Source Climate Variability and Predictability Drive Phenotypic Plasticity in Fremont Cottonwood
Hillary Cooper1*, Lela Andrews2, Jaclyn Corbin3, Iris Garthwaite4, Michael Eisenring5, Richard Lindroth6, Kevin Grady7, Catherine Gehring8, Kevin Hultine9, Thomas Whitham10, Gery Allan11, Rebeca Best12
1Center for Adaptable Western Landscapes, Northern Arizona University, Flagstaff, AZ
2Tecan Genomics, Inc., Redwood City, CA
3USDA Animal and Plant Health Inspection Service, Washington D.C.
4U.S. Fish and Wildlife Service, Reno, NV
5Forest Entomology, Swiss Federal Research Institute WSL, Birmensdorf, Switzerland
6Department of Entomology, University of Wisconsin-Madison, Madison, WI
7School of Forestry, Northern Arizona University, Flagstaff, AZ 786001
8Department of Biological Sciences and Center for Adaptable Western Landscapes, Northern Arizona University, Flagstaff, AZ
9Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix, AZ
10Department of Biological Sciences and Center for Adaptable Western Landscapes, Northern Arizona University, Flagstaff, AZ
11Department of Biological Sciences and Center for Adaptable Western Landscapes, Northern Arizona University, Flagstaff, AZ
12School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ
Rapid climate change is raising urgent questions about which species, populations, and genotypes are prepared to persist, and by what mechanisms. In the American Southwest, riparian systems already impacted by prolonged drought 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 cottonwoods’ 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. Previous research has demonstrated significant correlations between trait plasticity and mean climate. In this study, we specifically address the hypothesis that populations that have experienced more variable and predictable climates over their evolutionary history will exhibit increased trait plasticity compared to populations with relatively stable and/or unpredictable 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 and predictability using rho, a temporal autocorrelation coefficient. 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 source climate, but also with climate variability and predictability. 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.