Foliar quality of co-occurring mallee eucalypts: balance of primary and secondary metabolites reflects past growing conditions

Foliar quality for herbivores is determined by the balance of primary and secondary metabolites which is dependent on leaf age which in turn is determined by the periodicity of flushing and the rate of abscission. We conducted a 10-month longitudinal study in northeastern Victoria, Australia, of the quality of fully expanded leaves of Eucalyptus gracilis, E. socialis, E. dumosa and E. incrassata. We measured N and available N (AvailN) as well as sideroxylonals and other formylated phloroglucinol compounds (FPCs) and related changes in concentration to tree phenology (flushing and flowering) and climatic conditions (temperature and rainfall). Concentrations of N, sideroxylonals and other FPCs differed significantly between species and with time as well as among trees within a species. AvailN also differed significantly between species and among trees within a species and with time for E. gracilis. Our analyses indicated that tree phenology affected N concentrations in E. gracilis only while climatic conditions affected N concentrations in E. gracilis, E. socialis and E. dumosa. Nitrogen concentrations in E. incrassata were unaffected by phenological or climatic factors. Tree phenology affected concentrations of sideroxylonals in E. gracilis and E. dumosa while climatic conditions affected concentrations of the same in E. socialis, E. dumosa and E. incrassata. The concentration of primary metabolites in expanded leaves of these eucalypts was relatively consistent compared to the concentration of quality reducing secondary metabolites which varied with the conditions experienced when leaves were expanding. Our results show that the foliar quality of leaves can be highly variable, with variations mediated by phenology and climate. These variations are likely to partially explain fluctuations in the abundance of chewing insect herbivores reliant on expanded leaves.