Changes in Reproduction and Architecture in Flowering Dogwood, Cornus florida, after Attack by the Dogwood Club Gall, Resseliella clavula

Abstract

Plants exhibit a range of responses in growth and reproduction to attack by herbivores from loss, to no effect, to overcompensation. We assessed herbivore impact on branch architecture, growth, and sexual reproduction by flowering dogwood, Cornus florida, for four years after attack by a specialist stem gall-forming fly, Resseliella clavula. We estimated the immediate and long-term consequences of herbivory for vegetative growth and flower and fruit production on marked shoots that were galled or ungalled at the outset of our study. The response of dogwood trees to herbivory was complex with an initial negative effect on flower and fruit production, followed by compensatory shoot elongation and inflorescence production after three years, and a tendency toward overcompensation after four years. When observations on fruit production were terminated at the end of year three, fruit production on galled shoots was still suppressed relative to ungalled shoots. Attack by the gall-former in the initial year of the study caused an immediate reduction in shoot survival and a 46% decrease in inflorescence and 79% loss in fruit production relative to ungalled shoots. After four years, compensation in both vegetative growth and inflorescence production was detected. A tendency toward overcompensation within the fourth year was indicated by 24% greater shoot elongation and 90% more inflorescence production on galled shoots than on ungalled shoots. Compensation in vegetative growth on surviving galled shoots involved increases in the average length and the number of vegetative modules produced. Reproductive compensation occurred because a greater number of vegetative modules gave rise to reproductive buds on galled shoots than on ungalled shoots in the third and fourth years of the study. Our results suggest that in long-lived species, the initial response of the plant may not be an adequate measure of the impact of herbivores on plant fitness. Responses to herbivores may extend for several years beyond the year of attack, and compensatory or overcompensatory responses may only become apparent after several growing seasons.