A cornerstone of plant defense theory is that adaptation to the abiotic environment sets the template for defense strategies, imposing a fundamental tradeoff between plant growth and defense. Yet, while this tradeoff is commonly found among species occupying divergent resource habitats, it may not occur across populations of single species. We hypothesize that among populations, growth and defense can both increase across gradients as resources become more available. We evaluated whether plastic and genetically based plant growth and defense traits co-varied across 18 populations of showy milkweed (Asclepias speciosa) inhabiting a large east-west climatic gradient. To test our hypothesis, we measured a suite of traits related to growth strategy (height, stem production, reproduction, SLA, leaf nitrogen, and δC13) as well as defense (latex, cardenolides, and trichomes) in natural populations and in a common garden.
Results/Conclusions
In the natural populations, herbivore numbers and damage increased across the climate gradient towards warmer sites with longer growing seasons. There were also strong positive clinal patterns in several traits related to growth and defense. In a common garden, positive clines related to climatic origin were only evident for defense traits, but not for traits related to growth. We found mostly neutral or positive correlations among growth and defense traits, particularly in the natural populations. Thus, rather than growth and defense tradeoffs, it appears that climate drives positive correlations among traits through variation in resource availability and herbivore pressure. Importantly, these patterns of positive relationships between growth and defense are opposite what much of inter- and intraspecific plant defense theory predicts, suggesting that alternative frameworks are required for understanding intra- versus interspecific variation in plant defense.