When parasites bite hardest: mistletoe effects on oak radial growth peak near climatic optima

Hemiparasitic mistletoes reallocate host water and carbon, yet it remains unclear when their costs are greatest and whether parasites shift cambial timing or mainly constrain the amount of wood produced. We tested whether European mistletoe ( Loranthus europaeus ) alters the timing or magnitude of radial growth in ring-porous Quercus robur , and whether impacts peak under climatic stress or near hydrothermal optima for host performance. We continuously monitored oak trees across age classes, canopy positions, and infection statuses using 15-min electronic dendrometers over four growing seasons (2020–2023), paired with air temperature, soil temperature, and volumetric soil moisture. We quantified cambial phenology (onset, peak, cessation), seasonal infection-effect curves (non-infected minus infected), climate–growth correlations, and temperature–moisture response surfaces describing the realised hydrothermal niche of growth. Growth phenology was conserved among years and categories (onset DOY ~ 120–140; peak ~150–220; cessation ~250–270), and infection did not shift these phases. In contrast, mistletoe reduced the amplitude of cambial increment, most clearly in solitary, well-lit trees and in high-growth years. Suppression peaked at moderate air temperatures (~10–18 °C) with adequate soil moisture and diminished under suboptimal combinations of heat or cold and dryness, indicating that host–parasite competition is strongest near conditions that maximise host growth. Short-term climate sensitivities were broadly similar across infection status: during the peak season, growth was typically negatively related to temperature and positively related to soil moisture, and temperature–moisture niches were comparable among categories. However, young, solitary, non-infected trees tracked midsummer moisture pulses more strongly than infected trees, suggesting that infection shifts hosts from a resource-tracking to a stress-limited growth regime in exposed microclimates. European mistletoe thus imposes its greatest penalty near the host’s hydrothermal optimum, limiting wood production rather than growth timing, and identifies the environmental window in which parasite impacts are most consequential for forecasting oak productivity under shifting temperature–moisture regimes.