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Contributed Talk

Session: : Evolution: Selection And Adaptation 1

COS 105-4 - Climate contrasts and variability as drivers of selection on phenotypic plasticity in Fremont cottonwood

Tuesday, August 8, 2023
2:15 PM – 2:30 PM PDT
Location: B116
Abstract: Rapid climate change is raising urgent questions about which species and genotypes are prepared to persist, and by what mechanisms. In the American Southwest, riparian systems already impacted by climate change are often characterized by cottonwood trees, a foundation species that supports diverse communities and modulates water and carbon fluxes along riverways. Therefore, the success of riparian forests may largely depend on how well cottonwoods track changing environments through genetic adaptation and/or phenotypic plasticity. Given the rapid pace of climate change in the Southwest and long generation time of forest trees, phenotypic plasticity likely plays a crucial role in cottonwood’s climate response and population persistence.



Using a replicated common garden experiment, we assessed the degree of phenotypic plasticity in a suite of phytochemical traits across three environments and multiple years. We then tested if the magnitude of plasticity was related to source population climate variability. Previous research has demonstrated significant correlations between both traits and trait plasticity and mean climate. In this study, we specifically address the hypothesis that populations that have experienced more variable climates over their evolutionary history will exhibit increased trait plasticity compared to populations with relatively stable climates. We use bioclimatic variables from the past 120 years to estimate climate variability of each population using decadal coefficients of variation for each climate variable. Finally, using a genetic dataset of > 9,000 SNPs, we tested whether tree traits and trait plasticity were under selection using QST-FST comparisons and climate-phenotype regressions.



We found strong evidence of climate-driven divergent selection on some traits and trait plasticities. Population differences in phytochemical means and plasticities were correlated not just with mean source climates, but also with annual climate variability. For some traits, increased plasticity was correlated with increased climate variation, supporting the hypothesis of evolved plasticity under increased environmental variability. However, other traits showed the opposite response.



These results illustrate how large climatic gradients can act as an agent of selection to differentiate population-level traits and trait plasticities. Considering historic climate regimes may help us identify populations best suited to future environments, including those with phenotypic plasticity in those traits that are important for both tree survival and ecosystem processes.