Phenotypic variation in a Populus model system: Implications for ecological interactions across scales of age, space and time

Richard L. Lindroth, Christopher Cole, Michael Eisenring and Jennifer F. Riehl, Dept. of Entomology, University of Wisconsin-Madison, Madison, WI, Olivia L. Cope, Dept. of Entomology, Michigan State University, East Lansing, MI, Kennedy Rubert-Nason, Natural and Behavioral Sciences, University of Maine Fort Kent - Madison, Fort Kent, ME, Michael Eisenring, Forest Entomology, Swiss Federal Research Institute WSL, Birmensdorf, Switzerland, Ken Keefover-Ring, Depts. of Botany and Geography, University of Wisconsin-Madison, Madison, WI, Clay Morrow, Dept. of Forest and Wildlife Ecology, University of Wisconsin-Madison-Madison, Madison, WI, Eric L. Kruger, Dept. of Forest & Wildlife Ecology, University of Wisconsin-Madison, Madison, WI

Background/Question/Methods
Species in the genus Populus function as foundation species in forest habitats throughout much of North America. The extraordinary ecological and evolutionary success of the genus is linked to its unique secondary chemistry, which exhibits striking genotypic, developmental and environmental variation. Here we report on results from an array of long-term common garden, field and laboratory studies that measured diverse tree functional traits to address questions such as: What biochemical/physiological mechanisms underlie tradeoffs between growth and defense? What are the consequences of intraspecific trait variation in Populus for associated organisms (insects, mammals, microbes) and ecosystem function? How do genetic, environmental, and GxE factors shape expression of defense over space and time? And what are the consequences of environmentally-mediated trait tradeoffs for divergent evolutionary trajectories of forest stands?
Results/Conclusions
The use of large-scale and long-term common garden studies of trembling aspen (Populus tremuloides) has afforded insights into trait tradeoffs – and their ecological and evolutionary consequences – that were previously unknown. Our results have revealed that metabolic turnover of salicinoid phenolic glycosides influences rates of tree growth, and that ontogenetic trajectories in chemical defense are genetically variable and likely adaptive. Genome wide association studies and gene expression studies are helping to identify key candidate genes associated with variance in defense traits and aspen-associated insect communities. In aspen, extraordinary phenotypic variation in key defense traits is determined primarily by genetics. Genotypic and environmental variation in Populus chemistry influences the fitness of herbivores and the diversity of associated insect communities. Finally, long-term evaluation of growth-defense tradeoffs in the context of intraspecific competition has shown how environmental variation interacts with genotypic tradeoffs to shift the genetic architecture of forest stands over time. In aggregate, these studies have improved our understanding of how functional trait interactions may influence ecological dynamics and evolutionary processes at the landscape scale.