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 relative to exotics. If exotic species experience enemy release, increased nutrient availability may thus benefit exotics adapted to high resource conditions and lead to increased exotic dominance. In an old field (Duke Forest, Durham, NC) dominated by herbaceous species, we manipulated soil resources (added NPK+micronutrients, 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 17 most abundant species. We expected community-level damage to be highest when soil resources were added and enemies were present. If exotic species experience release from enemies, we would also expect damage to be lower on exotics than natives. Additionally, if exotics are released from enemies, enemy presence should disadvantage natives and lead to increased exotic dominance.

Results/Conclusions

Soil resource addition interacted significantly with fungal and insect enemy exclusion to influence community-level damage. Plots receiving additional soil resources had higher community-level damage than plots to which soil resources were not added when fungal and insect enemies were present but there was no difference in damage when fungal and insect enemies were excluded (p=0.014). When we analyzed natives and exotics separately, we found similar effects of soil resources and fungal and insect enemy exclusion on community-level damage (natives, p=0.023; exotics, p=0.006). Although both groups experienced similar trends in damage, enemy damage to natives was greater across all treatments. When soil resources were added and fungal and insect enemies not excluded, natives experienced 8.76% leaf damage while exotics experienced 2.35% leaf damage, on average. Soil resource addition over 3.5 years decreased overall species richness (p<0.0001) and increased relative cover of exotics (p=0.039). However, 1.5 years of exclusion of fungal and insect enemies has not reversed exotic dominance. These results suggest that long-term differences in enemy pressure experienced by natives and exotics may contribute to the effects that soil fertility has on plant species composition and exotic dominance.