Compensation through rosette formation: the response of scarlet gilia (Ipomopsis aggregata: Polemoniaceae) to mammalian herbivory

Plants could potentially compensate for floral herbivory by regrowing flowering stalks and by forming additional vegetative stems. Because scarlet gilia (Ipomopsis aggregata (Pursh) V. Grant) is described as monocarpic, its ability to regrow multiple flowering stalks following the removal of its primary inflorescence has been cited as the species’ primary means of compensating for herbivory. However, ancillary rosette formation could also contribute to compensation in subsequent years. To determine if herbivory induces ancillary rosette formation and whether energy diverted to vegetative regrowth reduces reproductive output, we analyzed the response of scarlet gilia to elk herbivory in the Wenatchee National Forest of Washington State. Control plants were protected from herbivory by wire enclosures; clipped plants were hand-cut to simulate herbivory; and grazed plants were left vulnerable to elk herbivory. Ninety percent of plants that lost inflorescences regrew multiple flowering stalks; these plants produced fewer fruits and seeds than protected plants, indicating that scarlet gilia undercompensated for herbivory despite greater aboveground biomass. The plants that regrew multiple flowering stalks were also more likely to form ancillary rosettes, which could increase compensation over multiple seasons. Although herbivory reduced initial fecundity, grazing generated morphological changes that could enable the plant to achieve a greater degree of compensation over time.