Climate data
We obtained 30-year (1921–1950) averages of climate data for each
genotype source location from the 270 m resolution California Basin
Characterization Model (BCM) (Flint et al. 2013). These
mid-20th-century values were used instead of more
recent climate data because they more closely resemble the conditions
when the genotypes were establishing as seedlings.
For the GEA, we chose to focus on raw environmental variables rather
than environmental PCA axes, as several previous studies have done
(Eckert et al. 2010, 2015). This is because PCA associations can
be challenging to interpret if, for example, the axes include both
temperature and moisture variables. Instead, we used PCA (Fig. S4) to
select five environmental variables that have low correlation with one
another across tree source locations: mean climatic water deficit (CWD,
a measure of evaporative demand exceeding soil moisture); mean minimum
winter (December-February) temperature (TMIN); mean maximum summer (June
- August) temperature of summer (TMAX); mean monthly winter
precipitation (PPTW); and mean April 1st snowpack
(PCK4). Other climate variables considered but not included in the
analysis were actual evapotranspiration (AET), potential
evapotranspiration (PET), excess water, recharge, runoff, snowfall,
snowmelt, soil water storage, and snow sublimation.