Effects of mistletoe removal on growth, N and C reserves, and carbon and oxygen isotope composition in Scots pine hosts

Loss of Stomatal Regulation Sensitivity to CO2 and Reduced Xylem Hydraulic Conductivity Contribute to Long-Term Tree Decline and Mortality

Increasing aridity is a major threat to forests worldwide. Understanding tree functional constraints under drought and their impacts on resilience and mortality among species is crucial to assess the impacts of global change on forests. We analyzed the long-term drought and atmospheric CO2 responses in three Mediterranean co-occurring species with differing drought tolerances (Pinus pinaster < Pinus pinea < Juniperus oxycedrus). In this mixed forest, P. pinaster exhibited widespread mortality and mistletoe infection, P. pinea showed scattered mortality, and J. oxycedrus showed no decline. Using tree-ring data (1978-2016), we compared intrinsic water-use efficiency (iWUE) and xylem hydraulic traits in healthy and non-healthy individuals of both pine species and healthy junipers. Healthy P. pinaster trees produced a more hydraulically efficient xylem, with wider lumen tracheids, than non-healthy trees, whereas P. pinea showed no anatomical differences between health statuses. Healthy P. pinaster displayed greater anatomical plasticity, adjusting hydraulic conductivity and cell-wall thickness to water availability. Despite small differences in average iWUE, the response of iWUE to rising CO2 and drought differed between species and health statuses. J. oxycedrus and P. pinea showed steady iWUE increases, but P. pinea experienced periods of stagnation following an extreme drought, later recovering regardless of health status. In contrast, iWUE in P. pinaster plateaued for over 20 years after a decline-inducing drought, particularly in non-healthy, mistletoe-infected trees. Differences in iWUE response to CO2 and anatomical plasticity to drought may explain the contrasting mortality patterns among these coniferous species. Our results suggest a long-term decline spiral in P. pinaster induced by low hydraulic efficiency in drought-induced defoliated trees and limited physiological responses to rising CO2 and drought. Increasing drought stress makes pine recovery increasingly unlikely.

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.

Mitigating the Mistletoe Menace: Biotechnological and Smart Management Approaches

Mistletoes have been considered a keystone resource for biodiversity, as well as a remarkable source of medicinal attributes that attract pharmacologists. Due to their hemiparasitic nature, mistletoes leach water and nutrients, including primary and secondary metabolites, through the vascular systems of their plant hosts, primarily trees. As a result of intense mistletoe infection, the hosts suffer various growth and physiological detriments, which often lead to tree mortality. Because of their easy dispersal and widespread tropism, mistletoes have become serious pests for commercial fruit and timber plantations. A variety of physical and chemical treatment methods, along with silvicultural practices, have shaped conventional mistletoe management. Others, however, have either failed to circumvent the growing range and tropism of these parasitic plants or present significant environmental and public health risks. A biocontrol approach that could sidestep these issues has never achieved full proof of concept in real-field applications. Our review discusses the downsides of conventional mistletoe control techniques and explores the possibilities of biotechnological approaches using biocontrol agents and transgenic technologies. It is possible that smart management options will pave the way for technologically advanced solutions to mitigate mistletoes that are yet to be exploited.

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

Pine mistletoe is a hemiparasitic shrub that can produce its own photosynthates. There is a lack of knowledge about the interaction of mistletoe and host under varying environmental condition that might influence carbon gain and allocation. In a 13C-pulse labeling experiment with mature Pinus sylvestris (pine) infected by mistletoes grown in naturally dry or irrigated conditions, (1) mistletoe clusters were shielded from 13CO2 added, and (2) mistletoes or host needles were removed to manipulate the local assimilate and water availability. No 13C signal was found in shielded mistletoes, indicating no carbon transfer from the host to the mistletoe. When the pine needles were removed from girdled branches, no 13C signal was found in the host tissues, implying no carbon transfer from mistletoe to the host. However, mistletoes on needle-removed pine trees accumulated more labeled assimilates and had higher non-structural carbohydrate (NSC) concentrations only under naturally dry conditions but not in irrigated plots. Our results suggest that mistletoes show full carbon autonomy, as they neither receive carbon from nor provide carbon resource to the host trees. Moreover, the high assimilation capacity of mistletoes seems to be constrained by the host water use under dry conditions, suggesting that drought stress is not only negatively impacting trees but also mistletoes. Therefore, we conclude that the hemiparasites live on their own in terms of carbon gain which, however, depends on the water provided by the host tree.

