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).