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.