Divergent selection shapes population trait and plasticity
differences
We interpret cases when both the QST-FSTanalysis showed large divergences from neutral expectation and
phenotype-climate correlations were significant as strong evidence for
climate-driven selection. Cases with only one of these tests showing
population differences provide partial evidence for climate-driven
selection (Table 4). For instance, there were four cases showing
QST > FST but
non-significant trait-climate correlations. These inconsistencies
between the two tests could be due to divergent selection that is not
related to the climatic gradients we tested. There were also four cases
showing QST ≈ FST and significant
trait-climate correlations. In these cases, QSTconfidence intervals overlapping with FST could be due
to the bootstrap sampling of genotypes with less genetic variation
compared to the full sampling design, thus lowering the
QST estimate. There were also more significant
plasticity-climate correlations (4 out of 5) than significant
QST-FST differences for plasticity (2
out of 5). Finally, there was a range of results across the three
gardens within the QST-FST analysis
itself. Together, these tests provide a continuum of support for
selection on traits and trait plasticity, and highlight which traits may
be under the strongest selection and potentially the most important to
investigate under climate change.
We found the largest QST values for spring bud flush,
consistent with other studies showing high phenological divergence
across latitudinal clines (Hurme 1999; Howe et al. 2003; Hallet al. 2007; Evans et al. 2016). Spring bud flush is
highly differentiated among P. fremontii populations, with a
difference of up to eight weeks observed in flush timing (Grady et
al. 2015; Cooper et al. 2019; Blasini et al. 2020). We
also found large population differences in fall bud set timing of
~2-5 weeks across the common garden gradient, reflected
in moderate QST values in two out of the three gardens.
The strong population differences in phenology found here agree with
Fischer et al. (2017), who showed leaf phenology accounted for
>80% of the variation in tree and forest productivity
among Fremont cottonwood genotypes. We found larger population
differences in bud flush compared to bud set. This result is intriguing
given that spring bud flush is primarily governed by temperature, while
fall bud set is mostly cued by precise day length periods (Thomashow
2001; Howe et al. 2003). While day length is driven by latitude
and is constant from year to year, temperature can vary. The fixed
environmental cue of day length should allow populations to become
highly locally adapted and differentiated in bud set compared to a
variable environmental cue such as temperature. However, the strength of
photoperiod-driven selection on bud set may be relaxed in all but the
highest elevation populations, where the trade-off for longer growing
seasons is selected for in areas that very rarely or never experience
killing frosts (Howe et al. 2003). Both phenology traits also
showed strong relationships with provenance climate across the gardens,
except for bud flush in the coldest garden, where low temperatures
prevented an earlier flush in the southern, warm-adapted populations
(Fig. 3a).
Our detection of selection was dependent, in part, on the environmental
conditions of each garden. Bud flush, bud set, and SLA all exhibited
divergent selection (QST >
FST) in two out of three gardens (Fig. 4). Tree growth
traits exhibited even larger variation in QST among
gardens. For example, we observed high population differentiation in
height expressed in the hottest garden (QST = 0.44).
When populations were planted in the moderate and cool gardens, these
population differences diminished, but became more strongly predictable
from home climate (Fig 3). QST estimates for trunk
diameter also decreased with decreasing garden temperature. This
variability in QST across gardens suggests that
phenotypes shaped by selection pressures across a species’ range can be
expressed differently in different growing environments, with some
environments enhancing and others dampening population phenotypic
differences (Oke et al. 2015; Akman et al. 2021).
Particularly for growth traits, this may represent an interaction
between the selection pressures that have shaped existing variation
across the species range and novel selection pressures imposed in a
common garden experiment or under future climate change.
The larger population-level trait differences exhibited in the hottest
common garden for most traits (except SLA) could be driven by the
maladaptation of the cold-adapted, northern populations to the extreme
thermal conditions experienced in this hot 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).
Our QST-FST comparison revealed support
for divergent selection acting on phenotypic plasticity in bud flush and
tree height, and showed partial evidence for selection on plasticity in
the other three traits (Table 2; Fig. 5). This is in contrast to
previous studies that found no evidence of selection on trait plasticity
using QST-FST type comparisons (Lindet al. 2011, De Kort et al. 2016), and low overall support
for selection on plastic responses to temperature (Arnold et al.2019). Our results suggest that for some traits, differences in
plasticity among populations across a wide environmental gradient are
larger than expected from neutral genetics, where some populations show
minimal plasticity and others exhibit high plasticity. Conversely we
found some evidence for stabilizing selection in DRC plasticity,
indicating that the difference in the magnitude of plasticity for this
trait across our populations was smaller than expected by
FST, however it was not below the FSTconfidence interval in all 100 plasticity permutations. The mosaic of
natural selection acting on trait plasticity across our populations
shows how plasticity itself can evolve in response to different
climates.
