Important European Tree Species Research Articles

Quantifying Forest Cover Loss as a Response to Drought and Dieback of Norway Spruce and Evaluating Sensitivity of Various Vegetation Indices Using Remote Sensing

The Norway spruce is one of the most important tree species in Europe. This tree species has been put under considerable pressure due to the ongoing impacts of climate change. Meanwhile, frequent droughts and pest outbreaks are reported as the main reason for its dieback, resulting in severe forest cover loss. Such was the case with Norway spruce forests within the Kopaonik National Park (NP) in Serbia. This study aims to quantify, spatially and temporally, forest cover loss and to evaluate the sensitivity of various vegetation indices (VIs) in detecting drought-induced response and predicting the dieback of Norway spruce due to long-lasting drought effects in the Kopaonik NP. For this purpose, we downloaded and processed a large number of Landsat 7 (ETM+), Landsat 8 (OLI), and Sentinel 2 (MSI) satellite imagery acquired from 2009 to 2022. Our results revealed that forest cover loss was mainly driven by severe drought in 2011 and 2012, which was later significantly influenced by bark beetle outbreaks. Furthermore, various VIs proved to be very useful in monitoring and predicting forest health status. In summary, the drought-induced response detected using various VIs provides valuable insights into the dynamics of forest cover change, with implications for monitoring and conservation efforts of Norway spruce forests in the Kopaonik NP.

Unpunctual in diversity: The effect of stand species richness on spring phenology of deciduous tree stands varies among species and years.

Climatic drivers alone do not adequately explain the regional variation in budburst timing in deciduous forests across Europe. Stand-level factors, such as tree species richness, might affect budburst timing by creating different microclimates under the same site macroclimate. We assessed different phases of the spring phenology (start, midpoint, end, and overall duration of the budburst period) of four important European tree species (Betula pendula, Fagus sylvatica, Quercus robur and Tilia cordata) in monocultures and four-species mixture stands of a common garden tree biodiversity experiment in Belgium (FORBIO) in 2021 and 2022. Microclimatic differences between the stands in terms of bud chilling, temperature forcing, and soil temperature were considerable, with four-species mixtures being generally colder than monocultures in spring, but not in winter. In the colder spring of 2021, at the stand level, the end of the budburst period was advanced, and its overall duration shortened, in the four-species mixtures. At species level, this response was significant for F. sylvatica. In the warmer spring of 2022, advances in spring phenology in four-species stands were observed again in F. sylvatica and, less markedly, in B. pendula but without a general response at the stand level. Q. robur showed specific patterns with delayed budburst start in 2021 in the four-species mixtures and very short budburst duration for all stands in 2022. Phenological differences between monocultures and four-species mixtures were linked to microclimatic differences in light availability rather than in temperature as even comparatively colder microclimates showed an advanced phenology. Compared to weather conditions, tree species richness had a lower impact on budburst timing, but this impact can be of importance for key species like F. sylvatica and colder springs. These results indicate that forest biodiversity can affect budburst phenology, with wider implications, especially for forest- and land surface models.

Water Vapor Sorption Kinetics of Beech Wood Modified with Phenol Formaldehyde Resin Oligomers

Beech is an important tree species in Europe. This study aimed to elucidate the influence of four molecular weights of phenol formaldehyde (PF) resin (266, 286, 387, and 410 g/mol) on the sorption behavior of unmodified and modified beech wood samples using a dynamic vapor sorption (DVS) apparatus. The variations in the environmental relative humidity and moisture content (MC) of the samples were recorded, and the DVS isotherms were plotted from the equipment. During the sorption process, the MC of the modified samples decreased in comparison to that of the unmodified samples, and both apparently decreased with the increasing molecular weight of the PF resin. The DVS isotherm hysteresis plot illustrated a reduction in sorption hysteresis for the modified wood with varying PF resins compared to the unmodified samples. Based on the DVS isotherm adsorption and desorption plots, the decrease in the equilibrium of the MC can be attributed to there being fewer sorption sites in the modified samples, which exhibited the lowest hygroscopicity. Overall, the moisture sorption mechanism for both types of samples was clarified, highlighting a clear correlation between the molecular weight of the applied PF resin and its influence on moisture sorption behavior.

