Background/Question/Methods Phenotypic plasticity, the ability to alter one’s phenotype in response to changing environmental cues, is one mechanism by which species can respond to the challenges of climate change. In particular, plasticity in key phenological traits is critical to a species' ability to balance optimal growing season length while avoiding damage due to unfavorable environmental conditions; timing that is essential to growth, reproduction, and ultimately survival in extreme climate change events. However, how species respond to multiple climate stresses across broad environmental gradients is largely unknown. Using clonally replicated genotypes from 16 source populations of the riparian tree Fremont cottonwood (Populus fremontii), we evaluated the potential for adaptive phenotypic plasticity in trees reciprocally transplanted across three common gardens spanning an elevation range of ~2000 meters. These plantings encompass a climate transfer range of -10°C to +12°C mean annual temperature, simulating extreme heat or chilling events for some populations. We tested whether plasticity varied between traits, could be predicted by source environments, and is adaptive or maladaptive. Results/Conclusions We found that (1) There are significant genetic (G), environmental (E), and GxE components of variation for both bud set and bud flush across three gardens; (2) Phenotypic plasticity is correlated with provenance climate, where the southern populations exhibited up to four times greater bud flush plasticity and more than three times the bud set plasticity compared to the northern, frost-adapted populations; (3) The relationship between plasticity and survival is highly context dependent: it depends on the magnitude and direction of environmental transfer and the trait of interest. For example, high bud set plasticity for populations transplanted into the coldest garden was negatively correlated with survival, exhibiting non-adaptive plasticity, whereas higher bud flush plasticity of populations transplanted into the hottest, southern garden was adaptive. These results suggest there is an intrinsic limit to plasticity in the northern populations, potentially due to physiological adaptations to freezing winters as compared to southern populations, which rarely experience frost. These insights provide an enhanced understanding of genotype and population level plasticity in riparian ecosystems, and have implications for assisted migration of genotypes across the landscape during current and projected climate warming scenarios.
