits relative to the colder populations, as
previously reported in Cooper et al. (2019). Tree-level growth
traits showed the opposite pattern of increased plasticity in those
populations sourced from the colder, high elevation environments. SLA
showed a similar trend to phenology, with warmer populations exhibiting
higher plasticity compared to populations sourced from cooler climates,
but was not significant. Again, this may be due in part to the lower
sample size of seven populations that did not include populations from
the hottest, driest sites (Fig. 3a).