Evidence of climate-driven selection on tree traits and trait plasticity across the climatic range of a riparian foundation species

Selection on quantitative traits by heterogeneous climatic conditions can
lead to substantial trait variation across a species range. In the context
of rapidly changing environments, however, it is equally important to
understand selection on trait plasticity. To evaluate the role of
selection in driving divergences in traits and their associated
plasticities within a widespread species, we compared molecular and
quantitative trait variation in Populus fremontii (Fremont cottonwood), a
foundation riparian distributed throughout Arizona. Using SNP data and
genotypes from 16 populations reciprocally planted in three common
gardens, we first performed QST-FST analyses to detect selection on traits
and trait plasticity. We then explored the environmental drivers of
selection using trait-climate and plasticity-climate regressions. Three
major findings emerged: 1) There was significant genetic variation in
traits expressed in each of the common gardens and in the phenotypic
plasticity of traits across gardens, both of which were heritable. 2)
Based on QST-FST comparisons, there was evidence of selection in all
traits measured; however, this result varied from no effect in one garden
to highly significant in another, indicating that detection of past
selection is environmentally dependent. We also found strong evidence of
divergent selection on plasticity across environments for two traits. 3)
Traits and/or their plasticity were often correlated with population
source climate (R2 up to 0.77 and 0.66, respectively). These results
suggest that steep climate gradients across the Southwest have played a
major role in shaping the evolution of divergent phenotypic responses in
populations and genotypes now experiencing climate change.