Optimal theory for plant defense predicts evolutionary tradeoffs between the ability to defend and to compete. Thus, the relaxation of herbivory is expected to increase plants' competitive ability. Here, we tested for the tradeoff in Solidago altissima using genotypes that originated from long-term (12 years) herbivore-removal experiment, where half of the plots were continuously sprayed with insecticide (sprayed regime) and the rest unsprayed (control). Plants compete through resource exploitation and/or interference via allelopathy. Because S. altissima produces a known allelopathic compound, dehydromatricaria ester (DME), the system provides an unique opportunity to explore the mechanisms of increased competitive ability in response to the relaxation of herbivory.  
Results/Conclusions

In a common garden without competitors, sprayed genotypes grew faster and produced more root DME than control. Consistent with the tradeoff hypothesis, sprayed genotypes exhibited a pattern suggesting a higher competitive ability against a conspecific competitor than control genotypes. However, when the effect of DME was neutralized by the addition of activated carbon, sprayed genotypes competed poorly compared to control plants. Results suggest that increased levels of DME is favored for maintaining a high competitive advantage in sprayed plots where intraspecific competition might be more intense.