A functional interpretation of herbivory resistance in mesic grasslands

Abstract

Herbivore tolerance among species differed from the general assumption of greater tolerance in late-seral dominants and varied with defoliation intensity. Tolerance was greatest in two of the three species designated as mid-seral following three defoliations at the most severe intensity (90%) in a season. Therefore, the expression of greater tolerance of late-seral compared to mid-seral species at the moderate defoliation intensity (70%) was reversed based on the increased expression of tolerance in mid-seral species at the most severe defoliation intensity. Similar magnitudes of []N acquisition among the two late-seral species indicate more equable competitive ability compared to the mid-seral species, B. Laguroides. The increase in defoliation intensity from 70 to 90% represents a threshold where both tolerance and competitive ability are reduced in late-compared to mid-seral species. Expressions of herbivore tolerance were found to be generally recognized and they were influenced by more dynamic than defoliation intensity and neighborhood interactions. The origin and magnitude of regrowth was impotent in explaining differences in the expressions of tolerance under the different defoliation intensities. Species expressed varying abilities to regrow by leaf production from apical meristems versus initiation of new tillers from axillary buds following defoliation. The two late-seral species, S. nutans and S. scoparium, had the highest shoot production from apical meristems, while the mid-seral highest tiller recruitment following species, S. asper and B. Laguroides, had the the more severe defoliation intensity. Contrasting expressions of herbivore tolerance with increasing defoliation intensity may represent the point at which a trade-off occurs between competitive ability and herbivore tolerance in these two groups. The occurrence of similar herbivore tolerance among species at moderate defoliation intensities supports the role of selective herbivore in modifying the relative composition among species of varying seral status. However, increased expression of herbivore tolerance in mid-seral species may potentially contribute to herbivore-induced species replacement in medic grasslands under conditions of intense defoliation.