Collection sites and common gardens
To establish the common gardens, 16 populations of Populus
fremontii were collected throughout Arizona, encompassing the
environmental variation experienced by the Sonoran Desert ecotype, as
well as three populations located on the Mogollon Rim within the
Colorado Plateau region of northern Arizona (Fig. 2). These populations
group genetically with and have been alternatively identified as the
Mogollon Rim (Blasini et al. 2020) or Utah High Plateau ecotype
(Ikeda et al. 2017; Supplemental Table 1). This sampling design
does not include the third described ecotype of the Central California
Valley (Ikeda et al. 2017). Cuttings were taken from individual
tree genotypes located over 20 m away from each other to ensure
independent genotype sampling. Clonal replicates from 12 trees per
population were planted in the summer and fall of 2014 in each of the
three common garden sites after rooting in the greenhouse for
approximately four months.
The three replicated experimental common gardens span broad elevation
and climatic gradients, resulting in extreme climatic transfers for some
populations. The northernmost garden represents the cold edge of the
species’ climatic range. It is located adjacent to Canyonlands National
Park, Utah and is maintained by The Nature Conservancy’s Dugout Ranch.
The middle Arizona garden is located adjacent to the Agua Fria River in
Agua Fria National Monument and is maintained by the Arizona Game and
Fish Department. The southernmost garden is in Yuma, Arizona near Mittry
Lake, and is maintained by the Bureau of Land Management. These gardens
span over a 1500 m elevation difference, a 12°C mean annual temperature
range (10.7°C in Yuma, 17.2°C in Agua Fria, and 22.8°C in Canyonlands),
and a precipitation difference of ~350mm (Supplemental
Table 1). Each common garden was planted with 4,096 trees. These trees
were arranged into four replicated blocks to account for within-garden
environmental variance, with each block made up of 16 randomized
population-level plots. Each population plot had 64 trees, made up of
three to six replicates of the 12 genotypes collected for that
population. Plots were arranged in a randomized 8 x 8 grid, with trees
spaced 1.85m in each cardinal direction. The garden was designed using
population plots instead of fully randomized by genotype to assess
population-level effects on dependent community members such as
arthropods and mycorrhizae, as well as ecosystem-level traits like
carbon flux.
In order to examine the relationship between climate and traits, we
downloaded 30-year normals (1961-1990 means) for 21 abiotic climate
variables for each of the 16 provenance sites and the three common
gardens using the program ClimateWNA (Wang et al. 2012). Because
variation in both temperature and precipitation in the Southwest are
very strongly correlated with elevation, these current climate variables
are excellent proxies for the climates that trees have experienced
during their local evolutionary histories (r > 0.985 for
correlations between current MAT and MAP (WorldClim 2, Fick & Hijmans
2017) and those variables estimated from 6,000 or 22,000 years ago
(WorldClim 1.4, Hijmans et al. 2005). To create a multivariate
climatic index representing the environmental variation found throughout
the 16 provenances, the ClimateWNA variables plus elevation, latitude,
and longitude, were combined in a principal component analysis (PCA)
using labdsv (Roberts 2007) and vegan (Oksanen et
al. 2016) packages in the R statistical language (R Core Team 2014).