Precipitation, not CO2 enrichment, drives insect herbivore frass deposition and subsequent nutrient dynamics in a mature Eucalyptus woodland

Background and aims: Herbivorous insects are important nutrient cyclers that produce nutrient-rich frass. The impact of elevated atmospheric [CO ₂] on insect-mediated nutrient cycling, and its potential interaction with precipitation and temperature, is poorly understood and rarely quantified. We tested these climatic effects on frass deposition in a nutrient-limited mature woodland. Methods: Frass deposition by leaf-chewing insects and its chemical composition was quantified monthly over the first 2 years at the Eucalyptus free-air CO ₂ enrichment experiment and contrasted with leaf nitrogen concentration, rainfall and temperature. Results: Leaf-chewing insects produced yearly between 160 and 270 kg ha ⁻¹ of frass depositing 2 to 4 kg ha ⁻¹ of nitrogen. Frass quantity and quality were influenced by rainfall and average maximum temperatures. In contrast, elevated CO ₂ did not impact nitrogen concentrations in fully expanded leaves and frass deposition to the woodland floor. Conclusions: Two years of elevated CO ₂ did not alter nutrient transfer by leaf-chewing insects. This may be due to the low nutrient status of this ecosystem, duration of CO ₂ fumigation or climatic conditions. However, rainfall co-occurring with seasonally higher temperatures exerted strong effects on nutrient cycling, potentially through shifts in leaf phenology with consequences for insect population dynamics and insect-mediated nutrient transfer.