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

Abstract

1. Herbivore-induced changes in both leaf silicon-based defence and nutrient levels are potential mechanisms through which grazers alter the quality of their own grass supply. In tundra grasslands, herbivores have been shown to increase nutrient contents of grasses; yet, it is an open question whether they also increase grass silicon-based defence levels. Here, we asked if, and to what extent, herbivores affect silicon content and silicon:nutrient ratios of grasses found in tundra grasslands.<p> <p>2. We performed an herbivore-interaction field-experiment spanning four tundragrassland sites. At each site, we established reindeer-open and reindeer-exclusion plots in tundra-patches that had been disturbed or not by small rodents during the previous winter, for a total of 96 plots. We randomly collected over 1,150 leaf samples of inherently silicon-rich and silicon-poor grass species throughout a growing season and analysed silicon, nitrogen and phosphorus contents of each leaf. <p>3. Small-rodent winter disturbance did not affect grass silicon content, but increased grass quality (i.e. lowered silicon:nutrient ratios) by enhancing nutrient levels of both silicon-rich (+20%–22%) and silicon-poor (+26%–34%) grasses. Reindeer summer herbivory increased the quality of silicon-rich grasses by decreasing their silicon content (−7%). However, the two herbivores together offset both these quality increments in silicon-rich grasses, thus reducing their quality towards the level of those found in the absence of herbivores and further enhancing their silicon:nutrient ratios (+13%–22%) relative to silicon-poor grasses.<p> 4. Synthesis. We provide the first community-level, field-based assessment of how herbivory-driven changes in both leaf silicon-based defence and nutrient levels alter grass-forage quality in tundra grasslands. Herbivores did not promote a net silicon accumulation in grasses, but rather enhanced their overall quality. Yet, the magnitude of these quality increments varied depending on the herbivore(s)