Collection sites and common gardens
Sixteen populations of Populus fremontii were collected throughout Arizona, encompassing the environmental variation experienced by the Sonoran Desert ecotype (Ikeda et al. 2017), as well as three populations located on the Colorado Plateau, which fall into a recently described Mogollon Rim ecotype (Blasini et al. 2020; Supplemental Table 1). Cuttings were from individuals collected over 20 m distance from each other to insure independent genotype sampling. Clonal replicates of 12 genotypes per population were planted in the summer and fall of 2014 in three common garden sites after rooting in the greenhouse for ~4 months. Using this design, within and among population variation for phenotypic traits could be assessed.
The three experimental common gardens span wide environmental gradients (Fig. 2, Supplemental Table 1), resulting in extreme climatic transfers for some populations. The northernmost garden 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 located in Yuma, Arizona near Mittry Lake, and is maintained by the Bureau of Land Management. Each common garden is composed of four blocks, with each block made up of 16 randomized population plots. Each population plot has 64 trees composed of replicates of 12 genotypes, arranged in a randomized 8 x 8 grid, with trees spaced 1.85m in each cardinal direction. Collection and garden locations are presented in Fig. 2, and additional garden information can be found in Cooperet al. (2019) and Hultine et al . (2020b).
In order to examine the relationship between climate and traits, we downloaded 21 abiotic climate variables (1961-1990 means) for the 16 provenance sites and the three common gardens using the program ClimateWNA (Wang et al. 2012). To create a multivariate climatic trait representing the environmental variation found throughout the 16 provenances, the ClimateWNA variables plus elevation, latitude, and longitude, were analyzed in a principal component analysis (PCA) usinglabdsv (Roberts 2007) and vegan (Oksanen et al.2016) packages in the R statistical language (R Core Team 2014).