Induction of resistance in plants is a form of phenotypic plasticity that can function as a cost-saving strategy when enemies are scarce. The alternative strategy is continuously expressed constitutive resistance. These two defense strategies have long been observed to trade-off in comparisons between species and most notably so in a successional gradient where early, more inducible species are replaced by constitutively defended species over successional time. There is variation in the defense strategies also within species and examining these traits along a successional gradient can illuminate how temporally variable environments can maintain genetic variation in plant defense trait expression. We studied the changes in constitutive and induced herbivore resistance in Solidago altissima over old-field succession. Using large experimental plots that represent a chronosequence of early to mid succession (1-6 and 14 years) in combination with common garden experiments we examined changes in the herbivore community over succession, genetic changes in S. altissima in resistance to a specialist beetle Trirhabda virgata and secondary metabolites (phenolics and diterpene acids) that mediate resistance to herbivores.
Results/Conclusions
The herbivore abundances in the experimental plots on S. altissima changed in a predictable pattern with increasing loads from year one through six followed by a decrease in late succession. We show that resistance was inducible in the plants originating from intermediate succession (four or six years) which coincides with the largest loads of herbivores and presumably strongest selection pressure for resistance. There were no differences in constitutive resistance between any of the years. However, the composition of phenolic compounds and diterpene acids was affected by the year of succession suggesting that constitutive secondary metabolite production changes over the course of succession. Our study demonstrates that selection for resistance in plants can be highly dynamic and result in shifts of defense strategies over relatively short time periods. Such temporal shifts can be important in maintaining genetic variation for defense strategies within a plant species.