A paradigm in the plant defense literature is that defending against herbivores comes at a cost to growth, resulting in a growth-defense tradeoff. However, while there is strong evidence for growth-defense tradeoffs across species, evidence is mixed within species. Several mechanisms can account for this equivocal support within species, but teasing them apart requires examining growth-defense relationships both within and among populations, an approach seldom employed. We examined correlations between plant biomass (growth) and terpene production (defense) within and among populations of Monarda fistulosa, a perennial herb. We sampled populations from Montana and Wisconsin, regions that differ in resource availability characterized by different summer precipitation and associated abiotic conditions that influence plant productivity. We found negative, neutral, and positive growth-defense correlations, depending on the scale examined. Negative correlations occurred across populations originating from divergent regions, positive correlations occurred across populations originating from within the high-resource region, and neutral correlations were found within single populations. Collectively, these results challenge the general expectation of ubiquitous tradeoffs and support emerging views that resource availability (as it affects productivity) shapes the evolution of defense at different scales.

All data were collected by the authors. Missing data are represented by NA or blanks. See the article for details on the methodology.

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