Management implications
When local adaptation and phenotypic differentiation in forest trees are closely tied to variation in climate, populations may become increasingly maladapted as climate change continues (Shaw & Etterson 2012; Franks et al. 2014; Aitken & Bemmels 2015). However, the magnitude of climate change combined with the degree of genetic variability, heritability, and phenotypic plasticity of traits will all interact to determine the extent of adaptation or maladaptation. Maladaptation due to climate change is expected to be greatest in populations from the warmest extent of their range, while populations at the cold edge may benefit from slightly warmer temperatures (Aitken & Bemmels 2015). This expectation corresponds with the maladaptive phenological plasticity we have observed in southern populations vs. adaptive phenological plasticity in northern populations (Cooperet al. 2019). However, nuanced changes in temperature and precipitation patterns will produce novel genotype-climate associations, creating more complex climate responses compared to the poleward range shifts and vulnerable trailing edges traditionally associated with warming (Gougherty et al. 2021). Although our study does not encompass the full genetic and geographic range of Fremont cottonwood, our results of declining performance as climate transfer distance increases suggests that this species will likely experience maladaptation as local conditions become more arid, especially for southern populations that are close to their thermal tolerance (Aultet al. 2014; see Fig. 1). Because these trees are important foundation species of riparian systems, selecting genotypes with sufficient performance under warming conditions is essential for the persistence of associated communities and ecosystems.