Spruce Research Articles

Large-Area UAS-Based Forest Health Monitoring Utilizing a Hydrogen-Powered Airship and Multispectral Imaging

Abstract. Climate change is threatening forest ecosystems worldwide by inducing various abiotic and biotic disturbances. In Europe, the European spruce bark beetle (Ips typographus L.) poses a significant threat, causing serious mortality in mature Norway spruce (Picea abies H. Karst.) stands. Rapidly evolving remote sensing technologies offer valuable tools for monitoring forest health, enabling timely management operations. This study presents a novel approach for large-area forest health monitoring using Uncrewed Aircraft Systems (UAS) and multispectral imaging. The research focuses on a hydrogen-powered Beyond Visual Line of Sight (BVLOS) airship for efficient monitoring of disturbances caused by I. typographus. A specific challenge is training machine learning models capable of covering wide areas. Our objective was to study the potential of deep learning models, including transfer learning and fine-tuning techniques, in developing the scalability and accuracy of UAS-based monitoring for detecting individual spruce trees and classifying their health. The approach was empirically evaluated in a study site in North Karelia, Finland. A multispectral image dataset was collected over a 1.3 km2 test area in May 2023 in a BVLOS setting operated from a command centre 75 km away. The results indicated that employing transfer learning significantly improved classification accuracy compared to training models from scratch, showing potential for implementing scalable machine learning methods for large-area UAS surveys. The best model yielded F1-scores of 0.936 for healthy, 0.955 for dead, and 0.817 for non-spruce classes. Furthermore, the results indicated that BVLOS airships offered high accuracy while reducing emissions and labour associated with UAS monitoring.

Reaction of Wood Ants to a Large-Scale European Spruce Bark Beetle Outbreak in Temperate Forests

In the Białowieża Forest, Norway spruce is the preferred host tree species for wood ants, both in coniferous and mixed stands; thus, spruce mortality as a consequence of a continuous spruce bark beetle outbreak in the Białowieża Forest since 2012 could have severe consequences for wood ant colonies, as well as their vitality and distribution. The main aim of this study was to assess whether the bark beetle outbreak had any effects on wood ant nest density and abundance and whether we could find any factors affecting the dead spruce distribution around ant nests. A re-inventory of active and abandoned wood ant nests in our study area was conducted from April to July 2022, using the same procedure as the previous inventory performed in 2016. The wood ant nest density was 0.11 per ha and remained practically the same relative to that determined in 2016. Our results indicate that, despite the importance of spruce for wood ants, in situations when only part of the spruce trees die, natural forest disturbances such as bark beetle outbreaks can actually have positive effects because more light can reach the forest floor, thereby promoting the establishment of new nests.

Legacy effect of 25 years reduced atmospheric sulphur deposition on spruce tree nutrition

AbstractBackgroundSince the mid‐1990s, sulphur (S) pollution was drastically reduced in Central Europe. Over time, this has led to a distinct reduction in S availability for Norway spruce (Picea abies Karst.), which is still the most important timber species in Central European forestries.AimsDetermination of the Norway spruce nutritional status of former strongly affected areas by S pollution (Saxony) with different degrees of liming by assessing their foliar element contents and comparing them to regions remote from historical high S deposition.MethodsSites were selected based on levels of S deposition in the 1970–1990s with historical high deposition in Saxony (NE Germany), low deposition at Schluchsee (SW Germany) and Davos (Switzerland) as a clean air reference. Needles were sampled in late autumn 2019/2020 and elemental contents determined. Additional historical data on foliar S contents were available.ResultsHistorical data showed a clear decrease in foliar S contents in the Saxonian sites over the last 25 years, independent of liming. No difference between all study sites was found in the most recent sampling, whereas S together with other macronutrients strongly indicates deficiencies for forest growth and health.ConclusionsAfter 25 years of reduced S deposition, S nutrition became low for Norway spruce trees in Saxony, whereas soil parent material determines the overall tree nutritional status with respect to other nutrients. As such, no difference between sites with historical high, low or no S deposition was found. Further studies should focus on the mineralization of organic S in the topsoil to understand if S is effectively recycled within the forest ecosystem and on the effect of other diminishing nutrients such as Mg and P.

