Mistletoe load is associated with higher water-use efficiency but lower carbohydrate reserves in deciduous host trees under seasonal drought

Net photosynthesis, intrinsic water-use efficiency (WUE), leaf δ¹³C, and non-structural carbohydrate (NSC) pools were quantified in 288 individual trees representing four deciduous host species across a natural Viscum album load gradient at three elevational sites during pre-drought, peakdrought, and early-recovery phases. Intrinsic WUE increased with increasing mistletoe load in all seasons (Pearson r=0.23-0.40, n=288 per season, all P<0.001), with the strongest association observed during peak drought (r=0.40). High-load trees showed approximately 27% higher mean intrinsic WUE than uninfected trees at peak drought. Total NSC declined with load during drought (r=-0.73), driven mainly by starch depletion; soluble sugars remained relatively stable. Load was also associated with lower net photosynthesis (r=-0.39 at peak drought). Cluster-robust linear models confirmed significant load and season effects, and load × season interactions for WUE and NSC. The WUE load slope differed among hosts (load × host, P=0.008); for δ¹³C and NSC, the interaction was not significant when seasons were pooled (P=0.17 and 0.76). Within each season, higher V. album loads were associated with lessnegative leaf δ¹³C. Overall, higher mistletoe loads were associated with a higher WUE operating point and progressive starch depletion amplified during seasonal drought.