EFFECTS OF SIMULATED HERBIVORY IN 3 OLD FIELD COMPOSITAE WITH DIFFERENT INFLORESCENCE ARCHITECTURES

The effects of simulated herbivory (early or late defoliation and cutt ing of the flowering shoot) on the growth and reproduction of three sp ecies of monocarpic composite forbs (Crepis pulchra, Picris hieracioid es and C. foetida) with different inflorescence architectures were stu died in experimental plots. For the three species studied, early defol iation had no significant effect on subsequent growth. In contrast, la te defoliation, occurring at the start of the season of drought, had a negative effect on growth and reproduction in the two Crepis species, particularly C. foetida, but had less effect on P. hieracioides. Sexu al biomass was more clearly affected by late defoliation than was vege tative biomass, although the effects differed markedly among species p ossibly as a result of differences in phenology. Clipping the flowerin g shoot removed about 3 times less biomass than late defoliation and h ad little effect on vegetative biomass. It had much greater effects on the sexual biomass in P. hieracioides and C. pulchra, and resulted in the production of many shoots sprouting from the rosette, allowing th e treated plants to regain a vegetative biomass close to that of contr ol plants. Clipping did however lead to the production of shorter shoo ts and a reduction in the number of capitula formed. In C. foetida, mu ch branching occurred even when the main shoot was not cut; the archit ecture of individual plants was therefore only slightly changed by cli pping the apical bud and the sexual biomass of this species was not af fected by ablation of the flowering shoot. Overcompensation was found in only two families of C. pulchra for vegetative biomass. No over-com pensation was found for sexual biomass, despite an increase in the num ber of flowering shoots in C. pulchra and P. hieracioides following cl ipping. However situations close to compensation for the vegetative bi omass in the three species and in P. hieracioides for the sexual bioma ss were recorded. The response of the three study species to simulated herbivory were related to their architecture and to the time of defol iation.