Contribution of flexible allocation priorities to herbivory tolerance in C4 perennial grasses: an evaluation with 13C labeling.

The ability of plants to rapidly replace photosynthetic tissues following defoliation represents a resistance strategy referred to as herbivory tolerance. Rapid reprioritization of carbon allocation to regrowing shoots at the expense of roots following defoliation is a widely documented tolerance mechanism. An experiment was conducted in a controlled environment to test the hypothesis that herbivory-sensitive perennial grasses display less flexibility in reprioritizing carbon allocation in response to defoliation than do grasses possessing greater herbivory tolerance. An equivalent proportion of shoot biomass (60% dry weight) was removed from two C4 perennial grasses recognized as herbivory-sensitive, Andropogon gerardii and Schizachyrium scoparium, and two C4 perennial grasses recognized as herbivory-tolerant, Aristida purpurea and Bouteloua rigidiseta. Both defoliated and undefoliated plants were exposed to 13CO2 for 30 min, five plants per species were harvested at 6, 72 and 168 h following labeling, and biomass was analyzed by isotope ratio mass spectrometry. The tallgrass, A. geraiddii, exhibited inflexible allocation priorities while the shortgrass, B. rigidiseta, exhibited flexible allocation priorities in response to defoliation which corresponded with their initial designations as herbivory-sensitive and herbivory-tolerant species, respectively. A. gerardii had the greatest percentage and concentration of 13C within roots and lowest percentage of 13C within regrowth of the four species evaluated. In contrast, B. rigidiseta had a greater percentage of 13C within regrowth than did A. gerardii, the greatest percentage of 13C within new leaves of defoliated plants, and the lowest concentration of 13C within roots follwing defoliation. Although both midgrasses, S. scoparium and A. purpurea, demonstrated flexible allocation priorities in response to defoliation, they were counter to those stated in the initial hypothesis. The concentration of 13C within new leaves of S. scoparium increased in response to a single defoliation while the percentage and concentration of 13C within roots was reduced. A. purpurea was the only species in which the percentate of 13C within new leaves decreased while the percentage of 13C within roots increased following defoliation. The most plausible alternative hypothesis to explain the inconsistency between the demonstrated responsiveness of allocation priorities to defoliation and the recognized herbivory resistance of S. scoparium and A. purpurea is that the relative ability of these species to avoid herbivory may make an equal or greater contribution to their overall herbivory resistance than does herbivory tolerance. Selective herbivory may contribute to S. scoparium's designation as a herbivorysensitive species even though it possesses flexible allocation priorities in response to defoliation. Alternatively, the recognized herbivory resistance of A. purpurea may be a consequence of infrequent and/or lenient herbivory associated with the expression of avoidance mechanisms, rather than the expression of tolerance mechanisms. A greater understanding of the relative contribution of tolerance and avoidance strategies of herbivory resistance are required to accurately interpret how herbivory influences plant function, competitive interactions, and species abundance in grazed communities.