Interactions between winter and summer herbivory affect spatial and temporal plant nutrient dynamics in tundra grassland communities

Abstract

In the long‐term, herbivores can alter nutrient dynamics in terrestrial ecosystems by changing the functional composition of plant communities. Here, we ask to what extent herbivores can affect plant‐community nutrient dynamics in the short‐term. We provide theoretical expectations for immediate effects of herbivores on tundra‐grassland plant‐community nutrient levels throughout a single growing season and empirically evaluate these predictions. We established an experiment within two forb‐dominated and two grass‐dominated tundra‐grassland communities. We selected tundra‐patches disturbed by small rodents during the previous winter, and neighbouring undisturbed tundra‐patches. Within each tundra‐patch, we set up a reindeer‐open and a reindeer‐exclusion plot. Throughout the summer, we randomly collected over 2800 leaf samples from 34 vascular plant species/genera and analysed their nitrogen and phosphorus contents. Plant‐community nutrient levels were consistently higher in tundra‐patches affected by small rodents, both across tundra‐grassland types and throughout the growing season. Forbs and grasses growing in small‐rodent disturbed tundra‐patches had 11% and 25% higher nutrient content, respectively, compared to undisturbed tundra‐patches. Reindeer affected only grasses growing in grass‐dominated tundra‐grasslands and the outcome was dependent on small‐rodent winter disturbance. Reindeer increased grass nitrogen content in undisturbed tundra‐patches (+7%) and weakened the positive effects of small rodents in disturbed tundra‐patches (from 25% to 15% higher nutrient content [both nitrogen and phosphorus]). By enhancing plant nutrient levels throughout a single growing season, herbivores were key, immediate modifiers of plant‐community nutrient dynamics in tundra‐grasslands. Higher nutrient contents still detected in senescent leaves at the end of the summer in herbivore‐affected tundra suggest that herbivory is accelerating short‐term tundra‐grassland nutrient cycling rates. Our findings from tundra‐grassland communities align with theoretical expectations of positive herbivore effects on nutrient cycling in relatively productive ecosystems.

Location of studies and evidence of effects of herbivory on Arctic vegetation: a systematic map

“…Similarly, outbreaks of invertebrate herbivores can influence the livelihood of northern communities [5] through their dramatic impacts on vegetation and the subsequent cascading effects on local wildlife [6,7]. Herbivores also play an important ecological role in Arctic tundra ecosystems by influencing soils [8] and plant communities [9,10], with consequences for ecosystem processes [11]. Moreover, herbivores have the potential to offset some of the effects of ongoing warming in tundra ecosystems by counteracting increases in deciduous shrub cover [12,13] or advances of the treeline [14].…”

Forage quality in tundra grasslands under herbivory: Silicon‐based defences, nutrients and their ratios in grasses

“…This could be particularly relevant in high-latitude regions, such as the tundra, where herbivores have a short time-window to access plants and hence any changes in plant quality may have large consequences for their fitness (Beard et al, 2019;Doiron et al, 2015;White, 1983). We have recently shown that, within a single summer, mammalian herbivores enhance plant nutrient dynamics in tundra grasslands (Petit Bon, Inga, et al, 2020). Here, we ask to what extent herbivores modify the quality of tundra grasses by altering their Si-content in relation to their nitrogen (N) and phosphorus (P) levels.…”

“…During peak abundances, which can occur both in summer and winter, small rodents visibly disturb the vegetation (Hambäck et al, 1998;Ims & Fuglei, 2005). Such disturbance also represents a major pathway through which small rodents affect tundra plant-community nutrient levels (Petit Bon, Inga, et al, 2020;Tuomi et al, 2018). In contrast, Rangifer (reindeer/caribou) show largescale spatial dynamics through their migratory behaviour and their impacts by grazing and trampling on vegetation are more likely to be seasonal (Bernes et al, 2015;Tuomi et al, 2021).…”

UAV reveals substantial but heterogeneous effects of herbivores on Arctic vegetation

“…Given that vole and lemming populations experience population fluctuations across the Arctic 22 , rodent peaks should be important disturbance events in almost the entire biome. The slow growth rates of low productive Arctic vegetation 23 , result in the Arctic tundra being a mosaic of patches in different successional stages with implications for plant community composition and plant nutrient content 36 . Quantifying the heterogeneity of rodent impacts using UAVs makes it possible to estimate the importance of rodents on fundamental aspects of Arctic ecosystems, i.e.…”