1. Accumulation of silica (Si) by plants can driven by (1) herbivore pressure (and therefore, plant-herbivore interactions) (2) geo-hydrological cycles or (3) a combination of (1) and (2), with (1-3) possibly affecting Si concentration with a 1-year delay.

2. To identify the relative significance of (1-3) we analysed the concentration of Si in fibrous tussock sedge (Carex appropinquata), the population density of the root vole (Microtus oeconomus) and the ground water level, over 11 years.

3. The largest influence of autumn Si concentration in leaves (Sileaf) was the level of the current year’s ground water table, which was positive and accounted for 13.3% of its variance. The previous year’s vole population density was weakly positively correlated with Sileaf and alone explained 9.5% of its variance.

4. The only variable found to have a positive, significant effect on autumn Si concentration in rhizomes (Sirhiz) was the current year spring water level explaining as much as 60.9% of its variance.

5. We conclude that the changes in Si concentration in fibrous tussock sedge are predominantly driven by hydrology, with vole population dynamics being secondary.

6. Our results provide only partial support for the existence of plant-herbivore interactions, as we did not detect the significant effects of Si tussock concentration on the vole density dynamics. This was mainly due to low level of silification of sedges, which was insufficient to impinge herbivores.

7. Future studies on plant-herbivore interactions should therefore aim at disentangling whether anti-herbivore protection is dependent on threshold values of herbivore population dynamics. Furthermore, studies on Si accumulation should focus on the effect of water-mediated Si availability.

Appendix S1

Raw data for the model depicted in Figure 1, Figure 2 and Tables 1- 3.