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Despite an increased focus on multiscale relationships and interdisciplinary integration, few macroecological studies consider the contribution of genetic-based processes to landscape-scale patterns. We test the hypothesis that tree genetics, climate, and geography jointly drive continental-scale patterns of community structure, using genome-wide SNP data from a broadly distributed foundation tree species (Populus fremontii S. Watson) and two dependent communities (leaf-modifying arthropods and fungal endophytes) spanning southwestern North America. Four key findings emerged: (1) Tree genetic structure was a significant predictor for both communities; however, the strength of influence was both scale- and community-dependent. (2) Tree genetics was the primary driver for endophytes, explaining 17% of variation in continental-scale community structure, whereas (3) climate was the strongest predictor of arthropod structure (24%). (4) Power to detect tree genotype—community phenotype associations changed with scale of genetic organization, increasing from individuals to populations to ecotypes, emphasizing the need to consider nonstationarity (i.e., changes in the effects of factors on ecological processes across scales) when inferring macrosystem properties. Our findings highlight the role of foundation tree species as drivers of macroscale community structure and provide macrosystems ecology with a theoretical framework for linking fine- and intermediate-scale genetic processes to landscape-scale patterns. Management of the genetic diversity harbored within foundation species is a critical consideration for conserving and sustaining regional biodiversity. [ABSTRACT FROM AUTHOR]

• We used a common garden experiment to study drought sensitivity in non-native Tamarix • Some Tamarix populations are more sensitive to soil water deficits than others. • Drought resiliency in Tamarix increases with root area to leaf area ratios • Freeze-thaw exposure 'non-adaptively' reduces drought sensitivity in Tamarix Rapidly colonizing species often thrive in a wide-range of conditions due to a high degree of phenotypic plasticity that results in populations of "general purpose" genotypes. Alternatively, species with high genetic variation could rapidly respond to forces of selection such that a local population evolves traits that provide an advantage in its local environment. We tested this generalist versus local adaptation paradigm in a widely distributed, recently introduced riparian tree / shrub, Tamarix spp. Using an eighty-day, mid-summer common garden drought experiment, we tested three inter-related hypotheses: 1) stomatal sensitivity to soil water depletion is lower in genotypes from a low-elevation, ephemerally flowing river than genotypes from a low-elevation, perennially flowing river, indicative of local adaptation to hydrological conditions, 2) stomatal sensitivity to soil water depletion is lower in genotypes from high-elevations with regular freeze-thaw exposure than genotypes sourced from locations with no freeze-thaw exposure, indicative of local adaptation to temperature conditions, and 3) differences among genotypes in drought sensitivity are correlated with differences in fine root area to leaf area ratios (A r : A l). For the most part, results did not support hypothesis 1, but largely supported hypotheses 2 and 3. Specifically, sap-flux scaled canopy transpiration (E l) declined 14 days earlier in the low-elevation populations after drought initiation compared to E l of the highest elevation population with the highest freeze-thaw exposure. Root area to leaf area ratios of the two low-elevation populations were also only 42% and 55% of A r : A l in the highest elevation population. Results indicate that the high degree of reported Tamarix hybridization since its introduction to North America has largely produced a swarm of "generalist" genotypes in terms of drought sensitivity. Nevertheless, rapid changes in ecohydrologic conditions may result in some Tamarix populations becoming maladapted sooner to reductions in available water than others in the western US. [ABSTRACT FROM AUTHOR]