Survival secrets of mistletoes: high drought tolerance in canopy habitats

The interaction between mistletoes and hosts impacts tree performance and mortality under climate change. However, little is known about the hydraulic performance and drought resistance of mistletoes, and their potential impacts on hosts. Here, we measured 21 functional traits related to hydraulics and drought resistance of eight mistletoe-host species pairs. We found that mistletoes were more drought tolerant compared with their hosts, characterized by more negative midday leaf water potentials during the dry season, turgor loss point (ranging from -1.81 to -2.48 MPa) and water potential at 12% loss of conductivity (ranging from -0.97 to -2.94 MPa), higher Huber values, sapwood density and vessel density, and lower leaf size, specific leaf area, vein density and stomatal density. Meanwhile, mistletoes were less hydraulically efficient compared with their hosts, demonstrated by lower leaf-specific hydraulic conductivity, sapwood-specific hydraulic conductivity and hydraulically weighted vessel diameter. Paradoxically, mistletoes showed lower water-use efficiency (as indicated by more negative stable carbon isotope values). Notably, trait associations between mistletoes and hosts differed, with mistletoes showing stronger correlations among functional traits, both within leaf traits and between leaf and stem traits. This suggests divergent ecological strategies between mistletoes and their hosts. However, no trade-off between hydraulic efficiency and safety was observed across the mistletoes and hosts examined. High plasticity in hydraulic traits was also found in mistletoes, with water potential at 12, 50 and 88% loss of conductivity varying significantly and intraspecifically across host species. Furthermore, trait correlations in mistletoes were driven by both intraspecific and interspecific variation, with interspecific variation being more important. These findings highlight the response capacity of mistletoes, enabling them to adjust their hydraulic strategies based on host-specific conditions. This study provides insights into mistletoe water use, drought resistance and potential responses to changing environmental conditions.