Transcriptome profiling of Fraxinus excelsior genotypes infested by emerald ash borer

European ash, Fraxinus excelsior is facing the double threat of ongoing devastation by the invasive fungal pathogen, Hymenoscyphus fraxineus and the imminent arrival of the non-native emerald ash borer (EAB), Agrilus planipennis. The spread of EAB which is currently moving westwards from European Russia and Ukraine into central Europe, poses an additional substantial threat to European ash, F. excelsior. While the molecular basis for resistance or variation in resistance among European ash genotypes is heavily investigated, comparatively little is known about the molecular ash traits involved in resistance against EAB. In this study we have gathered transcriptomic data from EAB inoculated genotypes of F. excelsior that have previously shown different levels of susceptibility to EAB. Resultant datasets show differential gene expression in susceptible and resistant genotypes in response to EAB infestation. This data will provide important information on the molecular basis of resistance to the EAB and allow the development of management plans to combat a pending threat of a culturally and ecologically important European tree species.

Silicon modifies leaf nutriome and improves growth of oak seedlings exposed to phosphorus deficiency and Phytophthora plurivora infection.

Beneficial effects of silicon (Si) on plants have primarily been studied in crop species under single stress. Moreover, nutrient acquisition-based responses to combination of biotic and abiotic stresses (a common situation in natural habitats) have rarely been reported, in particular in conjunction with soil amendments with Si. Pedunculate oak (Quercus robur L.), one of the ecologically and economically most important tree species in Europe, is facing a severe decline due to combined stresses, but also problems in assisted regeneration in nurseries. Here, we studied the effect of Si supply on the leaf nutriome, root traits and overall growth of 12-weeks-old oak seedlings exposed to abiotic stress [low phosphorus (P) supply], biotic stress (Phytophthora plurivora root infection), and their combination. The application of Si had the strongest ameliorative effect on growth, root health and root phenome under the most severe stress conditions (i.e., combination of P deficiency and P. plurivora root infection), where it differentially affected the uptake and leaf accumulation in 11 out of 13 analysed nutrients. Silicon supply tended to reverse the pattern of change of some, but not all, leaf nutrients affected by stresses: P, boron (B) and magnesium (Mg) under P deficiency, and P, B and sulphur (S) under pathogen attack, but also nickel (Ni) and molybdenum (Mo) under all three stresses. Surprisingly, Si affected some nutrients that were not changed by a particular stress itself and decreased leaf Mg levels under all the stresses. On the other hand, pathogen attack increased leaf accumulation of Si. This exploratory work presents the complexity of nutrient crosstalk under three stresses, and opens more questions about genetic networks that control plant physiological responses. Practically, we show a potential of Si application to improve P status and root health in oak seedlings, particularly in nurseries.

Trees Adjust the Shape of Branch Unions to Increase Their Load-Bearing Capacity

The likelihood of branch union failure often needs to be assessed in tree risk assessment. Most of the guidance used in practice is based on the shape of these forks, specifically the shape (“U” or “V”), the angle between the branches, the presence of lateral bulges, and the aspect ratio of the branches. This study extends previous studies with a novel approach to the biomechanical analysis of fork shape and contributes results from destructive tests on two important European tree species, using comparatively large trees. Surprisingly, many samples deviated from the expected pattern of constant or decreasing cross-sectional area from the trunk beyond the fork. The results show three mechanisms that counteract the potential weakening at a bifurcation, two of which have not been documented before: an increase in section modulus from the stem base to where the stems part, an increase in section modulus caused by lateral bulging, and an increase in section modulus in the branches caused by an adjusted shape. Neither the shape of the forks nor the amount of included bark had a significant impact on their strength. Like several previous studies, the results of this study caution against the use of simple rules to assess the likelihood of branch union failure. The increasing availability of “digital twins” of urban trees may help us to use these results to assess the shape of branch unions in a quantitative way.

Patterns of genetic variation and the potential origin of sweet chestnut (Castanea sativa Mill.) stands far from its natural northern distribution edge

Abstract Castanea sativa Mill. is an important European tree species that has been used for timber and nut production since Greek and Roman times. In Germany, the species accounts for only a small proportion of forest stands but it is attracting increasing interest due to its possible potential for climate tolerance and resilience. However, the status of German sweet chestnut stands in terms of genetic diversity and adaptive potential is poorly investigated. We took a representative sample of 520 individuals from 13 stands in northern Germany (Mecklenburg Western Pomerania; MWP) and genotyped these at 11 genomic SSRs and EST-SSRs. In addition, we compared our data with the available literature, focusing on the possible origin of the MWP populations. The analysis of genetic structure identified three distinct genetic groups within MWP, suggesting different origins or seed mixtures of various provenances. The comparison with reference data at the EST-SSR FIR059 suggests that the MWP stands originate from the western gene pool. The genetic variation observed in most MWP stands is at approximately the same level as stands in the natural range. Therefore, these stands with high diversity seem to be particularly suitable for further production of seed and plant material in MWP. This study contributes to the understanding of the genetic diversity and origin of sweet chestnut stands outside their natural distribution in MWP. It provides valuable knowledge for the evaluation and conservation of genetic resources of this tree species, which is expected to receive more attention under warmer and drier conditions.

