Inconsistent annual compensation for floral herbivory by an iterocarpic thistle.

Abstract

Plants experiencing steep reproductive losses from herbivores should favor strategies promoting tolerance or resistance to that herbivory. However, the degree to which such strategies succeed in improving plant fitness under natural conditions needs further evaluation, especially for iterocarpic species. We tested whether reproductive effort by the iterocarpic Cirsium undulatum Spreng. (Wavyleaf thistle) provided within-season tolerance for floral herbivory through response to apical damage. We imposed apical damage and manipulated floral herbivory on later-flowering, non-apical flowerheads for two seasons. We asked: (1) is there evidence of compensatory potential to tolerate apical flowerhead damage? If so, (2) does the amount of herbivore pressure on non-apical flowerheads influence the magnitude of any compensatory response; and (3) is the response to apical damage sufficient to increase plant seed production under ambient floral herbivory over the flowering season? Plants showed compensatory potential for apical head loss; apical damage increased seed contributions from later, lower positioned flowerheads. Further, the intensity of subsequent herbivore pressure influenced compensation outcomes. Equitable seed production under both levels of ambient herbivory occurred only in the year in which plants were larger and insect pressure was lower. Finally, the response to apical damage was sufficient to compensate for apical seed loss, but it did not consistently increase overall annual seed production under ambient floral herbivory. Although this iterocarpic species can compensate for apical damage, tolerance for floral herbivory varied between years.