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 NH4NO3,, 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.