The bigger the tree the better the seed – effect of Sessile oak tree diameter on acorn size, insect predation, and germination

Sessile oak is one of the ecologically and economically most important tree species in Europe. Recently, the importance of this species has been particularly highlighted in the context of climate change, where it is expected that these species will play a significant role within their natural range and beyond. To regenerate and expand the range of the sessile oak forests, a large amount of healthy acorns is needed. The acorn production and the acorns themselves are endangered by a large number of abiotic and biotic factors. As oak trees of different sizes do not produce the same amount of seed of the same quality, we investigated how the sessile oak tree diameter affects the acorn size, insect infestation, and germination rate. On the other side, as the acorn size influences the attack rates of the most significant acorn pest – Curculio glandium (Marsham, 1802), and the germination rates of both the damaged and healthy acorns we also investigated how the acorn dimensions influence the insect damage and germination rates, and how the insect damage influences the germination rates. We determined that the oak tree size influences some oak seed characteristics that are important for forest regeneration, while it does not affect others. A greater acorn yield was recorded on the bigger oak trees. The tree dimensions also had a significant influence on the acorn size. Greater average length, as well as greater average acorn diameter, were recorded on bigger trees. Although the tree size affects the acorn size, it does not influence the acorn insect predation rates or its germination rates. These parameters are affected by the acorn size itself. Bigger acorns had a greater germination rate. Insects preferred smaller acorns and had a significantly negative influence on the germination rate.

Tree species growth response to climate in mixtures of Quercus robur/Quercus petraea and Pinus sylvestris across Europe - a dynamic, sensitive equilibrium

Quercus robur/Quercus petraea and Pinus sylvestris are widely distributed and economically important tree species in Europe co-occurring on mesotrophic, xeric and mesic sites. Increasing dry conditions may reduce their growth, but growth reductions may be modified by mixture, competition and site conditions. The annual diameter growth in monospecific and mixed stands along an ecological gradient with mean annual temperatures ranging from 5.5 °C to 11.5 °C was investigated in this study. On 36 triplets (108 plots), trees were cored and the year-ring series were cross-dated, resulting in year-ring series of 785 and 804 trees for Q. spp. and P. sylvestris, respectively. A generalized additive model with a logarithmic link was fit to the data with random effects for the intercept at the triplet, year and tree level and a random slope for the covariate age for each tree; the Tweedie-distribution was used. The final model explained 87 % of the total variation in diameter increment for both tree species. Significant covariates were age, climate variables (long-term mean, monthly), local competition variables, relative dbh, mixture, stand structure and interactions thereof. Tree growth declined with age and local density and increased with social position. It was positively influenced by mixture and structural diversity (Gini coefficient); mixture effects were significant for P. sylvestris only. The influence of potential evapotranspiration (PET) in spring and autumn on tree growth was positive and non-linear, whereas tree growth sharply decreased with increasing PET in June, which proved to be the most influential month on tree growth along the whole ecological gradient. Interactions of PET with tree social position (relative dbh) were significant in July and September for Q. spp. and in April for P. sylvestris. Interactions of climate with density or mixture were not significant. Climatic effects found agree well with previous results from intra-annual growth studies and indicate that the model captures the causal factors for tree growth well. Furthermore, the interaction between climate and relative dbh might indicate a longer growth duration for trees of higher social classes. Analysis of random effects across time and space showed highly dynamic patterns, with competitive advantages changing annually between species and spatial patterns showing no large-scale trends but pointing to the prevalence of local site factors. In mixed-species stands, the tree species have the same competitivity in the long-term, which is modified by climate each year. Climate warming will shift the competitive advantages, but the direction will be highly site-specific.

