Inconsistent annual compensation for floral herbivory by an iterocarpic thistle

Authors: West, Natalie; LOUDA, SVATA - University Of Nebraska

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2021
Publication Date: 10/19/2021
Citation: West, N.M., Louda, S.M. 2021. Inconsistent annual compensation for floral herbivory by an iterocarpic thistle. American Journal of Botany. 108(10):1889-1901. https://doi.org/10.1002/ajb2.1744.
DOI: https://doi.org/10.1002/ajb2.1744

Interpretive Summary: Insect floral herbivores can drastically reduce plant seed production, and can drive selection for plant strategies that can compensate for seed lost to damage. However, such compensation may not always be beneficial for plants that flower multiple times (iterocarps). These responses can compete with other resource investments, and plants may be able to recoup fitness losses in years with when herbivore abundance is low. We examined whether plants redistributed flowering and seed production in response to damage, and if this sufficiently compensated for seed lost to floral herbivores, in two years. We found that plants had the capacity to compensate for experimentally imposed damage both years, but only fully compensated for floral herbivory impacts on seed production in one year. These findings provide needed insight into tradeoffs in how plants allocate resources to counteract herbivore damage within years to maximize their lifetime fitness

Technical Abstract: • Premise of the study: Plants experiencing steep reproductive losses from herbivores should favor plant strategies that promote tolerance or resistance to herbivory damage. However, the degree to which such strategies succeed in altering 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) in response to apical damage provided within-season tolerance for fitness losses to insect floral herbivores. • Methods: We manipulated the strength of apical dominance and the level of cumulative floral herbivory on later-flowering, non-apical flower heads for two seasons. We asked: (1) Did C. undulatum under strong insect herbivore pressure have the potential to tolerate apical flower head loss through increased reproductive effort from later flowering heads?; (2) if so, to what extent did such tolerance increase total seed reproduction over the flowering season? • Key Results: First, plants did compensate for apical head loss, measured as increased seed contribution from later, lower positioned flower heads when cumulative herbivory was experimentally reduced. Second, despite evidence of compensatory potential for apical head loss, positive fitness consequences occurred only in the year that plant size, and so seed production potential, was lower and insect herbivory pressure was higher. • Conclusions: This iterocarpic species can potentially tolerate apical damage, but only fully compensate for floral herbivory under some conditions. Lower levels of the variable floral herbivory and better seasonal growing conditions limit the contribution of that tolerance to plant annual reproductive fitness.