Seedling sampling
In August 2019, we sampled 90 seedlings per garden (3 siblings/maternal tree) representing 30 maternal trees originating from 10 populations (3 maternal trees/population) (Fig. 1) across the P. strobiformis-P. flexilis hybrid zone (Menon et al ., 2018). To evaluate the selective pressures experienced by the seedlings in the gardens relative to that in their native range we used climate variables at 1 km resolution from ClimateWNA v6.1 (Wang et al ., 2016). Population-level estimates of climate variables were obtained by averaging values across the three maternal trees selected per population. During 2019, the high elevation cold garden experienced drier and colder conditions relative to the values noted across a 60-year period in the native range of the sampled hybrid populations. The low elevation warm garden, experienced warmer and drier conditions relative to the sampled populations and relative to the cold garden as well (Fig. S1). The seedlings in each garden were further exposed to a 50% experimental reduction in growing season precipitation for three growing seasons prior to sampling in 2019 (see Bucholz et al . (2020) for details).