Mismatch between Annual Tree-Ring Width Growth and NDVI Index in Norway Spruce Stands of Central Europe

Presently, the forests of one of the most economically important tree species in Europe—Norway spruce [Picea abies (L.) Karst.]—have been disrupted and are in rapid decline due to a combination of several natural factors: extreme drought, heatwaves, and secondary damage caused by bark beetle outbreaks. The vulnerability of these forests has increased considerably over the past decade, and remote sensing methods can theoretically improve the identification of endangered forest stands. The main objective was to determine the relationship between remotely sensed characteristics of vegetation (using the normalized difference vegetation index—NDVI) and annual tree-ring growth in 180 trees through precipitation and air temperature. The research was conducted at six research plots in lowland spruce forests (319–425 m a.s.l.) in the central Czech Republic. No significant correlation between NDVI and annual ring width was observed. The primary factor limiting radial growth was lack of precipitation in the growing season; subsequently, spruce trees reacted negatively to air temperatures. A higher correlation with NDVI was observed on sites susceptible to drought, but overall, NDVI and RWI did not show similarities. This result describes that NDVI is a poor indicator for identifying low radial growth in Norway spruce stands on non-native localities in the studied area.

Molecular plasticity to soil water deficit differs between sessile oak (Quercus Petraea (Matt.) Liebl.) high- and low-water use efficiency genotypes.

Water use efficiency (WUE) is an important adaptive trait for soil water deficit. The molecular and physiological bases of WUE regulation in crops have been studied in detail in the context of plant breeding. Knowledge for most forest tree species lags behind, despite the need to identify populations or genotypes able to cope with the longer, more intense drought periods likely to result from climate warming. We aimed to bridge this gap in knowledge for sessile oak (Quercus petraea (Matt.) Liebl.), one of the most ecologically and economically important tree species in Europe, using a factorial design including trees with contrasted phenotypic values (low and high WUE) and two watering regimes (control and drought). By monitoring the ecophysiological response, we first qualified genotypes for their WUE (by using instantaneous and long-term measures). We then performed RNA-seq to quantify gene expression for the three most extreme genotypes exposed to the two watering regimes. By analyzing the interaction term, we were able to capture the molecular strategy of each group of plants for coping with drought. We identified putative candidate genes potentially involved in the regulation of transpiration rate in high-WUE phenotypes. Regardless of water availability, trees from the high-WUE phenotypic class overexpressed genes associated with drought responses, and in the control of stomatal density and distribution, and displayed a downregulation of genes associated with early stomatal closure and high transpiration rate. Fine physiological screening of sessile oaks with contrasting WUE, and their molecular characterization (i) highlighted subtle differences in transcription between low- and high-WUE genotypes, identifying key molecular players in the genetic control of this trait and (ii) revealed the genes underlying the molecular strategy that evolved in each group to potentially cope with water deficit, providing new insight into the within-species diversity in drought adaptation strategies.

Spatial species distribution models: Using Bayes inference with INLA and SPDE to improve the tree species choice for important European tree species

Species distribution models (SDMs) are a standard tool for predicting species occurrence under climate change. Despite its limitations, SDMs have been widely used to assist forest management decisions in their choice of future tree species. The accuracy of SDMs is often affected by heterogeneous occurrence data caused by different forest inventory schemes, historical processes, climate and insect calamities and more. These processes bias the relationships modelled between species occurrence and climate. However, numerous studies have shown that explicit modelling of spatial effects may improve model accuracy. In this study, we applied the Integrated Nested Laplace Approximation (INLA) and Stochastic Partial Differential Equations (SPDE) algorithms as a means to accomplish Bayes inference for Generalized Additive Models (GAMs) with explicit modelling of spatial effects. Our results show that including spatial effects in GAM-based SDMs improved model performance and accuracy leading to more reliable predictions for the species Favorability. Conditional predictions that remove spatial effects from the models allow us to distinguish core and marginal ecological ranges with less bias through forest management, which may support the tree-species choice for climate-resilient forests.

