The effects of soil nutrients on foliar herbivory and disease on native and exotic old field species

Background/Question/Methods

Foliar fungal pathogens and insect herbivores can be important regulators of plant populations and communities.  Enemies can alter plant success by reducing biomass, leaf longevity and photosynthetic capacity. While increased nutrient availability allows plants to increase growth, increased nutrient concentrations in leaves lead to increased severity of fungal diseases and insect herbivory. This may reduce the competitive ability of native species and allow exotic species to invade more readily. If exotic species experience enemy release, increased nutrient availability may thus benefit exotic species adapted to high resource conditions. In an old field (Duke Forest, Durham, NC) dominated by herbaceous species (primarily grasses), we manipulated soil resources (added NPK, control), vertebrate consumer presence (exclusion fences, control) and fungal pathogen and insect herbivore presence (application of biocides, control). We visually estimated percent damage to leaf tissue for the 13 most abundant species in this experiment. We expected enemy damage to be lowest on plants in unfertilized plots and among exotic species.

Results/Conclusions

The effects of nutrients on enemy damage differed among functional groups and between native and exotic species. Across all species we found that soil nutrient addition did not affect enemy damage to leaf tissue (F_1,27.9=0.30, p=0.59). Biocide application reduced enemy damage across all species (F_1,50.4=4.23, p=0.045).When we analyzed exotic species separately, we found no significant main effects of nutrient addition (F_1,20.8=0.01, p=0.9) or biocide application (F_1,16=1.81, p=0.20). Enemy damage to all grasses was significantly affected by the interaction between nutrient addition and biocide application (F_1,11=6.84, p=0.02). Plots receiving additional nutrients had higher enemy damage than control plots and enemy damage differed substantially more between biocide application and control plots under high nutrients than low nutrient conditions. However, there was no significant interaction between nutrients and biocide application among non-leguminous forb species (nutrient x biocide interaction, F_1,16.2=1.09, p=0.31) nor did any main effects contribute significantly to the model. These results suggest that differences between species of different functional groups may affect the manner in which their enemies respond to changes in nutrient availability. Future work will determine whether differences in enemy damage result in changes at the community level (species abundance, biomass) especially the relative success of native and exotic species.