Leaf fluctuating asymmetry (FA), a deviation from bilateral symmetry, may be a useful biomarker of maladaptation. We explored the potential of leaf fluctuating asymmetry (FA) of a foundation tree species, Fremont cottonwood (Populus fremontii), to reflect environmental stress between two climate extremes and its implications for assisted migration, plant performance and mortality. This is the first study that has compared FA between natural populations with cloned genotypes in common gardens. We quantified the heritability of FA by comparing 16 populations reciprocally planted in two common gardens representing the upper and lower temperature extremes where these trees naturally occur.
Results/Conclusions
Four patterns emerged. 1. Trees growing in the wild were, on average, 40% more symmetrical than trees growing in common gardens at most temperature based transfer distances, suggesting that plants are locally adapted. 2. Trees with a high temperature difference (>6°C) between their home site to the garden displayed 1.5x higher FA on average, but those shifted relatively small transfer distances (<2°C) were generally not significantly different from their origin sites. Intermediate distances showed a gradient of asymmetry. 3. Greater symmetry was correlated with several measures of relative fitness including diameter at root crown, biomass and mortality suggesting that FA is both ecologically and evolutionarily indicative of local adaptation. 4. Asymmetry as a measure of environmental tolerance exhibits significant broad-sense heritability in which some genotypes are innately more plastic than others (high elevation garden H2B =0.324 and low elevation garden H2B =0.22). We posit that FA results from environmental stressors, which can be an important tool for assessing the efficacy of assisted migration to mitigate the impacts of climate change.