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

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. The authors asked if, and to what extent, herbivores affect silicon content and silicon:nutrient ratios of grasses found in tundra grasslands.

The authors performed an herbivore-interaction field-experiment spanning four tundra-grassland sites. They 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.

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) involved and differed between silicon-rich and silicon-poor grasses. Such differential herbivory-induced changes in grass quality between and within tundra-patches may mediate plant–herbivore interactions by altering herbivore forage patterns and food choices. In tundra-patches utilized by both herbivores, the quality of silicon-rich grasses was further decreased relative to that of silicon-poor grasses. This could provide an advantage against herbivory, potentially being one of the pathways through which tundra-grassland vegetation states dominated by silicon-rich grasses are maintained by herbivores.