Mistletoes and their eucalypt hosts differ in the response of leaf functional traits to climatic moisture supply

Trade-offs between photosynthesis and the costs of resource capture inform economic strategies of plants across environmental gradients and result in predictable variation in leaf traits. However, understudied functional groups like hemiparasites that involve dramatically different strategies for resource capture may have traits that deviate from expectations. We measured leaf traits related to gas exchange in mistletoes and their eucalypt hosts along a climatic gradient in relative moisture supply, measured as the ratio of precipitation to pan evaporation (P/E_p), in Victoria, Australia. We compared traits for mistletoes vs. hosts as functions of relative moisture supply and examined trait-trait correlations in both groups. Eucalypt leaf traits responded strongly to decreasing P/E_p, consistent with economic theory. Leaf area and specific leaf area (SLA) decreased along the P/E_p gradient, while C:N ratio, leaf thickness, N per area, and δ¹³C all increased. Mistletoes responded overall less strongly to P/E_p based on multivariate analyses; individual traits sometimes shifted in parallel with those of hosts, but SLA, leaf thickness, and N per area showed no significant change across the gradient. For mistletoes, leaf thickness was inversely related to leaf dry matter content (LDMC), with no relationship between SLA and mass-based N. In mistletoes, reduced costs of transpiration (reflecting their lack of roots) and abundant succulent leaf tissue help account for observed differences from their eucalypt hosts. Trait-based analysis of atypical functional types such as mistletoes help refine hypotheses based on plant economics and specialized adaptations to resource limitation.