Replicated Common Gardens Reveal High Variation in Traits and Phenotypic Plasticity among Fremont Cottonwood Populations with Consequences for Adaptation to Future Climates
Hillary Cooper1*, Kevin Grady2, Jacob Cowan3, Rebecca Best4,
Gerard Allan5, and Thomas Whitham6
1 Department of Biological Science, Northern Arizona University, Flagstaff, AZ USA; hfc5@nau.edu
2School of Forestry, Northern Arizona University, Flagstaff, AZ USA; kevin.grady@nau.edu
2School of Forestry, Northern Arizona University, Flagstaff, AZ USA; kevin.grady@nau.edu
3School of Forestry, Northern Arizona University, Flagstaff, AZ USA; jac865@nau.edu
4School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ USA; Rebecca.Best@nau.edu
5Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ USA; Gery.Allan@nau.edu
6Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ USA; Thomas.Whitham@nau.edu
One mechanism to meet the challenge of rapidly shifting environments is phenotypic plasticity: altering phenotype in response to environmental change. Here, we investigated the magnitude, direction, and consequences of changes in key phenology and growth traits in a widespread riparian tree species, Populus fremontii. Using replicated genotypes from 16 populations from throughout Arizona, and reciprocal common gardens at hot, warm, and cool sites, we identified four major findings: 1) There are significant genetic (G), environmental (E), and GxE components of variation for functional traits across three common gardens; 2) The magnitude of phenotypic plasticity for phenology is correlated with provenance climate, where trees from hotter, southern populations exhibited up to four times greater plasticity compared to the northern, frost-adapted populations; 3) Populations exhibit local adaptation where mortality increases as the climate transfer distances between provenance and garden increase; and 4) The relationship between plasticity and survival depends on the type of environmental stress (i.e. heat or freezing) and how particular traits have evolved in response to that stress. Trees transferred to warmer climates generally showed small to moderate shifts in an adaptive direction, a hopeful result for climate change. Trees experiencing cooler climates exhibited large, non-adaptive changes, suggesting smaller transfer distances for assisted migration. Understanding the magnitude and adaptive nature of phenotypic plasticity of multiple traits responding to multiple environmental cues is key to guiding restoration management decisions as climate continues to change.