THE INFLUENCE OF BELOW GROUND HERBIVORY AND PLANT COMPETITION ON GROWTH AND BIOMASS ALLOCATION OF PURPLE LOOSESTRIFE

Experiments investigating plant-herbivore interactions have primarily focused on above-ground herbivory, with occasional studies evaluating the effect of below-ground herbivores on plant performance. This study investigated the growth of the wetland perennial Lythrum salicaria (p urple loosestrife) under three levels of root herbivory by the weevil Hylobius transversovittatus and three levels of plant competition by t he grass Phleum pratense in a common garden. Plant growth, flowering p henology, and biomass allocation patterns of purple loosestrife were r ecorded for two growing seasons. During the first year, root herbivory reduced plant height; plant competition delayed flowering; and the in teraction of root herbivory and plant competition resulted in reductio ns in plant height, shoot weight and total dry biomass. Plant competit ion or larval feeding did not affect the biomass allocation pattern in the first year. These results indicate the importance of interactions of plant competition and herbivory in reducing plant performance - at least during the establishment period of purple loosestrife. In the s econd growing season, root herbivory reduced plant height, biomass of all plant parts, delayed and shortened the flowering period, and chang ed the biomass allocation patterns. Plant competition delayed flowerin g and reduced the dry weight of fine roots. The interaction of root he rbivory and plant competition delayed flowering. Root herbivory was mo re important than plant competition in reducing the performance of est ablished purple loosestrife plants. This was due, in part, to intense intraspecific competition among the grass individuals effectively prev enting shoot elongation of P. pratense and resulting in a carpet like growth.