Do Abies religiosa Seedlings That Survived Hot/Dry Range-Edge Conditions Exhibit Genetic Evidence of Selection?

Sacred fir (Abies religiosa) is the only roosting tree for monarch butterfly populations overwintering in Mexico, but its geographic range is expected to shrink due to climate change. A common garden experiment carried out 2019-2022 showed high seedling survival at 3400 and 3000 m, but only 6% survival at 2600 m (representing possible warmer and drier future conditions). We tested whether these survivors were genetically different from the starting population (represented by the seedlings from high-survival gardens) using PCAdapt. We also conducted a Genotype-Environment Association (GEA) test using climate information from source populations to identify SNP variants associated with climate gradients. PCAdapt identified 2202 outlier SNPs whose allele frequencies may have been impacted by selection. LFMM₂ identified 33 SNPs associated with 5 source population climate gradients, especially winter precipitation. Minor allele frequencies (MAF) in both categories showed more fluctuation in survivors. This suggests that local climate conditions may select for genetic differences in Abies religiosa on short as well as long timescales, but fully understanding these responses requires more complete genome or transcriptome data, as well as validation by further field tests.