Two arctic tundra graminoids differ in tolerance to herbivory when grown with added soil nutrients

Abstract

Greater soil nutrients are thought to increase graminoid tolerance to herbivory (within-season regrowth following herbivore damage) by enabling new growth in response to defoliation, but these responses vary among graminoid species. We studied how two arctic graminoids, Eriophorum vaginatum (L.) in moist acidic tussock (MAT) tundra and Hierochloe alpina (Roem. & Schult.) in dry heath (DH) tundra, tolerate both experimental and natural herbivory when fertilized with nitrogen and phosphorus. Fertilization reduced the tolerance and recovery (regrowth in subsequent years) of defoliated Eriophorum relative to plants growing without fertilization. In contrast, fertilized Hierochloe plants regrew well following defoliation. These opposing results may be due to differential abilities of these two species to access other resources in MAT and DH when fertilized. Herbivory may be affecting access to light of Eriophorum in the more productive, closed canopy MAT, in spite of its ability to grow bigger when fertilized. Hierochloe may not face such a limitation in the more open DH community. Alternatively, the different responses between these two species could be caused by environmental differences between the two communities. Regardless, our results suggest that although high tolerance is thought to be a functional characteristic of graminoids, tolerance to herbivory varies within and between species with soil nutrient availability and thus caution must be exercised in predicting responses to herbivory within this plant growth form.