Rangewide patterns of climate adaptation and phenotypic plasticity in western larch

Background/Question/Methods

: Shifting climates are disrupting historical patterns of local adaptation in tree populations, introducing a pressing need to understand the risks posed by climate change and mitigation strategies such as assisted gene flow. Climate niche modeling projects that areas of future suitable climate for western larch (Larix occidentalis Nutt.) will stretch far north and west of its contemporary distribution, making it a desirable candidate for assisted migration trials. It was the first conifer species approved to be operationally planted north of its native range in British Columbia, and these plantings have demonstrated its reforestation potential on many sites throughout the province. Previous common garden studies indicate weak geographic clines for growth and other adaptive traits compared to co-occurring evergreen conifer species, suggesting populations may possess high phenotypic plasticity and standing genetic variation. However, no prior studies have leveraged a combination of phenotype- and genotype-environment associations to test patterns of local adaptation to climate among populations across the extent of its current distribution. We assessed phenotypic trait data on growth, phenology, cold hardiness of 52 natural and 28 selectively-bred populations as seedlings in common garden experiments at the UBC campus in Vancouver, British Columbia. Moderate drought treatments were imposed in 2020 and severe drought treatments are planned for summer 2021 and to assess population-level plasticity (GxE) and correlations of phenotypic traits among treatments. We have also generated exome-capture sequence data for pooled samples from 45 populations for genotype-environment association analyses to identify the strength of and climatic drivers of local adaptation.

Results/Conclusions

: Preliminary analyses of total height growth suggest relatively weak differentiation among natural populations, but selectively-bred populations have faster height growth compared to natural populations. This is likely due to genetic differences for plastic late-season indeterminate growth. Populations at the geographic center of the range show comparable height growth to the selectively-bred populations. Southern natural populations have lower fall cold hardiness compared to central and northern populations, though variation exists among populations regionally, which may be explained by local variation in elevation and minimum winter temperatures. Results from analyses of phenotypic traits including GxE among treatments will be combined with genomic data to assess vulnerabilities related to climate change and inform assisted gene flow strategies for matching seed sources with emerging climates.