Monitoring weekly δ 13С variations along the cambium-xylem continuum in the Canadian eastern boreal forest.

Intra-annual variations of carbon stable isotope ratios (δ13C) in different tree compartments could represent valuable indicators of plant carbon source-sink dynamics, at weekly time scale. Despite this significance, the absence of a methodological framework for tracking δ13C values in tree rings persists due to the complexity of tree ring development. To fulfill this knowledge gap, we developed a method to monitor weekly variability of δ13C in the cambium-xylem continuum of black spruce species [Picea mariana (Mill.) BSP.] during the growing season. We collected and isolated the weekly incremental growth of the cambial region and the developing tree ring from five mature spruce trees over three consecutive growing seasons (2019-2021) in Simoncouche and two growing seasons (2020-2021) in Bernatchez, both located in the boreal forest of Quebec, Canada. Our method allowed for the creation of intra-annual δ13C series for both the growing cambium (δ13Ccam) and developing xylem cellulose (δ13Cxc) in these two sites. Strong positive correlations were observed between δ13Ccam and δ13Cxc series in almost all study years. These findings suggest that a constant supply of fresh assimilates to the cambium-xylem continuum may be the dominant process feeding secondary growth in the two study sites. On the other hand, rates of carbon isotopic fractionation appeared to be poorly affected by climate variability, at an inter- weekly time scale. Hence, increasing δ13Ccam and δ13Cxc trends highlighted here possibly indicate shifts in carbon allocation strategies, likely fostering frost resistance and reducing water uptake in the late growth season. Additionally, these trends may be related to the black spruce trees' responses to the seasonal decrease in photosynthetically active radiation. Our findings provide new insights into the seasonal carbon dynamics and growth constraints of black spruce in boreal forest ecosystems, offering a novel methodological approach for studying carbon allocation at fine temporal scales.

Behavioral responses of predatory flies of the genus Medetera Fischer von Waldheim (Diptera: Dolichopodidae) and the tree-killing beetle Ips typographus L. (Coleoptera: Scolytinae) to odor compound blends

Key MessageMedetera (Fischer von Waldheim) flies, natural enemies of the spruce bark beetle Ips typographus (L.), were attracted to synthetic blends of compounds produced by infested spruce trees. A subset of trapped specimens revealed sixteen Medetera species. Most abundant were M. signaticornis, M. infumata, and M. prjachinae. Only blends containing beetle-produced compounds significantly attracted Medetera spp. and I. typographus.ContextFly species of the genus Medetera (Fischer von Waldheim) (Diptera: Dolichopodidae) represent one of the most important groups of natural enemies of the Eurasian bark beetle Ips typographus (L.), which infests Norway spruce Picea abies (L.) Karst. In a previous study, we showed that adult Medetera flies exploit semiochemicals to find beetle-infested trees however, the exact nature of those attractive compounds has not yet been determined.AimsThe aim of this follow-up study was to investigate the behavioral responses of Medetera spp. and I. typographus, to different combinations of semiochemicals.MethodsIn this study, 22 volatile compounds identified from I. typographus-infested Norway spruce were divided into five groups (A–E) based on being primarily produced by the bark beetle I. typographus (group A), bark beetle-associated microorganisms (groups B and C), or spruce tree (groups D and E). The effect of the compounds in these groups in the attraction of Medetera species and I. typographus was tested in two different subtractive field trapping assays.ResultsIn the first subtractive assay, the full blend (ABCDE), and the blends lacking microbial compounds of group C, or spruce tree compounds of group D led to significant attraction of Medetera flies. Morphological identification of a subset of the specimens collected revealed that sixteen species were attracted to the synthetic blends, with M. signaticornis Loew being the most abundant. In the second subtractive assay, high attraction of Medetera flies and I. typographus was found for a 12-component synthetic blend.ConclusionThe insights gained provide a basis for developing synthetic attractants to facilitate monitoring of Medetera flies. Future testing and optimization of these attractants will enhance our ability to monitor, conserve and utilize Medetera flies, thereby enabling us to better protect forests from the damaging effects of spruce bark beetles.

