Plant competition and slug herbivory: Effects on the yield and biomass allocation pattern of Poa annua L

Abstract

A factorial greenhouse experiment was conducted to examine the overall impact of slug herbivory, plant density and neighbour iden- tity on the yield and allocation pattern of the short-lived grass Pea cmnua. The effects of slug herbivory were compared in experimental plants at two densities in two types of culture: pure and mixed with Arabidopsi.~ thalianu. All the plant parts of P annua showed significantly greater bio- mass in mixed than in pure stands, and in low compared to high density stands. Slug herbivory consistently reduced the biomass of reproductive organs and the percentage of total biomass allocated to reproduction, whereas density had no effect on allocation. In mixed stands, the proportion of biomass allocated to reproductive organs and leaves was greater and lesser, respectively, than in pure stands. There were significant interactions between herbivory and culture for root biomass and for the percentage of biomass allocated to root and shoots, and between herbivory, culture and density for shoot, stem, leaf and total biomass. Slug herbivory reduced root biomass in pure stands, whereas it did the opposite in mixed stands. The root:shoot ratio did not vary between mixed and pure stands in the absence of slugs, while when slugs were present pure stands had lower root/shoot ratio than mixed stands. Except for reproductive structures, all the plant parts showed greater biomass in grazed than in ungrazed low density mixed stands, whereas the general trend in high and low density pure stands, and in high density mixed stands was that herbivory decreased biomass. The study indicates that the competitive ability of plants is conditioned by a variety of factors. Plant density, the identity of competitors, and occurrence of herbivory are all relevant factors, the particular combination of which will alter competitive interactions. 0 Elsevier, Paris