Résumé
The resource availability hypothesis predicts that plants adapted to infertile soils have high levels of anti-herbivore leaf defenses. This hypothesis has been mostly explored for secondary metabolites such as phenolics, while it remains underexplored for silica-based defenses. We determined leaf concentrations of total phenols and silicon (Si) in plants growing along the 2million-year Jurien Bay chronosequence, exhibiting an extreme gradient of soil fertility. We found that nitrogen (N) limitation on young soils led to a greater expression of phenol-based defenses, whereas old, phosphorus (P)-impoverished soils favored silica-based defenses. Both defense types were negatively correlated at the community and individual species level. Our results suggest a tradeoff among these two leaf defense strategies based on the strength and type of nutrient limitation, thereby opening up new perspectives for the resource availability hypothesis and plant defense research. This study also highlights the importance of silica-based defenses under low P supply.