The ecophysiology of a neotropical mistletoe depends on the leaf phenology of its tree hosts

PREMISE

Mistletoes parasitize many hardwood and softwood tree species; however, they play key roles in forest ecosystems. Adult individuals of Psittacanthus schiedeanus take up water and xylem nutrients from both deciduous and evergreen host trees, suggesting the ability to modify its physiology according to the availability of host resources. Yet, there is little information regarding the effects of mistletoes on their host trees from the eophyll stage to reproductive phases of the parasite.

METHODS

Taking advantage of the fact that P. schiedeanus can reach sexual maturity in 1 year, we investigated its physiological performance during development on deciduous (Liquidambar styraciflua) and evergreen (Quercus germana) host trees in a cloud forest in eastern Mexico. Variables related to chlorophyll fluorescence, carbon assimilation, photosynthetic pigments, and nitrogen, phosphorus, and carbon contents of the parasite and non-infected and infected hosts were analyzed in a nursery experiment.

RESULTS

Mistletoe had lower water-use efficiency and higher transpiration rates than the host species did. Despite the fact that P. schiedeanus obtained resources from species with differing phenology and resource availability, the parasite steadily improved its CO₂ assimilation, electron transport rate, and nutrient content from seedling establishment to adult life stages. Mistletoe decreased the photosynthetic reactions of carbon metabolism in the deciduous host, photosynthetic light reactions in the evergreen host, and nutritional status of both host species, mostly in the evergreen host.

CONCLUSIONS

The hypothesis that mistletoes adjust their physiology according to the availability of host resources could extend to the early growth of the parasite.

Synergistic abiotic and biotic stressors explain widespread decline of Pinus pinaster in a mixed forest

Global change potentially increases forest vulnerability. Different abiotic and biotic factors may interact to cause forest decline and accelerated tree mortality. We studied a mixed Mediterranean continental forest where Pinus pinaster Ait. (maritime pine) shows widespread decline to analyse the role of different abiotic and biotic factors on health status and growth dynamics both at the individual and plot levels. We also analysed stand composition and regeneration of tree species to check whether there is a change in species dominance. Fungal pathogens were seldom present and we detected no pervasive fungi or insect infestation and no presence of pathogens like Heterobasidion or Phytophthora. Infection of hemiparasite plants like Viscum album L. (mistletoe) can reduce leaf area and its abundance is generally considered an expression of host decline. Yet, the existence among declining trees of high defoliation levels without mistletoe, but not vice versa, suggests that defoliation in response to some abiotic stressor could be a predisposing factor preceding mistletoe infection. Compared to healthy trees, declining and dead trees exhibited higher defoliation rates, smaller needles and lower recent growth with steeper negative trends. Dead and declining trees showed similar negative growth trends since the early 1990s droughts, which we interpreted as early warning signals anticipating mortality of currently declining trees in the near future. Mortality of maritime pine extending across all size classes, the lower presence of this species in the smallest size classes and its lack of regeneration suggest it is potentially losing its current dominance and being replaced by other co-occurring, more drought-tolerant species. Our results unravelled that maritime pine decline seems to be mainly driven by a combination of predisposing and inciting abiotic factors (microenvironment and drought stress) and biotic factors (mistletoe). The absence of widespread fungal pathogens suggests that they may have a minor role on pine decline acting only eventually as contributing factors. Although there could be other interrelations among factors or other biotic agents at play, our results strongly suggest that water stress plays a major role in the decline process of the dominant species on an ecosystem with strong land-use legacies.

Impact of Common Mistletoe (Viscum album L.) on Scots Pine Forests—A Call for Action

Common mistletoe is increasingly mentioned as contributing not only to the decline of deciduous trees at roadside and in city parks, but to conifers in stands. The presence of Viscum in fir stands has been known for many years, but since 2015 has also been the cause of damage to pine. In 2019, mistletoe was observed on 77.5 thousand hectares of Scots pine stands in southern and central Poland. Drought resulting from global climate change is implicated as an important factor conducive to weakening trees and making them more susceptible to the spread of mistletoe and other pests. This paper presents an overview of the latest information on the development of this semi-parasitic plant in Poland, its impact on tree breeding traits and raw material losses, as well as current options for its prevention and eradication.

