DIRECT AND INDIRECT EFFECTS OF DROUGHT ON COMPENSATION FOLLOWING HERBIVORY IN SCARLET GILIA

Abstract

Compensation following herbivory is an important element of plant defense; however, variation in compensation under naturally stressful conditions has yet to be evaluated directly. During Arizona's worst drought on record, we explored compensation following ungulate herbivory in a typically overcompensating population of scarlet gilia (Ipomopsis aggregata) near Flagstaff, Arizona, USA. In a natural survey of browsed and unbrowsed plants, we documented severe undercompensation. We tested two factors that potentially contributed to this atypical pattern: (1) the direct effect of drought stress on the capacity to compensate (following the typical single bout of herbivory) and (2) the indirect effect of drought stress on compensation through increased ungulate browsing. Using a 2 × 2 factorial experiment (with or without supplemental water, with or without clip-simulated herbivory typical of non-drought years), we found that water availability limited compensation (direct effect). In a second experiment examining the indirect effects of drought, we found that ungulates browsed scarlet gilia at unprecedented levels during the drought year, resulting in severe undercompensation. We conclude that, although water was an important limiting factor for compensation during the drought year, the overriding factor determining fitness achieved through compensation was the indirect effect of drought, i.e., drought-year-associated ungulate browsing.