Mapping pathogenic fungi in decayed Norway spruce stands: Insights from harvester positional data

Abstract Root and butt rot caused by pathogenic fungi in the genera Heterobasidion and Armillaria is a pressing issue in managed Norway spruce forests. The disease results in financial losses for the forest owners and reduces the volume of wood that can be used in long‐lived products. Pathogenic wood decay fungi spread either with the aid of airborne spores or via mycelial growth among neighbouring trees, the latter leading to clustering (tendency of decayed trees to be in close proximity relative to their neighbouring trees) of decay‐affected trees in forests. Understanding the spatial patterns of the decay‐affected trees at the forest stand level is vital for designing management strategies to address this problem. We examined decay clustering in 273 clear‐cut Norway spruce stands in Norway using harvester‐recorded data on spatial occurrence of decayed and healthy Norway spruce trees. We tested clustering using three global‐cluster tests that account for population density and distribution, evaluating clustering without identifying specific cluster locations. The proportions of clustered and non‐clustered stands differed depending on the statistical test used for clustering assessment, resulting in overall agreement of 32.8% for clustered and 36.9% for non‐clustered. Clustered stands exhibited a median cluster distance (maximum distance between the decay‐affected trees within a cluster) of 12 m (Inter‐Quantile Range, IQR, 6–20 m) and a median of 6 (IQR 3–16) nearest neighbour trees (number of decayed trees forming a cluster), estimates comparable with prior studies focused on assessment of trees infected by mycelial spread of the same fungal individual. The decay incidence in the clustered stands was 16.24%, while the non‐clustered stands had a butt‐rot incidence of 20.97%. In clustered stands the average number of trees per hectare was higher (693) than in non‐clustered stands (553). Synthesis and applications: Our study demonstrates that Norway spruce stands display a diverse range of spatial patterns of butt rotted trees. We found that higher densities of Norway spruce trees probably facilitate the vegetative spread of pathogenic wood decay fungi, leading to clustering of decay‐affected trees. To disrupt the spread of decay fungi between tree generations, precision planting of trees other than Norway spruce around infested stumps of prior generation trees has been recommended by earlier studies. We discussed the potential of using harvester‐derived geoposition data for butt‐rotted trees upon planning and execution of forest regeneration.

Exploring forest changes in an Ips typographus L. outbreak area: insights from multi-temporal multispectral UAS remote sensing

AbstractUncrewed Aerial Systems (UAS) offer a versatile solution for monitoring forest ecosystems. This study aimed to develop and assess an individual tree-based methodology using multi-temporal, multispectral UAS images to track changes caused by the European spruce bark beetle (Ips typographus L.). The approach encompassed four key steps: (1) individual tree detection using structure-from-motion point clouds, (2) tree species classification, (3) health classification of spruce trees as healthy, declined, or dead, and (4) change detection, identifying fallen/removed trees and alterations in tree health status. The developed methodology was employed to quantify changes in a bark beetle outbreak area covering 215 hectares in southeastern Finland during 2019–2021. The dataset included two managed and two conserved forest areas. The uncertainty estimation demonstrated the overall accuracies ranging from 0.58 to 0.91 for individual tree detection, 0.84 for species classification, and 0.83–0.96 for health classification, and a F1-score of 0.91 for the fallen or removed tree detection. Maps and statistics were produced, containing information on the health of the spruce trees in the area and information on changes, including trees that died during monitoring and those that fell or were removed from the forest. The results demonstrated successful control of the outbreak in the managed stands, evidenced by moderate tree mortality. Conversely, in the conserved stands, the outbreak resulted in dramatic tree mortality. This method serves stakeholders by enabling large-scale outbreak impact monitoring, facilitating timely risk assessment, and validating bark beetle outbreak management strategies.

