There Is No Carbon Transfer Between Scots Pine and Pine Mistletoe but the Assimilation Capacity of the Hemiparasite Is Constrained by Host Water Use Under Dry Conditions

Mistletoe-induced carbon, water and nutrient imbalances are imprinted on tree rings

Mistletoes are xylem-tapping hemiparasites that rely on their hosts for water and nutrient uptake. Thus, they impair tree performance in the face of environmental stress via altering the carbon and water relations and nutritional status of trees. To improve our understanding of physiological responses to mistletoe and ongoing climate change, we investigated radial growth, stable carbon and oxygen isotopic signals, and elemental composition of tree rings in silver fir (Abies alba Mill.) and Scots pine (Pinus sylvestris L.) forests infested with Viscum album L. We compared temporal series (1990-2020) of basal area increment (BAI), intrinsic water-use efficiency (iWUE), oxygen isotope composition (δ18O), nutrient concentrations and stoichiometric ratios between non-infested (NI) and severely infested (SI) fir and pine trees from populations located close to the xeric distribution limit of the species in north-eastern Spain. The SI trees showed historically higher growth, but the BAI trend was negative for more than three decades before 2020 and their growth rates became significantly lower than those of NI trees by the mid-2010s. Mistletoe infestation was related to an enhanced sensitivity of radial growth to vapour pressure deficit (atmospheric drought). The SI trees showed less pronounced iWUE increases (fir) and lower iWUE values (pine) than NI trees. The lower tree-ring δ18O values of SI trees may be the result of several superimposed effects operating simultaneously, including leaf-level evaporative enrichment, source water isotopic signals, and anatomical and phenological differences. We observed a deterioration of potassium (K) nutrition in tree-ring wood of both species in SI trees, along with accumulation of manganese (Mn). We suggest that such nutritional patterns are driven by the indirect effect of mistletoe-induced drought stress, particularly in pine. The combined analyses of different physiological indicators imprinted on tree rings provided evidence of the progressive onset of carbon, water and nutrient imbalances in mistletoe-infested conifers inhabiting seasonally dry regions.

Water status and macronutrient concentrations, but not carbon status, of Viscum album ssp. album are determined by its hosts: a study across nine mistletoe-host pairs in central Switzerland

Introduction

European mistletoe, Viscum album L., is a hemiparasite that can infect various tree species, yet our understanding of its physiological interactions with host species is limited.

Methods

Nine mistletoe-host pairs (i.e. V. album ssp. album growing on nine different broadleaf tree species) under different growth conditions in central Switzerland were selected to examine the carbon, water and nutrient relationships between mistletoe and its hosts. We measured leaf morphological traits, isotopic compositions (δ13C and δ15N), concentrations of non-structural carbohydrates (NSC) and specific compounds (i.e. mobile sugars and starch), and macronutrients (i.e. N, P, K, Ca, Mg, S) in leaf and xylem tissues of both mistletoe and its hosts.

Results and Discussion

There were only non-significant relationships between NSC concentrations in mistletoe and in its host species across the nine mistletoe-host pairs, suggesting the carbon condition of V. album ssp. album is determined by both the heterotrophic carbon transfer and self-photosynthetic capacity among different mistletoe-host pairs. However, mistletoe leaf morphological traits (single leaf area and mass, and leaf mass per unit leaf area) did not change across the nine mistletoe-host pairs, and mistletoe leaf δ13C, water content and macronutrient concentrations were linearly correlated with those in the host leaves. Macronutrients showed accumulations in mistletoe across the nine pairs. Further, tissue N concentrations were significantly higher in mistletoe grown on N-fixing hosts than on non-N-fixing hosts. Finally, leaf N:P in mistletoe was significantly correlated with the ratio in the host across the nine mistletoe-host pairs. Overall, our results indicate strong relationships between mistletoe and its hosts for water- and nutrient-related traits, but not for carbon-related traits, which demonstrates that V. album ssp. album can adjust its physiology to survive on different deciduous tree species hosts and under different site conditions.