Investigating how genotype and environment influence the growth and drought response physiology of quaking aspen trees

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Kelly L. Kerr and William R.L. Anderegg, School of Biological Sciences, University of Utah, Salt Lake City, UT

Background/Question/Methods
Many tree species will likely be unable to migrate or adapt quickly enough to keep pace with climate change, but may be able to acclimate to rapid environmental change through phenotypic plasticity. Determining the extent to which phenotypic variation results from plasticity, local adaptation, or a combination of the two remains a challenge. We investigated within-species variation in drought response traits for quaking aspen (Populus tremuloides) using both a sapling common garden and in-situ mature forest stands to answer the following questions: 1) To what extent are traits influenced by genotype and environment? 2) Is there a trait-mediated trade-off between growth under wet conditions and survival under drought conditions? Aspen propagules were grown from root segments collected from five geographic populations across Utah and Colorado and planted into a common garden during spring 2020 on the University of Utah campus. Growth and morphological and physiological drought response traits were measured in saplings in the common garden (summer 2021) and mature stands (summer 2020) in the field. Growth was also quantified in mature stands using tree cores.

Results/Conclusions
Preliminary conclusions provide evidence that among the aspen populations investigated here, within-species variation in drought response traits was due to both phenotypic plasticity and local adaptation. At the end of the 2020 growing season aspen saplings in the common garden showed significant differences in growth, whereby saplings that originated from cooler, wetter climates grew more. We also observed variation in measured drought response traits among the common garden populations, indicating that genotype does influence trait expression. Among the ­in-situ mature forest stands, we observed substantial variation in morphological and physiological traits. Stands that occurred in cooler, wetter climates exhibited higher rates of leaf area-specific conductivity and less percent loss of hydraulic conductance in situ during summer 2020. Interestingly, stands that occurred in warmer, drier climates exhibited higher leaf area during summer 2020 which may have been due to earlier leaf out in response to warmer-than-average spring temperatures. Results from this work will help improve predictions of how tree species/populations will respond to future climate change and will inform forest management and conservation efforts.