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. Résumé :On croit qu'une meilleure teneur en nutriment du sol augmente la tolérance des plantes gramoinoïdes à l'herbivorie (repousse au cours de la saison suite au dommage par les herbivores) en permettant le rejet de nouvelles pousses en réaction à la défoliation, mais ces réactions varient selon les espèces graminoïdes. Les auteurs ont étudié deux graminoïdes arctiques, l'Eriophorum vaginatum (L.) venant en touffes humides acides (THA) en toundra et l'Hierochloe alpina (Roem. & Schult.) venant sur landes sèches (LS) en toundra, tolérant toutes les deux l'herbivorie naturelle et expérimentale lorsqu'on les fertilise avec de l'azote et du phosphore. La fertilisation réduit la tolérance et la repousse (années subséquentes) des Eriophorum défoliés comparativement aux plantes venant sans fertilisant. Au contraire, les plants de l'Hierochleum fertilisés repoussent bien suivant une défoliation. Ces résultats opposés peuvent être dus à des capacités différentes chez ces deux espèces à accéder à d'autres ressources dans les THA et LS lorsque fertilisées. L'herbivorie peut possiblement affecter l'accès à la lumière chez l'Eriophorum dans la canopée fermée des THA, en dépit de sa capacité à mieux se développer, lorsque fertilisée. L'Hierochloe peut ne pas rencontrer une telle limitation dans la communauté plus ouverte des LS. Alternativement, les réactions différentes entre ces deux espèces pourraient être causées par des différences environnementales entre les deux communautés. Quoi qu'il en soit, les r...

Small herbivores with big impacts: Tundra voles (Microtus oeconomus) alter post‐fire ecosystem dynamics

“…Our study suggests that selective foraging by tundra voles may play a secondary role to post‐fire nutrient release, further decreasing graminoid abundance. Tussocks have less tolerance to herbivory under enriched soil nutrients; grazed tussocks are disadvantaged by both loss of nutrients from rhizome damage and light limitation from damaged tillers and competitive growth of overtopping, non‐palatable shrubs (Gough et al, 2012; Johnson & Gough, 2013).…”

Mammalian herbivory exacerbates plant community responses to long-term increased soil nutrients in two Alaskan tundra plant communities

“…Evidence suggests that the effects of these herbivores may be relatively slight when averaged across years, although further study is needed to better document their impacts on the plant communities and to tease out differences in herbivore pressure by the different mammals . For example, the only individual plant species focused on here that responded positively to herbivore exclusion at ambient soil nutrients was Eriophorum in MAT; as discussed earlier, this species is targeted by small mammals, although it can regrow following tiller removal (e.g., Chapin 1980) and thus can withstand some herbivory each year yet persist (Johnson and Gough 2013).…”

“…In DH where bare space is more readily available, the community transitioned from one dominated by evergreen shrubs and lichens to a grassland when fertilized, and these changes were exacerbated by the presence of mammalian herbivores, mainly through the stimulation of growth of a grazing-tolerant tussock-forming grass (Johnson and Gough 2013). In the more closed-canopy MAT community, the tussock-forming sedge was negatively affected by nutrient addition and the presence of herbivores because an unpalatable dwarf deciduous shrub was able to take advantage of the nutrients and mammalian activity and grow taller and larger, overtopping the tussocks (Gough et al 2012b).…”