Natural disturbance history regime and the development of the dark coniferous forest in the Southern Sikhote-Alin

Age shifts in fir–spruce forests and the potential influence of ongoing climate changes upon them are pressing issues. The study focuses on a stand of undisturbed fir–spruce forest located in the southern Sikhote-Alin at an altitude of 530 m a.s.l. A permanent sample plot was established there in 2001, and a revision was conducted in 2022–2023 with wood samples (cores) obtained. The research has revealed significant deviations in the stand development from the previously described pattern of age shifts in virgin fir–spruce forests. Firstly, it was established that there have been no major stand disturbances over the last 242 years, and only minor ones occurring in 1831–1850 and 1931–1950 were identified. About 50% of Jezo spruce trees survived at least 2 minor disturbances during their development. Secondly, it is shown that Jezo spruce does not form a single-age generation within the studied stand. It was also found that at the time of revision, the stand was in a situation preceding a new stand disturbance. An important result obtained is that a stable trend of decreasing growth has been observed in 75% of Jezo spruce trees since 2001. This trend might not be associated with age shifts within the stand but could be attributed to the spruce’s response to global climate changes. Such a trend could significantly impact the age shifts in fir– spruce forest stands.

Towards scalable wide area UAS monitoring of forest disturbance using hydrogen powered airships

ABSTRACT Uncrewed Aircraft Systems (UAS) have been recognized as powerful tools for providing monitoring data supporting mitigating and adapting to various abiotic and biotic forest disturbances. However, their suitability and commercial viability have been significantly limited by the necessity for operation primarily within visual line of sight (VLOS). Efforts are currently ongoing to establish frameworks for beyond visual line of sight (BVLOS) UAS operation. This study represents the first successful application of hydrogen-powered BVLOS airship UAS technology for monitoring forest disturbances. The test area covered 1.3 km2 of conserved forest area in North Karelia, Finland, seriously affected by bark beetles and other disturbance agents. Monitoring flights were conducted in spring, summer, and autumn in BVLOS setting operated from a command centre situated approximately 75 km distance from the test area. Deep learning models were applied to study the tree health in the disturbance area, with the special focus on examining the scalability of machine learning models. Multispectral imagery outperformed RGB imagery in classifying trees as healthy, infested, and dead spruce trees, as well as non-spruce tree classes. With multispectral imagery, the F1-scores were 0.88–0.94, 0.80, 0.92–0.96, and 0.75–0.82 for healthy, infested, and dead spruce trees, and non-spruces, respectively. Transfer learning and fine-tuning, which adapted previously trained models to new study areas, improved results compared to training models from scratch and thus contributed to the development of scalable UAS remote sensing methods in mitigating forest disturbances on a larger scale.

Environmental Impact Assessment of a Plant Cell-Based Bio-Manufacturing Process for Producing Plant Natural Product Ingredients

Purpose: This study employed a Life Cycle Assessment (LCA) methodology to evaluate the environmental impacts of a novel plant cell-based biomanufacturing process for producing plant natural product ingredients. The primary purpose was to assess the relative sustainability of the process and to provide insights into potential areas of improvement in the biomanufacturing process. Method: The LCA method used an MS Excel (Ver. 2407) -based approach with a cradle-to-gate system boundary covering raw material sourcing (A1), raw material transportation (A2), and product extract manufacturing (A3) stages. Energy use and material inventory data are presented for different unit operations, and environmental impact factors were obtained from the Ecoinvent database. The study included a Material Circularity Index (MCI) calculation to assess the circularity of the biomanufacturing process for the production of saponin emulsifiers that are normally extracted from the woody tissue of the Chilean soapbark tree (Quillaja saponaria). Comparative analyses were performed against a wild-harvest approach for plant tannin extraction from spruce (Picea abies) tree bark. Key Results: The environmental impact assessment focused on determining relative Global Warming Potential (GWP), Acidification Potential (AP), Freshwater Eutrophication (FE), Particulate Matter Formation (PMF), and Ozone Depletion Potential (ODP). Results indicated that the extract manufacturing stage (A3) contributed significantly to adverse environmental impacts, with varying levels of effects based on the energy source used. Comparative analysis with the wild harvest approach highlights the lower environmental impact of the alternative biomanufacturing process. The biomanufacturing process showed a 23% reduction in GWP, AP, and FE and a 25% reduction in PMF and ODP relative to the wild harvest approach. However, the MCI for the biomanufacturing process was estimated to be 0.186, indicating a low material circularity. Conclusions: The results revealed that the extract manufacturing stage, particularly energy consumption, significantly influences the relative environmental impacts of the alternative production processes. Different energy sources exhibit varying effects, with renewable energy sources showing lower environmental impacts. The Material Circularity Index indicated a low circularity for the biomanufacturing process, suggesting opportunities for improvement, such as incorporating recycled or reused materials. Compared with the tannin extraction process, the plant cell-based biomanufacturing process demonstrated lower environmental impacts, emphasising the importance of sustainable practices and the use of renewable energy sources in future plant natural product sourcing. Recommendations include implementing more sustainable practices, optimising raw material choices, and extending product life spans to enhance circularity and overall environmental benefits.

