Interactions between herbivory and nitrogen availability alter invasive plant dynamics in forest understory communities

Background/Question/Methods

Nitrogen (N) availability can impact both invasive plant abundance and herbivore pressure on plants, yet the interaction of these two factors has not often been investigated. Nitrogen deposition has increased N availability in semi-urban forest ecosystems and likely has contributed to the proliferation of invasive understory plant species. In this study, our goal was to determine how varying N availability altered forest understory plant community composition and herbivore damage on native and invasive plants. At Ridley Creek State Park in southeastern Pennsylvania, we set up a nitrogen availability experiment using soil carbon (C) and N additions. Ten plots received each of the following treatments: high N, 20 g/m² of ammonium nitrate (NH₄NO₃), medium N, 10 g/m² of  NH₄NO_3,, decreased N, 750 g/m² of sucrose, and control, deionized water. We assessed soil N pools, invasive and native plant diversity and percent cover, and herbivore damage over a period of two years. 

Results/Conclusions

We found that C additions did decrease soil N availability, though the time of year determined the intensity of this effect. Native plant species cover was 30% higher in carbon addition plots than in high nitrogen addition plots. Invasive plant species richness was significantly lower in high N plots than in moderate N plots, possibly indicating N saturation in the high N plots. Herbivores consumed more invasive plant species (in proportion to their cover) in the N addition plots than in the control or C addition plots. Our results seem to suggest that N availability and herbivory can interact to have detrimental effects on invasive plant species. Our results are important to projecting plant community structure and soil N cycling in a future where N deposition continues to increase.