Assessing the Impacts of Herbivory on Plant Silica Accumulation across a Global Network of Grasslands

Abstract:

Plants, especially grasses, have a profound impact on the biogeochemical cycling of silicon. Silicic acid (Si(OH)₄) in soil water is absorbed by plant roots, transported via the transpiration stream, and deposited as solid silica (SiO₂) phytoliths in leaf tissue. Evidence indicates that plant phytolith accumulation may have evolved as an anti-herbivore strategy, and modern studies reveal that these silica particles are abrasive to animal mouthparts and can interfere with digestion. Furthermore, several studies have shown that grasses have the ability to respond to insect and mammal herbivory by modifying the amount of silicon they absorb from soil, a property known as inducible defense. However, herbivory studies remain largely limited to a laboratory setting, and research in natural systems has only been conducted at a regional spatial scale. To address whether these localized patterns persist at the global scale, we utilized data from a network of 40 grassland sites occurring on six continents. Vegetation samples including grasses, forbs, and litter, were collected in and out of 6m x 6m herbivore exclosures by a team of collaborating scientists for an on-going research effort known as the Nutrient Network (NutNet). We utilized near infrared spectroscopy (NIRS) to create a calibration for plant silica which allowed for the rapid analysis of more than 1000 samples. Preliminary analyses indicate that silica content of grasses was higher outside of exclosures, where herbivores had access to vegetation. Our data reveal that herbivores play a significant role in modifying plant silicon uptake, and hence, the rates of silicon cycling in grasslands across the globe.