Characterisation of nuclear microsatellite markers for Fraxinus excelsior L. and their transferability to six related species

Abstract Common ash (Fraxinus excelsior L.) is an economically and ecologically important tree species in Europe acutely threatened by ash dieback. Here, we present a new set of Simple Sequence Repeat markers for F. excelsior and six related ash species based on published in silico designed primers. Forty SSR markers, mainly from genic regions, were tested for amplification and polymorphism and characterized in two ash populations in Germany. Transferability of polymorphic markers to six related Fraxinus species (F. angustifolia, F. ornus, F. quadrangulata, F. pennsylvanica, F. americana and F. biltmoreana) was also assessed. Eighteen markers, predominantly from genic regions including three markers closely linked to candidate genes for ash dieback, were successfully amplified and polymorphic in F. excelsior, of which between 10 to 17 were transferable to one of the six related species. High genetic diversity was found in the two ash populations (Na = 7.8 and 6.9, He = 0.71 and 0.68), while low genetic differentiation between populations (FST = 0.025) was observed. The newly characterised SSR markers extend the set of genetic markers available for F. excelsior and six other ash species for future studies on the genetic diversity and structure of ash populations.

Drought Stress Described by Transcriptional Responses of Picea abies (L.) H. Karst. under Pathogen Heterobasidion parviporum Attack

The major threats to the sustainable supply of forest tree products are adverse climate, pests and diseases. Climate change, exemplified by increased drought, poses a unique threat to global forest health. This is attributed to the unpredictable behavior of forest pathosystems, which can favor fungal pathogens over the host under persistent drought stress conditions in the future. Currently, the effects of drought on tree resistance against pathogens are hypothetical, thus research is needed to identify these correlations. Norway spruce (Picea abies (L.) H. Karst.) is one of the most economically important tree species in Europe and is considered highly vulnerable to changes in climate. Dedicated experiments to investigate how disturbances will affect the Norway spruce—Heterobasidion sp. pathosystem are important, in order to develop different strategies to limit the spread of H. annosum s.l. under the predicted climate change. Here, we report a transcriptional study to compare Norway spruce gene expressions to evaluate the effects of water availability and the infection of Heterobasidion parviporum. We performed inoculation studies of three-year-old saplings in a greenhouse (purchased from a nursery). Norway spruce saplings were treated in either high (+) or low (−) water groups: high water group received double the water amount than the low water group. RNA was extracted and sequenced. Similarly, we quantified gene expression levels of candidate genes in biotic stress and jasmonic acid (JA) signaling pathways using qRT-PCR, through which we discovered a unique preferential defense response of H. parviporum-infected Norway spruce under drought stress at the molecular level. Disturbances related to water availability, especially low water conditions can have negative effects on the tree host and benefit the infection ability of the pathogens in the host. From our RNA-seq analysis, 114 differentially expressed gene regions were identified between high (+) and low (−) water groups under pathogen attack. None of these gene pathways were identified to be differentially expressed from both non-treated and mock-control treatments between high (+) and low (−) water groups. Finally, only four genes were found to be associated with drought in all treatments.

High-frequency stable isotope signals in uneven-aged forests as proxy for physiological responses to climate in Central Europe.

Picea abies (L.) Karst. and Fagus sylvatica (L.) are important tree species in Europe, and the foreseen increase in temperature and vapour pressure deficit (VPD) could increase the vulnerability of these species. However, their physiological performance under climate change at temperate and productive sites is not yet fully understood, especially in uneven-aged stands. Therefore, we investigated tree-ring width and stable isotope chronologies (δ13C/δ18O) of these two species at 10 sites along a climate gradient in Central Europe. In these uneven-aged stands, we compared the year-to-year variability of dominant and suppressed trees for the last 80years in relation to the sites' spatial distribution and climate. δ18O and δ13C were generally consistent across sites and species, showing high sensitivity to summer VPD, whereas climate correlations with radial growth varied much more and depended on mean local climate. We found no significant differences between dominant and suppressed trees in the response of stable isotope ratios to climate variability, especially within the annual high-frequency signals. In addition, we observed a strikingly high coherence of the high-frequency δ18O variations across long distances with significant correlations above 1500km, whereas the spatial agreement of δ13C variations was weaker (~700km). We applied a dual-isotope approach that is based on known theoretical understanding of isotope fractionations to translate the observed changes into physiological components, mainly photosynthetic assimilation rate and stomatal conductance. When separating the chronologies in two time windows and investigating the shifts in isotopes ratios, a significant enrichment of either or both isotope ratios over the last decades can be observed. These results, translated by the dual-isotope approach, indicate a general climate-driven decrease in stomatal conductance. This improved understanding of the physiological mechanisms controlling the short-term variation of the isotopic signature will help to define the performance of these tree species under future climate.

