Relative herbivory tolerance and competitive ability in two dominant: subordinate pairs of perennial grasses in a native grassland
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We evaluated herbivory tolerance and competitive ability within two dominant: subordinate pairs of C4, perennial grasses at each of two sites to determine the contribution of these processes to herbivore-induced species replacement. Herbivory tolerance was assessed by cumulative regrowth from defoliated plants of each species and competitive ability was evaluated by relative uptake of a 15N isotope placed into the soil between paired species in the field. Herbivory tolerance was similar for the dominant and subordinate species in both plant pairs and defoliation intensity had a greater influence on herbivory tolerance than did defoliation pattern. Both species within the Sorghastrum nutans: Schizachyrium scoparium pairs exhibited comparable nitrogen acquisition from a 15N enriched pulse with or without defoliation. In contrast, S. scoparium acquired more 15N than did its subordinate neighbor, Bothriochloa laguroides when this species pair was undefoliated. Uniform defoliation of this species pair at a defoliation intensity removing 70% of the shoot mass accentuated this response further demonstrating the greater competitive ability of the dominant compared to the subordinate species. Although the 90% defoliation intensity reduced nitrogen acquisition by the dominant relative to the subordinate species, B. laguroides, it did not reduce nitrogen acquisition by the dominant below that of the subordinate neighbor. The occurrence of similar herbivory tolerance among dominant and subordinate species indicates that selective herbivory suppressed the greater competitive ability, rather than the greater herbivory tolerance, of the dominant grasses in this experimental design. These data suggest that interspecific competitive ability may be of equal or greater importance than herbivory tolerance in mediating herbivore-induced species replacement in mesic grasslands and savannas.