Multifunctional Evaluation of Spruce–Fir Forest Based on Different Thinning Intensities

Multifunctional Evaluation of Spruce–Fir Forest Based on Different Thinning Intensities

Geochemical fingerprinting of Norway spruce from the Eastern Carpathians: Sr isotopic and multi-elemental signatures

Ensuring efficient wood traceability within procurement chains is essential for establishing sustainable forest management and minimizing environmental damage in countries that produce and export timber. While some progress has been made with key legislative reforms to tackle this issue, the effectiveness of law enforcement still relies on the availability of appropriate analytical tools to determine the provenance of wood. This study documents the Sr isotopic and multi-elemental signatures of Norway spruce trees in the Eastern Carpathians, Romania – an area known for intensive forest logging. The research focused on trees from Rodna, Rarău, and Călimani, analyzing tree rings from 1970 to 2020 to assess temporal variability and develop site-specific geochemical fingerprints. In conjunction with tree samples, corresponding soil, groundwater, and river samples were analyzed, providing a comprehensive view of the bioavailable 87Sr/86Sr ratios in these ecosystems. The study found a strong correlation between the Sr isotope ratios in the wood and those in corresponding water and soil samples, confirming that the wood accurately mirrors local geochemical signatures. The Sr isotope ratios were consistent across most of the cores from trees within the same site but varied among sites within the same region. Principal Component Analysis of the Sr isotopic and multi-elemental data highlighted closer clustering of samples from the Călimani area, though distinguishing the samples from the Rodna and Rarău regions proved more difficult due to overlapping data points. This study broadens our understanding of geochemical variability and its implications. The insights gained provide a valuable foundation for future research aimed at enhancing wood traceability methods.

Mobile laser scanning as reference for estimation of stem attributes from airborne laser scanning

The acquisition of high-quality reference data is essential for effectively modelling forest attributes. Incorporating close-range Light Detection and Ranging (LiDAR) systems into the reference data collection stage of remote sensing-based forest inventories can not only increase data collection efficiency but also increase the number of attributes measured with high quality. Therefore, we propose a model-based forest inventory method that uses reference data collected by a car-mounted mobile laser scanning (MLS) system along boreal forest roads. This approach is used for the estimation of diameter at breast height (DBH) and stem volume at the individual tree-level from airborne laser scanning (ALS) data. In addition, we compare the estimates obtained using the proposed method with the ones derived from reference data collected by traditional field inventory of 265 field plots systematically distributed over the study area. The accuracy of the estimates remained comparable regardless of the reference dataset used for estimation of DBH and stem volume. When using the field inventory dataset for model training, the root mean square error (RMSE) of DBH estimates were 4.06 cm (18.8 %) for Norway spruce trees, 6.3 cm (29.6 %) for Scots pine and 8.61 cm (55.9 %) for deciduous trees. Similarly, when evaluating predictions based on the MLS dataset as reference, RMSEs were equal to 3.97 cm (18.4 %) for Norway spruce, 6.12 cm (28.8 %) for Scots pine, and 8.98 cm (58.3 %) for deciduous trees. In general, biases were below 1 cm for most species classes, with the exception of deciduous trees. The accuracy of stem volume also had RMSEs varying across different tree species. For the estimates based on traditional field inventory, the RMSEs were 0.176 m3 (38.8 %) for Norway spruce, 0.228 m3 (52.4 %) for Scots pine and 0.246 m3 (158 %) for deciduous trees. When using the MLS dataset as a reference, the RMSEs were equal to 0.176 m3 (38.8 %), 0.228 m3 (52.4 %), and 0.246 m3 (158 %) for Norway spruce, Scots pine, and deciduous trees, respectively. Car-mounted MLS demonstrated its potential as an efficient alternative for collecting reference data in remote sensing-based forest inventories, which could complement traditional methods.

