The impact of serpentine soils on plant traits and plant–herbivore interactions

Serpentine soils, known for high heavy metal content and low nutrients, create harsh conditions for plants and herbivores. Although previous research has explored how serpentine soils affect herbivory, most studies have been restricted to a specific region, and the mechanisms involved remain unclear. Here, we conducted a large-scale study across 78 plant species from California, Spain, and Sweden, comparing insect herbivory and leaf traits (chemical, nutritional, and physical) in serpentine and non-serpentine habitats within each region. We also analyzed soil properties—pH, texture, nutrient content, and heavy metal concentrations—to investigate their potential role in mediating soil type effects on herbivory. Our results showed that plants growing in serpentine soils experienced lower herbivory levels and exhibited higher concentrations of phenolic compounds, lower nutrient content, and thicker leaves, compared to their non-serpentine counterparts. Additionally, elevated pH and heavy metal levels in serpentine soils were associated with reduced herbivory, increased concentrations of phenolic compounds, and decreased specific leaf area (SLA) and nitrogen content. However, no indirect link between soil properties and herbivory via leaf traits was found, suggesting that bottom-up effects of soil on plants and herbivores operate independently. Collectively, these findings enhance our understanding of the mechanisms linking plant–soil specialization and aboveground plant–herbivore interactions.