Mixed vs. monospecific mountain forests in response to climate change: structural and growth perspectives of Norway spruce and European beech

Mixed forests play a key role in terms of stability, production potential and adaptation to climate change. Norway spruce [PA, Picea abies (L.) Karst] and European beech (FS, Fagus sylvatica L.) are among the most important tree species in Europe. The aim of the study was to determine the influence of the species composition of these two tree genera on the production, structure, diversity and growth of mixed Fageto-Piceetum acidophilum stands in the Krkonoše Mountains, in the Czech Republic. The following 5 variants (ratios) of mixture were compared in 6 replications (30 research plots in total): PA 100%, PA 75:25 FS, PA 50:50 FS, PA 25:75 FS and FS 100%. Based on 178 tree core samples, the research also focused on the influence of climatic factors (temperature, precipitation) and air pollution (SO2, NOX, AOT40F) on the radial growth of these tree species of particular variants. Mixed forests showed a timber production higher by 7.7% (-10.8 to 31.5%) in comparison to spruce monocultures, and by 47.3% (21.9–79.7%) compared to beech monocultures. The largest production as well as the highest diameter increment were documented in PA 75:25 FS (656 m3 ha−1). In addition, this variant had the lowest extreme decreases/fluctuations in radial growth in both tree species. Over the last 50 years, the increment in beech increased by 7.9% and by 2.5% in spruce. The cyclical behavior in the radial growth of both tree species occurred in the short-term solar cycles of 9–11 and long-term periods of 50–75 years, while the spruce showed higher cyclic intensity. The concentration of both SO2 and NOX had a significant negative effect on the radial growth of spruce. In both tree species, the negative effect of air pollution lessened with their decreasing share in the stand. Similarly, precipitation and temperature had a more significant effect on the growth of monospecific variants in both tree species, especially in beech. Temperatures, when compared to precipitation, had a greater effect on the radial growth of both tree genera, especially during the vegetation period. In terms of diversity, mixed stands achieved significantly higher structural (diameter, height, crown) differentiation and overall diversity compared to monospecific variants. In general, mixed stands can achieve higher production potential, diversity and especially resistance to climate extremes and air pollution in relation to climate change in the water-sufficient highland and mountain areas of the Czech Republic. Differences between mixed stands vs. monocultures, i.e. the effect of tree species mixing, depend on suitable ratios of tree species and their spatial pattern.

The GenTree Leaf Collection: Inter‐ and intraspecific leaf variation in seven forest tree species in Europe

AbstractMotivationTrait variation within species can reveal plastic and/or genetic responses to environmental gradients, and may indicate where local adaptation has occurred. Here, we present a dataset of rangewide variation in leaf traits from seven of the most ecologically and economically important tree species in Europe. Sample collection and trait assessment are embedded in the GenTree project (EU‐Horizon 2020), which aims at characterizing the genetic and phenotypic variability of forest tree species to optimize the management and sustainable use of forest genetic resources. Our dataset captures substantial intra‐ and interspecific leaf phenotypic variability, and provides valuable information for studying the relationship between ecosystem functioning and trait variability of individuals, and the response and resilience of species to environmental changes.Main types of variable containedWe chose morphological and chemical characters linked to trade‐offs between acquisition and conservation of resources and water use, namely specific leaf area, leaf size, carbon and nitrogen content and their ratio, and the isotopic signature of stable isotope 13C and 15N in leaves.Spatial location and grainWe surveyed between 18 and 22 populations per species, 141 in total, across Europe.Time periodLeaf sampling took place between 2016 and 2017.Major taxa and level of measurementWe sampled at least 25 individuals in each population, 3,569 trees in total, and measured traits in 35,755 leaves from seven European tree species, i.e. the conifers Picea abies, Pinus pinaster and Pinus sylvestris, and the broadleaves Betula pendula, Fagus sylvatica, Populus nigra and Quercus petraea.Software formatThe data files are in ASCII text, tab delimited, not compressed.

