Abstract
Abstract 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. We performed an herbivore‐interaction field‐experiment spanning four tundra‐grassland 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. 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. 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) 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 utilised 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.
Highlights
Grasslands, including managed rangelands and pastures, cover approximately 40% of the Earth's land surface (Gibson, 2009) and have long been recognised as important hotspots for plant–herbivore interactions (Bardgett & Wardle, 2003)
The extent to which these herbivores, that often co-occur in space, but less often in time (Ims et al, 2007; Ravolainen et al, 2011), concomitantly alter Si-based defence and N and P levels in tundra grasses will determine changes in the quality of their pastures. We studied both absolute changes in grass Si-based defence and changes relative to nutrient levels following winter disturbance by small rodents and summer herbivory by reindeer in tundra grasslands
Our findings clearly indicate that herbivory does not cause a net increase in Si-based defence levels of grasses found in these tundra grasslands
Summary
Grasslands, including managed rangelands and pastures, cover approximately 40% of the Earth's land surface (Gibson, 2009) and have long been recognised as important hotspots for plant–herbivore interactions (Bardgett & Wardle, 2003). Massey and Hartley (2006) found that experimentally enriched grass Si-content reduced the amount of nitrogen that field voles Microtus agrestis could absorb from foliage, leading to a decline in the growth rate of both juveniles and mature females These findings indicate that changes in Si-content relative to levels of essential nutrients in forage grasses are likely to be critical for herbivores utilising grassland ecosystems. The extent to which these herbivores, that often co-occur in space, but less often in time (Ims et al, 2007; Ravolainen et al, 2011), concomitantly alter Si-based defence and N and P levels in tundra grasses will determine changes in the quality of their pastures We studied both absolute changes in grass Si-based defence and changes relative to nutrient levels following winter disturbance by small rodents and summer herbivory by reindeer in tundra grasslands. The enhanced grass N-and P-contents (Petit Bon, Inga, et al, 2020) made us expect altered grass quality (Si:N and Si:P ratios) to be dependent on the magnitude of changes in both leaf Si and nutrient levels
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