COS 46-4
Testing growth-defense trade-off among 14 willow and poplar species along a hydrological gradient
Species distributions are limited by both abiotic and biotic factors. A trade-off between growth and defense, caused by adaptions to high or low nutrient availability (Coley et al. 1986), could reinforce habitat specialization among closely-related species. It is still unclear if it could also promote species co-existence within the same habitats, and if the phenotypic plasticity of defense traits could alter the trade-off along environmental gradients. We conducted a reciprocal transplant experiment with an insect exclusion treatment using 14 co-occurring willow and poplar species with contrasting distributions along a hydrological gradient to address two questions: 1) Does the growth-defense trade-off promote co-existence by reducing performance differences among species? 2) Does plastic response of defense traits influence plant performance along the hydrological gradient?
We established 20 pairs of common gardens in the field. Each pair contained a wetland and an upland garden. In each garden we planted two individuals of each species, to which we applied a real and a sham cage (cage with openings) and measured survival and growth for two years. In the second year, we also measured herbivory as percentage of damaged leaves and collected leaves to analyze defense compounds (condensed tannins and phenolic glycosides).
Results/Conclusions
Analyses of growth and herbivory showed a significant species effect of herbivory, such that species were differentially impacted by herbivores. The cage treatment significantly reduced herbivory damage. Among species, the differences in herbivory between real and sham cage treatment was positively correlated with differences in relative growth rates. This evidence suggests that species differ in their resistance to herbivory, which may, in turn, have metabolic consequences that impact growth. In contrary to our hypothesis, however, we did not find evidence supporting a growth-defense trade-off among species: herbivory damage and relative growth rates were not significantly correlated, when examined in the same habitats (i.e. upland or wetland gardens only) or across the hydrological gradient. We also found that herbivory damage was higher in the upland gardens than the wetland gardens, and there was no interaction effect between garden types and species. These results suggest that herbivory varied along the hydrological gradient, but defense traits may not show a plastic response to the gradient as hypothesized.