Effect of Fungus Meripilus giganteus (Pers.) P. Karst. on Occurrence and Development of False Heartwood and Rot in Fagus sylvatica L. Round Wood

Common beech is one of the most widespread and important European tree species, widely used in timber industry and for energy production. Under specific set of complex factors, it facutatetivly develops false heartwood, which considerably decreases market demand and value of processed logs. Due to its properties, false heartwood is more susceptible to attack of wood decay fungi, which leads to further loss of wood quality and value. One of the most common fungi able to cause heart rot in beech is Meripilus giganteus, known for its spread in the basal parts of the tree, where it can affect most valuable sections of round wood. The aims of this study were to monitor the distribution of fungus and appearance of its fruiting bodies in research area, and to analyze the extent of fungus impact on shape and size of false heartwood and occurrence and length of rot in infected trees, while taking into account the observed stem damage as a possible influential factor. Fruiting bodies occurrence, life span and position on a tree were monitored during a six-year period. For trees with confirmed infection, stem damage was evaluated and appointed to one of four size classes. False heartwood shape and share in associated stem cross-section were observed and measured on 1–4 cross-sections per tree at different heights, and compared between infected and uninfected trees. If present, length of wood decay extent on butt-log was measured. The obtained results confirmed increased susceptibility of mature trees to infection, which seemed to occur mostly via roots from where mycelium spread into stem base. It was found that Meripilus giganteus has a significant impact on enlargement and change of FH shape from cloud- to star-like, up to approximately 5 m of the stem height, thus causing devaluation of the first assortments. The presence of rot was confirmed on the majority of infected trees, extending averagely 0.5 m into the first processed log, causing the loss of utilizable volume and thus the value of round wood. Stem damage category showed no significant effect on false heartwood or rot, supporting the prevailing impact of the fungus.

From the comfort zone to crown dieback: Sequence of physiological stress thresholds in mature European beech trees across progressive drought

Drought responses of mature trees are still poorly understood making it difficult to predict species distributions under a warmer climate. Using mature European beech (Fagus sylvatica L.), a widespread and economically important tree species in Europe, we aimed at developing an empirical stress-level scheme to describe its physiological response to drought. We analysed effects of decreasing soil and leaf water potential on soil water uptake, stem radius, native embolism, early defoliation and crown dieback with comprehensive measurements from overall nine hydrologically distinct beech stands across Switzerland, including records from the exceptional 2018 drought and the 2019/2020 post-drought period. Based on the observed responses to decreasing water potential we derived the following five stress levels: I (predawn leaf water potential >−0.4 MPa): no detectable hydraulic limitations; II (−0.4 to −1.3): persistent stem shrinkage begins and growth ceases; III (−1.3 to −2.1): onset of native embolism and defoliation; IV (−2.1 to −2.8): onset of crown dieback; V (<−2.8): transpiration ceases and crown dieback is >20%. Our scheme provides, for the first time, quantitative thresholds regarding the physiological downregulation of mature European beech trees under drought and therefore synthesises relevant and fundamental information for process-based species distribution models. Moreover, our study revealed that European beech is drought vulnerable, because it still transpires considerably at high levels of embolism and because defoliation occurs rather as a result of embolism than preventing embolism. During the 2018 drought, an exposure to the stress levels III-V of only one month was long enough to trigger substantial crown dieback in beech trees on shallow soils. On deep soils with a high water holding capacity, in contrast, water reserves in deep soil layers prevented drought stress in beech trees. This emphasises the importance to include local data on soil water availability when predicting the future distribution of European beech.

Tree species diversity improves beech growth and alters its physiological response to drought

Beech trees growing in biodiverse patches in Belgium have higher radial growth and are less physiologically sensitive to drought than those in monocultures. Forest diversification therefore alleviates the negative effects of drought on beech. Common beech, a widespread and economically important tree species in Europe, is known to be drought sensitive. For ensuring its survival under increasing future drought conditions, we need to advance our understanding on the relationships between drought and its growth performance. Diversifying forests has been proposed as a management strategy to mitigate the effects of drought, because a more complete use of the available water is expected. We made use of a tree species diversity gradient in Belgium to study if beech trees growing in diverse forest patches are more resistant and resilient to drought than beech trees in monocultures. Combining dendrochronological and stable carbon (δ13C) and oxygen isotope (δ18O) data allowed for studying the effect of tree species diversity on the response of beech growth to drought regarding stem radial growth and physiological performance. Up to 62% enhanced stem radial growth strongly increased growth stability, and higher resistance to drought was observed for beech trees in diverse forest patches. Beech performs best in three-species mixtures, particularly those with oak and maple. In drought years, beech growth is more reduced in monocultures than mixtures. During these drought years, δ13C values increased, and the increase was weaker in beech trees of diverse stands compared to monospecific stands, indicating enhanced stomatal conductance and growth continuation in mixtures. δ18O patterns did not indicate a clear effect of diversity or the response of beech trees to drought. Overall, our results indicate that until now, still the positive effects of diversity on beech growth outperform the negative effects induced by drought.