Data from: Population variation, environmental gradients, and the evolutionary ecology of plant defense against herbivory

A central tenet of plant defense theory is that adaptation to the abiotic environment sets the template for defense strategies, imposing a tradeoff between plant growth and defense. Yet, this tradeoff, commonly found among species occupying divergent resource environments, may not occur across populations of single species. We hypothesized that more favorable climates and higher levels of herbivory would lead to increases in growth and defense across plant populations. We evaluated whether plant growth and defense traits co-varied across 18 populations of showy milkweed (Asclepias speciosa) inhabiting an east-west climate gradient, spanning 25° of longitude. A suite of traits impacting defense (e.g., latex, cardenolides), growth (e.g., size), or both (e.g., SLA, trichomes) were measured in natural populations and in a common garden, allowing us to evaluate plastic and genetically based variation in these traits. In natural populations, herbivore pressure increased towards warmer sites with longer growing seasons. Growth and defense traits showed strong clinal patterns and were positively correlated. In a common garden, clines with climatic origin were only recapitulated for defense traits. Correlations between growth and defense traits were also weaker and more negative in the common garden compared to the natural populations. Thus, our data suggest that climatically favorable sites likely facilitate the evolution of greater defense at minimal costs to growth, likely because of increased resource acquisition