Restoration of stand on the site of burnt spruce forest in the Central Forest reserve

With the warming of the climate, the intensification of droughts and the drying up of coniferous forests, the number of forest fires is also increasing. Therefore, the study of the post-fire dynamics of stands and prospects for reforestation on burnt areas is especially relevant. The research was carried out in the Central Forest State Natural Biosphere Reserve (southern taiga) on the burnt area formed in 1999 as a result of a thunderstorm. The purpose of the work was: to describe the state of vegetation 22 years after burnout; to find out the characteristics of the stand formed on the burnt area and the possibility of their use for reconstructing the history of disturbancies in old spruce forests. The objectives of the study included: monitoring vegetation changes in the 1999 burnt area; studying the growth of spruce undergrowth in the burnt area; finding out the number and ratio of preliminary and subsequent renewal of tree species; analyzing the distribution and amount of coals in the sample plots (depending on the intensity of burnout and the state of the pre-fire stand). To study the state of vegetation after the burnout of the blueberry-wood sorrel and wood sorrel – nemoral-herb spruce (Picea abies (L.) H. Karst.) forests, 14 sample plots (100 m2 each) were laid in the northwestern part of the burnt area. The sample plots covered 1.5 hectares. 22 years after the fire, a mixed small-leaved forest with a predominance of birch and a significant participation of spruce has been forming on the site of burnt spruce forests. Spruce is represented mainly by post-fire renewal. In the undergrowth, the ratio of pre- and post-renewal spruce is about 1 : 3. In the ground cover, a small number of nemoral herb species is recorded, and in the stand and undergrowth – almost complete absence of broad-leaved species. A lot of coals were found in soil digs in 4 sample plots (30%); there were no coals in 3 (20%) sample plots, and small coals in small amount were found in 7 (50%) sample plots. Assessing the possibilities of identifying the first post-fire generation of spruce in old stands, it must be said that none of the characteristics of a post-fire spruce stand is universal for its recognition. Only a set of features allows to identify the history of the stand. Distinctive features of the first post-old generation: the presence of coals in 30% of samples or more; a large admixture of birch and aspen; a large diversity of the aspen part of the stand; if there are broad-leaved species, they will be much younger than the spruce; the presence of a sharp decline, and then a rise in growths in a significant part of the oldest spruce trees; the main generation of spruce has initial radial growths on an average of 2.1 mm/year, and the absence of synchronous sharp rises of the growths at the beginning of life. The age of aspen most accurately corresponds to the year of the fire.

Dendrochemical Analysis as a Method to Identify Mineralized Zones: Insights from the Marathon Cu-PGE Deposit

The Marathon Cu-platinum-group element (PGE) deposit is located within the Eastern Gabbro Series of the Proterozoic Coldwell Alkalic Complex, 10 km north of Marathon, Ontario. The Cu-PGE mineralization occurs within the Two Duck Lake Gabbro, an olivine gabbro intrusion. This geochemical orientation study tested the trace element analysis of tree cores as a method of identifying mineralized versus background areas in the Marathon deposit, focusing on copper (Cu), sulfur (S), palladium (Pd), and silver (Ag). Tree core samples (n=25) were collected with an increment borer from black spruce (Picea mariana) trees and dated by counting growth rings. Fifteen cores, 12 near/overtop the deposit and three distal control samples, were analysed with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) over a 10-year interval (1992-2002). Data normalization to 88Sr like in other dendrochemical studies corrected for tree health and growth effects. 32S, 63Cu, and 105Pd were found to have the highest concentrations at 94.198, 2.224, and 0.055 ppm, respectively, in control sample 23KPA2-3004. This tree is located beside the Pic River and receives large amounts of run-off from the entire Eastern Gabbro Series, possibly enhancing the chemical composition of the tree core. Excluding this anomalous sample, significant concentrations of Cu and Ag were found moving SSW along the deposit, aligning with the direction of movement of the Pleistocene Laurentide Ice Sheet. While this study did not clearly identify mineralized zones in the Marathon deposit, it has identified the possibility of mineralized run-off in the Pic River affecting nearshore tree chemical composition. The influence of local terrain and glacial till must be also considered when using this method. Future research will continue to study the factors influencing the uptake of elements in trees and refine methods for detecting mineralization through tree core analysis.

