ROOT AND LEAF HERBIVORY ON LYTHRUM SALICARIA: IMPLICATIONS FOR PLANT PERFORMANCE AND COMMUNITIES

Abstract

Herbivores can have major direct impacts on plant performance, carbon and nitrogen content of plant tissues, nutrient cycling, and important ecosystem processes such as decomposition. Herbivory on a dominant plant can alter competitive interactions, which may have profound effects on plant community composition. We investigated the impact of herbivory by a root weevil (Hylobius transversovittatus) and a leaf beetle (Galerucella calmariensis) on purple loosestrife (Lythrum salicaria) in large walk‐in cages established in an invaded wetland in upstate New York, USA, from 1997 to 2000. We monitored how herbivory affected L. salicaria performance (growth and biomass allocation), litter dynamics, plant community composition, and changes in canopy temperature, humidity, and the amount of light penetrating through the canopy. Leaf herbivory had the greatest effect, reducing the height of L. salicaria shoots, reproductive effort, and aboveground biomass. Root herbivory also reduced L. salicaria leaf, stem, and inflorescence biomass and the number of stems but did not affect shoot height. Both root and leaf herbivore damage to L. salicaria resulted in increased light availability within the canopy but had no effect on canopy temperature or humidity. Whereas existing wetland plants grew larger in plots where L. salicaria was damaged by leaf herbivores, plant species richness did not vary among treatments. Leaf herbivory also accelerated decomposition of L. salicaria leaves and stems. Leaf and root herbivory did not interact, suggesting that (1) beetles did not compete for resources via their shared host plants and (2) attack by multiple herbivores did not increase or accelerate herbivore impact on L. salicaria over our four‐year study. Our results show that the competitive dominance of L. salicaria in North American wetlands can be curtailed by herbivores introduced as biological control agents, but it also demonstrates the need for long‐term investigations to capture cumulative herbivore impacts on plant individuals and communities.