Defoliation can affect plant competition in at least two ways: negative effects on the performance of defoliated plants and positive effects on nondefoliated plants due to neighbor defoliation. It has been suggested that these effects can alter competitive relations between plants, and this study was designed to test this in an experiment using Crofton weed as the invasive category, and its two neighbors Japanese brome and Dichrocephala as the native category. The plants were grown from seed and potted either singly or in pairs. The pairs consisted either of two conspecific plants (same pairs) or of one plant of each category (mixed pairs). Randomly preselected plants were defoliated twice in the growing season, at 9 wk and again at 5 wk before final biomass sampling. Competition reduced growth by 51 to 78%, with the plants of the native category consistently more affected than the invasive category. When grown singly, Crofton weed grew larger (average 67%) than the native species. As compared with plants grown singly, Crofton weed plants grown in mixed pairs were 40% smaller. The corresponding data for the native species were 78% smaller for Japanese brome and 73% smaller for Dichrocephala. The effects of neighbor defoliation differed among categories and competition. All the nondefoliated plants grown singly were larger than plants grown with a defoliated conspecific neighbor. For the corresponding relationship in mixed pairs, the native plants grown singly were significantly larger than the same native species grown together with a defoliated, invasive neighbor. Defoliation of Crofton weed reduced the growth of native neighbors and increased negative competitive effects on native plants. For the invasive species, however, Crofton weed plants grown in mixed pairs with neighbor defoliation compensated fully for the competitive effects, i.e., they were not significantly different from nondefoliated plants grown singly. Consequently, the effects of defoliation on competition between invasive Crofton weed and its native neighbors may depend on the ability to undertake compensatory growth, and probably on the allelopathic exudation of Crofton weed to herbivory. Our data suggest that herbivory by biocontrols can make an already superior competitor even stronger, especially if the biocontrol agent does not effectively damage or kill the target plant. Owing to complex interactions between competition and defoliation, the indirect effects of herbivory may be more complicated than currently conceived, and understanding the indirect effects of biocontrol agents before their release is crucial.
Nomenclature: Crofton weed; Eupatorium adenophorum Spreng. EUPAD; Japanese brome; Bromus japonicus Thunb. Ex. Murr. BROJA; Dichrocephala; Dichrocephala integrifolia (L. f.) Kuntze.