Tree Rings Mercury Controlled by Atmospheric Gaseous Elemental Mercury and Tree Physiology.

Tree rings are an emerging atmospheric mercury (Hg) archive. Questions have arisen, though, regarding their mechanistic controls and reliability. Here, we report contrasting tree-ring Hg records in three collocated conifer species: Norway spruce (Picea abies), Scots pine (Pinus sylvestris), and European larch (Larix decidua), which are from a remote boreal forest. Centennial atmospheric Hg trends at the site, derived from varved lake sediments, peats, and atmospheric monitoring, indicated a steady rise from the 1800s, peaking in the 1970s, and then declining. Prior to ca. 2005, larch and spruce tree rings reproduced the peak in the atmospheric Hg trend, while pine tree rings peaked in the 1930s, likely due to the prolonged sapwood period and ambiguity in the heartwood-sapwood boundary of pine. Since ca. 2005, tree rings from all species showed increasing Hg concentrations in the physiologically active outer rings despite declining atmospheric Hg concentrations. The good agreement between Hg and nitrogen concentrations in active tree-ring cells indicates a similar transport mechanism and cautions against their applicability as atmospheric Hg archives. Our results suggest that tree-ring Hg records are controlled by atmospheric Hg and tree physiology. We provide recommendations for using tree-ring Hg archives that take tree physiology into account.

Effect of bedrock, tree size and time on growth and climate sensitivity of Norway spruce in the High Tatras

AbstractTree growth is a multifaceted process influenced by various factors at different spatial and temporal scales, including intrinsic tree traits and environmental conditions. Climate factors have a significant impact on tree growth dynamics, while geological controls can also play a crucial role. However, our understanding of the interplay between these factors concerning tree growth is currently limited. This study focuses on Norway spruce (Picea abies [L.] Karst.), one of the economically most important coniferous tree species in Europe, to investigate the interplay of growth, climate, and environment at the forest and corresponding treeline sites in the High Tatra Mountains of Slovakia. Specifically, we developed chronologies of tree-ring width (TRW) and late-wood density (MXD) for different tree size classes across two limestone and granitic sites. Growth rates of Norway spruce trees have been increasing in forests since the 1930s and from the 1950s at treelines. Growth rates were consistently higher on limestone bedrock compared to granitic bedrock conditions. Variability of radial growth is primarily driven by climate at both geological settings with trees on granitic bedrock displaying more pronounced responses to climatic variables. We observed weakening (non-stationarity) in climate signals over time and across all size classes in both geological settings. The magnitude of these effects is small, but varies across size classes, with larger trees generally displaying stronger climate sensitivities compared to smaller ones. Therefore, our findings accentuate the potential implications of geological settings, climate, and environmental factors on the absolute growth and growth dynamics of Norway spruce, highlighting the need for further research to fully understand and manage forest ecosystems in mountainous regions.

