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Assimilation to the internal CO₂ (<i>ACi</i>) response curve and gas exchange parameters were quantified for four North American willows ((<i>Salix cordata</i> (COR), <i>S. discolor</i> (DIS), <i>S. eriocephala</i> (ERI), and <i>S. interior</i> (INT)) grown in a 2 × 2 factorial of atmospheric CO₂ and soil moisture treatments to see how they would respond to climate change factors. After the first year of greenhouse growth under said treatments, we saw no difference in the aboveground stem biomass between CO₂ treatments. Thus, in the second year, a second experiment on a subset of well-watered, coppiced willows was conducted in a 2 × 2 factorial of atmospheric CO₂ and soil fertilization (FERT) treatments. In both experiments, the maximum rate of carboxylation (<i>V_cmax</i>) significantly declined for all four species in response to elevated CO₂ (eCO₂). In response to a drought treatment (DRT), <i>V_cmax</i> declined, except for INT, which increased <i>V_cmax</i>. In both experiments, INT had the greatest <i>V_cmax</i>, maximum rate of electron transport (<i>J_max</i>), and triose phosphate utilization, followed by COR and ERI, with DIS having the lowest values. FERT resulted in a strong increase in assimilation (<i>A</i>) and stomatal conductance (<i>G_wv</i>) by 92 and 119%, respectively. <i>G_wv</i> is the primary driver and <i>A</i> is a minor driver of water use efficiency (WUE) under DRT. FERT mitigated the <i>V_cmax</i> and <i>A</i> downregulation in eCO₂, but eCO₂ did not mitigate the DRT downregulation effect. Differences between INT and the other three willows in a number of adaptive traits and responses related to drought may reflect the evolutionary origins of INT and the taxonomic group <i>Longifoliae</i> in the arid southwest USA and Mexico.