BEETLE FOLIVORY INCREASES RESOURCE AVAILABILITY AND ALTERS PLANT INVASION IN MONOCULTURES OF GOLDENROD

One way that insect herbivores can influence plant community structure is by altering the ambient availability of resources required by plan ts. To examine the importance of this mechanism, I tested the followin g three hypotheses in a field experiment in 1990: (H-1) Folivory by th e leaf beetle Trirhabda canadensis (Coleoptera: Chrysomelidae) increas es availability of light, water, and N in monocultures of goldenrod (S olidago missouriensis). This hypothesis was partially supported. Foliv ory by Trirhabda reduced leaf, root, and total biomass of goldenrod an d increased light penetration, soil water content, and soil nitrate co ncentration. However, Trirhabda grazing did not affect overall soil N availability. (H-2) The invasion of goldenrod monocultures by other pl ant species and the response of goldenrod to folivory reflects an incr eased availability of resources in monocultures grazed by Trirhabda. T his hypothesis was supported. Folivory by Trirhabda raised the probabi lity that a species would invade the experimental monocultures and inc reased the species richness of the invading plant assemblage. Relative growth rate of goldenrod and production of aboveground biomass by inv aders were higher in monocultures grazed by Trirhabda than in ungrazed ones. Prostrate and creeping forbs increased in relative abundance fo llowing folivory by Trirhabda. (H-3) Reduced N uptake by goldenrod or increased N mineralization accompanies increased N availability in gra zed monocultures. This hypothesis was also supported. Net N mineraliza tion and nitrification were higher in monocultures grazed by Trirhabda than in ungrazed monocultures. Folivory by Trirhabda larvae reduced N uptake by goldenrod, but that of adults did not. The impact of herbiv ores upon plant communities may be effected through increased availabi lity of important plant resources.