Cascading Effects of Plant Hormone-induced Trait Shifts in Alnus rubra on Aquatic and Terrestrial Ecosystem Function

While intraspecific trait variation can regulate ecosystem functions, there is limited understanding of whether this variation caused by inducible phenotypic shifts, as opposed to genetic hardwiring, can have such effects. Leveraging the complex inducible defense systems of plants, we found that treating young Alnus rubra trees with the plant hormones methyl jasmonate and salicylic acid, which are involved in plant defensive responses to herbivory and/or pathogen stress, triggered shifts in the composition of leaf secondary metabolites and nutrient reallocation among plant tissues. Induction of the jasmonic acid pathway triggered substantial changes among the dosage-dependent ellagitannins and a decline in leaf nitrogen, while induction of the salicylic acid pathway triggered greater restructuring among the cytotoxic diarylheptanoids. Concurrent induction of both pathways caused distinct changes in leaf composition compared to either pathway alone. We also found opposing seasonal trends of these two major classes of secondary metabolites with seasonal declines among diarylheptanoids and increases among ellagitannins. Further, these plant hormone-induced compositional changes in leaves had cascading effects on ecosystems, with litter mass loss rates in both aquatic and terrestrial ecosystems inhibited most by induction of the jasmonic acid pathway, least by induction of the salicylic acid pathway, and effected at intermediate levels by the combined induction of both pathways. These results demonstrate that inducible plant defenses that are mounted against terrestrial stressors can alter rates of litter mass loss in aquatic and terrestrial ecosystems.