Bark Thickness of Forest-forming Species: Modeling and Comparative Analysis

The need to assess the carbon-depositing capacity of a stand, an individual tree and its components is constantly emphasized in the world literature, since mitigation of the effects of climate change has become the highest priority. Since the carbon concentration differs in different tree species and in different components of the tree, including in the stem wood and bark, it is necessary to develop species-specific mathematical models for estimating the proportion of bark in trunks to improve the accuracy of knowledge about the carbon balance of forests. The relationship of bark thickness with both age and diameter of the tree trunk is known, but with respect to its regional variability, the data are contradictory. According to the actual data from 1100 model trees of five forest-forming species, allometric models of a mixed type have been developed. They include as independent variables the age and diameter of the trunk, as well as a binary variable characterising the influence of the growing area itself on the bark thickness of Scots pine, silver birch and Siberian larch, as well as the difference in the thickness of the bark between Siberian spruce and Siberian fir in case of joint growth in mature stands of the taiga zone. At a statistically significant level, it was found that the bark thickness of Scots pine and silver birch trees in the steppe zone is significantly greater compared to the taiga zone, and the bark thickness of Siberian larch trees in the forest-tundra is significantly greater compared to the steppe zone. The bark thickness of Siberian spruce trees is significantly less than that of Siberian fir trees. The ranking of species by bark thickness is included in the paper.

Divergent Response of Two Bark Beetle-Fungal Symbiotic Systems to Host Monoterpenes Reflects Niche Partitioning Strategies.

The successful establishment of bark beetle-fungus symbionts on plants is required to overcome host defenses. However, little is known about how different bark beetle-fungus symbionts adapt to different niches on the same host plant. Here, we investigated the niche partitioning mechanism of two co-occurring bark beetle-fungus symbiotic systems, Ips nitidus-Ophiostoma bicolor and Dendroctonus micans-Endoconidiophora laricicola, on Qinghai spruce (Picea crassifolia) tree. The lower niche of the spruce trunk inhabited by D. micans showed a higher content of monoterpenes than the upper niche of the trunk inhabited by I. nitidus. Dendroctonus micans showed greater tolerance and higher metabolic efficiency toward monoterpenes than I. nitidus. However, both beetle species showed a similar metabolic profile toward α-pinene, albeit with different levels of metabolites. Additionally, O. bicolor, transmitted by I. nitidus, showed a significantly higher tolerance to monoterpenes and pathogenicity to spruce trees than E. laricicola, transmitted by D. micans. In particular, monoterpenoid metabolites were observed to attenuate the inhibitory effect of high-dose α-pinene on E. laricicola, thus increasing its fitness in a high-dose monoterpene microhabitat. These results show that these two bark beetle-fungus symbionts have adapted to different niches, leading to fitness differences in niche distribution that are at least partly related to the different distribution of monoterpene concentration in the spruce trunk. This research provides a novel perspective for understanding the coevolution between bark beetle-fungus symbionts and their host plants.

From deadwood to forest soils: quantifying a key carbon flux in boreal ecosystems

Deadwood represents a dynamic carbon pool in forest ecosystems where microbial decomposition causes fluxes of CO2 to the atmosphere through respiration and organic carbon to the soil through leakage and fragmentation. This study characterises different stages of deadwood of Norway spruce (Picea abies). 35 Norway spruce trees were sampled and categorized on a 0–5 decay scale. For the 14 trees in classes 0–3, two stem discs were collected from two heights. For the 21 trees in classes 4 and 5, a single sample per tree was taken, because decay was relatively uniform throughout the stem. The relative amount of hemicellulose and cellulose declined moderately from decay class 1 to 3 and substantially from decay class 3 to class 4 but small amounts were still present in decay class 5. The relative lignin proportion increased substantially from decay class 3 to 4 and dominated in decay class 5. Relative carbon content increased from 50 to 56% during the decomposition process due to the increasing accumulation of lignin residuals being a typical signature of brown rot decay. A laboratory experiment including three species of brown rot fungi verified decomposition close to 70% of Norway spruce biomass and resulted in 55% carbon content. This was similar to the carbon content in decay class 4 and 5. A novel approach is presented to quantify the carbon flux from deadwood to the soil. First, we calculated the residual proportion of carbon in decayed wood compared to the initial carbon content of live trees. Subsequently, we extended the calculation to determine the amount of remaining carbon from non-decayed wood that was transferred to the soil during each decay class. The approach showed that Norway spruce wood decomposition under field conditions transfers at least 39–47% of the initial wood carbon to the soil carbon pool, depending on soil type. This strengthens the previously under-communicated fact that the carbon flux from deadwood to soil is higher from brown rot decomposition in boreal forests than the corresponding carbon flux in temperate and tropical forests where deadwood is more influenced by white rot fungi.