The mosaic of natural selection acting on trait plasticity across our
populations shows how plasticity itself can evolve in response to
different climates. We found significant plasticity-climate
relationships in phenology and growth traits, where the sign of the
correlation switched between these two types of traits (Fig. 3).
Specifically, we found trees sourced from colder environments were
significantly more plastic in height and DRC compared to the warm
provenance populations, but were not as plastic with regard to their bud
set and bud flush (Fig. 3). This is an example of a multivariate
plasticity response, where plasticity in one trait may be affecting the
plasticity in another trait (Nielsen & Papaj 2022). The higher
plasticity in phenology traits measured in populations from hotter
provenances is counterintuitive because colder source populations
experience much more predictable fall freezing events and higher yearly
temperature variation (see TD in Supplemental Table 1), and theory
predicts plasticity will increase under predictably variable
environments (Chevin & Lande 2010). However, our climate transfer of
southern Arizona populations to the northernmost cold garden represents
an extreme climate event (over 15°C colder in the coldest month for the
populations from the hottest source locations, Supplemental Table 1) far
outside of some populations’ normal temperature range, which can result
in large, maladaptive plastic responses (Chevin & Hoffman 2017). The
higher phenological plasticity seen in hot-adapted populations did not
translate into increased growth or growth plasticity, likely due to
maladaptive phenological plasticity that pushed these trees outside of
the appropriate growing season window (Cooper et al. 2019). The
high bud set plasticity of warm-adapted populations meant that these
trees did not set bud until late in the growing season, when freezing
temperatures damaged non-dormant tissues. The subsequent frost damage
translated to lower growth compared to cold-adapted trees that set buds
earlier in the season and avoided frost damage. The increased height of
the cold adapted populations in the coldest garden relative to the warm
populations produced the significant differences in height plasticity.
Therefore, our result of higher height plasticity in populations sourced
from cold locations can be partially explained by the warm populations’
maladaptive plasticity in phenology.
In comparing our results to previous findings of no divergent selection
on plasticity in other systems, it is important to consider both climate
means and variances. In this system, higher growth plasticity observed
in populations sourced from colder, high elevation locations could also
be due to adaptation to increased climate variability, compared to the
central and southern Arizona populations. Specifically, the temperature
difference between the mean warmest month and the mean coldest month was
the largest for the three populations collected on the Colorado Plateau
compared to the rest of the populations below the Mogollon Rim of the
Plateau (see TD in Supplemental Table 1). This follows the theory that
higher levels of plasticity should occur in more variable environments
(Lande 2009).
Finally, our estimates of heritability in both traits and trait
plasticity also indicate that these components of the phenotype can
evolve in response to selection, at least under some environmental
conditions. Broad-sense heritability values for the five traits were
moderate, with a mean value across all gardens of 0.21 (Table 2). Our
phenology heritability measures (H2 = 0.04-0.48 for
bud flush and H2 = 0.19-0.30 for bud set) were lower
than previously found in some Populus studies
(H2 = 0.94 for bud flush and H2 =
0.91 for bud set in P. trichocarpa x deltoides , Frewen et
al. 2000). Heritability values for growth traits (H2= 0.11-0.27 for height and 0.08-0.21 for DRC) and SLA
(H2 = 0.10-0.35) were fairly consistent with other
reported Populus estimates (H2 = 0.03-0.42 for
height and H2 = 0.09-0.25 for diameter at breast
height in P. tremuloides , Ding et al. 2020;
H2 ≈ 0.2-0.6 for SLA in P. nigra , Guet et
al. 2015). The range of heritability estimates for the same trait
across the three gardens highlights the environment-dependent nature of
heritability. This is especially apparent in our bud flush results,
where we found the lowest value in the cold garden (H2= 0.04) and the highest value in the warmest garden
(H2 = 0.48). There was also no trend toward higher or
lower heritability estimates in a particular common garden. These
results suggest that in some environments evolutionary potential is
limited but can increase as environmental conditions and associated
selection pressures change. Furthermore, these heritability increases
are not necessarily associated with a specific direction of change
(i.e., increasing or decreasing temperature). Broad-sense
heritability for the five trait plasticities ranged from 0.09-0.18, a
similar result to bud burst plasticity found in another riparian
deciduous tree, black alder (H2 = 0-0.129, De Kortet al. 2016). Our results of genetic variation in trait
plasticity combined with the evidence for selection based on
QST-FST analysis and non-zero
heritability estimates show selection on the heritable components of
phenotypic plasticity may lead to evolving plasticity across the
landscape among these Arizona populations of Fremont cottonwood.