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.
Highlights
Mammalian herbivores have long been recognized as fundamental drivers of the nutrient cycling in terrestrial ecosystems worldwide
Forbs and grasses differed in their nutrient contents and dynamics throughout the summer, a seasonal decrease in nutrient levels characterized both plant functional types (PFTs) (Fig. 4, 5 [Ro−/Re−], Table 2, 3)
Nutrient levels were consistently higher in forbs, the relative difference in PFT nutrient contents decreased throughout the growing season
Summary
Mammalian herbivores have long been recognized as fundamental drivers of the nutrient cycling in terrestrial ecosystems worldwide (reviewed by Pastor et al 2006, Harrison and Bardgett 2008, Sitters and Olde Venterink 2015). The plant nutrient-related traits that contribute to the palatability of foliage can govern the decomposability of plant litter (Grime et al 1996, Cornelissen et al 2004) Perhaps for this reason long-term herbivore-driven changes in ecosystem nutrient cycling of cold, nutrient-limited environments, such as (sub-)Arctic and alpine tundra, can be contingent on herbivore-induced changes in the functional composition of plant communities (reviewed by Stark 2007). Longterm herbivory can promote higher abundance of either palatable, nutrient-rich plant species (e.g. forbs and grasses) or less palatable, nutrient-poor plant species (e.g. shrubs), either accelerating (Olofsson et al 2004a, Tuomi et al 2018) or retarding (Pastor et al 1993, Grellmann 2002) nutrientcycling rates It remains an open question whether herbivores can cause immediate changes in tundra plantcommunity nutrient levels, indicative of accelerating, neutral or retarding effects on nutrient cycling rates
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