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The Impact of Root Herbivory as a Function of Plant Density and Competition: Survival, Growth and Fecundity of Centaurea maculosa in Field Plots

Heinz Muller-Scharer
Journal of Applied Ecology
Vol. 28, No. 3 (Dec., 1991), pp. 759-776
DOI: 10.2307/2404206
Stable URL: http://www.jstor.org/stable/2404206
Page Count: 18
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Impact of Root Herbivory as a Function of Plant Density and Competition: Survival, Growth and Fecundity of Centaurea maculosa in Field Plots
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Abstract

(1) Two inter-related experiments on the effect of root herbivory by the moth Agapeta zoegana on Centaurea maculosa, an economically important rangeland weed in North America, were done under various competitive environments to study interactions between root herbivory, plant density and competition, and to obtain information on the potential impact of this root feeder as a biological control agent. (2) Competition with grass was the most important single factor, reducing survival of rosettes to 17% of that of the control plants and strongly limiting plant growth, biomass and fecundity of the survivors, leading to a 94% reduction in biomass and a 83% reduction in seed output per area at the end of the experiment. (3) Increased Centaurea density only slightly reduced plant height, survival and fecundity under the conditions studied, but significantly reduced biomass and shoot numbers per plant. However, plants at high densities produced slightly increased biomass and significantly more seeds per area than controls. (4) Intermediate levels of herbivory had no significant impact on biomass or shoot number, either per plant or per area, but decreased plant height and reproductive mass, as well as survival of young rosettes. (5) The intraspecific gradient of plant responses to root herbivores was found to be influenced by the competitive environment and the degree of herbivory. In nutrientrich soils, plant reactions were well buffered from losses from intermediate levels of herbivory when plant density was low. The herbivore effect increased in plots with high density and was most detrimental when additional grass competition was present. Under competition with fescue, survival, shoot number and fecundity decreased linearly with increasing numbers of herbivores. (6) That low levels of herbivory increased survival, shoot number and biomass per area as compared with the control plots indicates that the relation between the applied stress and the plant's response to it is not a simple one. (7) The effect of these root herbivores as biological control agents will therefore be determined mainly by soil conditions and the competitive status of the weed. Under the generally poor soil conditions of the infested areas in North America, the main effect of A. zoegana on plant population dynamics will probably be through reduced rosette survival, as infested plants die relatively late in the growing season and will mainly be replaced by grass, if present.

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