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 2-million-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.