Role of Leaf Traits in Driving Genotypic Diversity-Mediated Associational Effects in Silver Birch

Trees growing in more diverse stands generally experience less herbivory than those in less diverse ones, potentially due to neighbourhood-mediated variations in traits which influence leaf palatability. While numerous studies have assessed leaf trait responses to species diversity, the influence of genotypic diversity on leaf traits, and subsequent effects on herbivory, remains poorly understood. We investigated two genotypes of silver birch (Betula pendula) growing in single-, 2-, 4- and 8-genotype mixture plots in the Satakunta birch clone diversity experiment in SW Finland. Our aim was to determine whether genotypic diversity causes leaf trait variation and whether these changes are linked to herbivory. We found no effect of genotypic diversity on specific leaf area (SLA) or concentrations of phenolic compounds, but increased canopy cover around the focal trees was associated with lower concentrations of some phenolics. Genotypic diversity had a significant effect on herbivory, with one of the genotypes suffering more herbivory in 2-genotype mixture plots than in single-genotype plots. Genotype identity was the strongest predictor of both leaf traits and herbivory. Two birch genotypes differed in concentrations of most phenolic compounds, SLA and herbivory. Leaf trait-herbivory relationships were also genotype specific, with only one of the two genotypes exhibiting a negative correlation between herbivory and phenolics. Our study demonstrates that genotypic diversity is a poor predictor of leaf traits and herbivory in silver birch and indicates the importance of genotype selection as a consideration when establishing herbivory-resilient forests.