Divergent selection has shaped population differences
We found the largest QST values for spring bud flush,
followed by fall bud set, consistent with other studies showing high
phenological divergence across latitudinal clines (Hurme 1999; Howeet al. 2003; Hall et al. 2007; Evans et al. 2016).
Spring bud flush is highly differentiated among P. fremontiipopulations, with a difference of up to eight weeks observed in flush
timing in three separate common garden studies (Grady et al.2015; Cooper et al. 2019; Davis et al. 2020). We also
found large population differences in fall bud set timing of
~2-5 weeks across the common gardens.
Bud flush and bud set also showed evidence of divergent selection
(QST > FST) in the
congeneric species, P. angustifolia (Evans et al. 2016),
as well as in P. tremula (Hall et al . 2007). In contrast
to these studies, however, we found bud flush had a higher degree of
population differentiation compared to bud set (Fig. 4). This result is
intriguing, since spring bud flush is primarily governed by the
accumulation of degree-days above a specific temperature, while fall bud
set is often linked to precise day length periods (Howe et al .
2003). While day length is driven by latitude and is constant from year
to year, temperature can vary each year. Our result of less
differentiated bud set timing compared to other Populus examples
could be because our study did not encompass as large a latitudinal
gradient. Evans et al. (2016) used three gardens spanning over 10
degrees of latitude and Hall et al. (2007) employed two gardens
approximately seven degrees apart, while this study used gardens just
over five degrees of latitude in separation (Supplemental Table 1). This
reduced latitudinal gradient means smaller day length differences
experienced across the gardens, perhaps resulting in reduced differences
in bud set expression.
Alternatively, our results of higher bud flush versus bud set
differentiation could be because our source populations span the winter
frost line. The southern populations in our study do not experience
predictable freezing winters, which could mean that there is more room
for temperature-related differentiation in bud flush earlier in a
risk-free spring, and/or less selection for differentiation in bud set
among populations south of the frost line late in the fall. Overall,
these two phenology traits showed both the highest degree of population
differentiation of all our traits and the strongest correlations with
provenance climate (highest QST values; Fig. 5). This
result is suggestive of climate as a strong driver of phenotypic
differences among these Arizona populations.