Mistletoe-induced growth reductions at the forest stand scale

The hemiparasite European mistletoe (Viscum album L.) adversely affects growth and reproduction of the host Scots pine (Pinus sylvestris L.) and in consequence may lead to tree death. Here, we aimed to estimate mistletoe-induced losses in timber yield applying the process-based forest growth model 4C. The parasite was implemented into the eco-physiological forest growth model 4C using (literature-derived) established impacts of the parasite on the tree's water and carbon cycle. The amended model was validated simulating a sample forest stand in the Berlin area (Germany) comprising trees with and without mistletoe infection. At the same forest stand, tree core measurements were taken to evaluate simulated and observed growth. A subsample of trees were harvested to quantify biomass compartments of the tree canopy and to derive a growth function of the mistletoe population. The process-based simulations of the forest stand revealed 27% reduction in basal area increment (BAI) during the last 9 years of heavy infection, which was confirmed by the measurements (29% mean growth reduction). The long-term simulations of the forest stand before and during the parasite infection showed that the amended forest growth model 4C depicts well the BAI growth pattern during >100 years and also quantifies well the mistletoe-induced growth reductions in Scots pine stands.

Mistletoe infestation mediates alteration of the phytohormone profile and anti-oxidative metabolism in bark and wood of its host Pinus sylvestris

European mistletoe (Viscum album L.) has largely infested Central European forests and causes high mortality probability particularly in dry years. However, little information is available about the consequences of mistletoe infestation for metabolic processes in bark and wood of its host, despite their important roles in infestation defense. We analyzed the tissue hydration, carbohydrate composition, phytohormone profile, reactive oxygen species and anti-oxidant levels in bark and wood of Scots pines (Pinus sylvestris L.), as dependent on mistletoe infestation. As a consequence of mistletoe infestation, host bark and wood showed impaired hydration and reduced total carbon content. In the bark, soluble sugar and lignin contents increased, apparently at the expense of holo-cellulose. Hydrogen peroxide accumulation was accompanied by increased glutathione and decreased reduced ascorbic acid levels. Mistletoe infestation mediated alteration of the phytohormone profile in bark and wood of its host. Cytokinins, jasmonic acid and abscisic acid levels increased in both tissues, whereas salicylic acid and indole-3-acetic acid, which were only detected in the bark, declined. The present results show that mistletoe infestation affects both the host's anti-oxidative defense system and the phytohormone profile after establishment of the xylem tapping haustorium. The significance of these processes for the development of the woody mistletoe stem and the haustorium is discussed.

Thinning effect on photosynthesis depends on needle ages in a Chinese fir (Cunninghamia lanceolata) plantation

Canopies in evergreen coniferous plantations often consist of various-aged needles. However, the effect of needle age on the photosynthetic responses to thinning remains ambiguous. Photosynthetic responses of different-aged needles to thinning were investigated in a Chinese fir (Cunninghamia lanceolata) plantation. A dual isotope approach [simultaneous measurements of stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes] was employed to distinguish between biochemical and stomatal limitations to photosynthesis. Our results showed that increases in net photosynthesis rates upon thinning only occurred in the current-year and one-year-old needles, and not in the two- to four-year-old needles. The increased δ¹³C and declined δ¹⁸O in current year needles of trees from thinned stands indicated that both the photosynthetic capacity and stomatal conductance resulted in increasing photosynthesis. In one-year-old needles of trees from thinned stands, an increased needle δ¹³C and a constant needle δ¹⁸O were observed, indicating the photosynthetic capacity rather than stomatal conductance contributed to the increasing photosynthesis. The higher water-soluble nitrogen content in current-year and one-year-old needles in thinned trees also supported that the photosynthetic capacity plays an important role in the enhancement of photosynthesis. In contrast, the δ¹³C, δ¹⁸O and water-soluble nitrogen in the two- to four-year-old needles were not significantly different between the control and thinned trees. Thus, the thinning effect on photosynthesis depends on needle age in a Chinese fir plantation. Our results highlight that the different responses of different-aged needles to thinning have to be taken into account for understanding and modelling ecosystem responses to management, especially under the expected environmental changes in future.