Insect herbivores can have dramatic effects on ecosystem dynamics.  While herbivores can affect litter decomposition rates, decoupling the effect of herbivore-mediated changes in litter chemistry from other factors such as physical removal or alteration of leaf material has been difficult.  The poplar petiole galler (Ectoedemia populella) forms a gall on the petiole at the base of the leaf lamina, which results in an unchanged leaf morphology compared with an ungalled leaf.  Thus, any observed differences in decomposition or nutrient release rates of the leaf tissue should be the result of gall-mediated changes in leaf chemistry.  Since E. populella caterpillars use both big-toothed aspen and quaking aspen as hosts, we decomposed galled and ungalled leaf litter from each species over a time course of 18 months. 

Results/Conclusions

Presence of the lepidopteran gall had strong effects on initial leaf chemistry, rates of mass loss, and decomposing litter nutrient dynamics.  Presence of the petiole stem gall reduced decomposition rates of the big-toothed aspen and quaking aspen leaf litters, but in temporally distinct ways.  galling decelerated the “early” stages of mass loss of P. grandidentata litter and the “later” stages of P. tremuloides litter.  After four months, mass loss of the P. grandidentata galled leaf litter was 6% lower than the mass loss to ungalled P. grandidentata litter; after 12 and 18 months, mass loss was 15% and 17% lower, respectively, in the galled P. tremuloides litter relative to the ungalled P. tremuloides litter. Galling had strong, species-specific effects on tannin concentrations and protein-binding activity in the senesced, undecomposed initial litter.  Tannin concentration changes during decomposition were influenced by galling, but the effect was small relative to the dramatic decline in litter tannin concentrations and protein-binding capacity that occurred over time.  Cellulose and lignin concentrations in senesced, undecomposed leaf tissue were 12% and 29% higher in galled litters relative to ungalled litters, independent of plant species.  During decomposition, cellulose concentrations segregated by plant species, but galling remained a significant factor in litter lignin concentrations. 

Our results clearly indicate that galling by E. populella slowed litter decomposition, but that decomposition rates were differentially influenced by species-specific interactions between the host plant species and the galling insect.  Because E. populella does not obviously alter leaf morphology, the patterns we discovered appear to result from altered chemistry in the leaf lamina, supporting the hypothesis that herbivore-induced changes in leaf chemistry can influence ecosystem dynamics.