Leveraging research infrastructure co-location to evaluate constraints on terrestrial carbon cycling in northern European forests.

Integrated long-term, in-situ observations are needed to document ongoing environmental change, to "ground-truth" remote sensing and model outputs and to predict future Earth system behaviour. The scientific and societal value of in-situ observations increases with site representativeness, temporal duration, number of parameters measured and comparability within and across sites. Research Infrastructures (RIs) can support harmonised, cross-site data collection, curation and publication. Integrating RI networks through site co-location and standardised observation methods can help answers three questions about the terrestrial carbon sink: (i) What are present and future carbon sequestration rates in northern European forests? (ii) How are these rates controlled? (iii) Why do the observed patterns exist? Here, we present a conceptual model for RI co-location and highlight potential insights into the terrestrial carbon sink achievable when long-term in-situ Earth observation sites participate in multiple RI networks (e.g., ICOS and eLTER). Finally, we offer recommendations to promote RI co-location.

Modelling the regional potential for reaching carbon neutrality in Finland: Sustainable forestry, energy use and biodiversity protection.

The EU aims at reaching carbon neutrality by 2050 and Finland by 2035. We integrated results of three spatially distributed model systems (FRES, PREBAS, Zonation) to evaluate the potential to reach this goal at both national and regional scale in Finland, by simultaneously considering protection targets of the EU biodiversity (BD) strategy. Modelling of both anthropogenic emissions and forestry measures were carried out, and forested areas important for BD protection were identified based on spatial prioritization. We used scenarios until 2050 based on mitigation measures of the national climate and energy strategy, forestry policies and predicted climate change, and evaluated how implementation of these scenarios would affect greenhouse gas fluxes, carbon storages, and the possibility to reach the carbon neutrality target. Potential new forested areas for BD protection according to the EU 10% protection target provided a significant carbon storage (426-452 TgC) and sequestration potential (- 12 to - 17.5 TgCO2eq a-1) by 2050, indicating complementarity of emission mitigation and conservation measures. The results of the study can be utilized for integrating climate and BD policies, accounting of ecosystem services for climate regulation, and delimitation of areas for conservation.

Estimating intraseasonal intrinsic water-use efficiency from high-resolution tree-ring δ13 C data in boreal Scots pine forests.

Intrinsic water-use efficiency (iWUE), a key index for carbon and water balance, has been widely estimated from tree-ring δ13 C at annual resolution, but rarely at high-resolution intraseasonal scale. We estimated high-resolution iWUE from laser-ablation δ13 C analysis of tree-rings (iWUEiso ) and compared it with iWUE derived from gas exchange (iWUEgas ) and eddy covariance (iWUEEC ) data for two Pinus sylvestris forests from 2002 to 2019. By carefully timing iWUEiso via modeled tree-ring growth, iWUEiso aligned well with iWUEgas and iWUEEC at intraseasonal scale. However, year-to-year patterns of iWUEgas , iWUEiso , and iWUEEC were different, possibly due to distinct environmental drivers on iWUE across leaf, tree, and ecosystem scales. We quantified the modification of iWUEiso by postphotosynthetic δ13 C enrichment from leaf sucrose to tree rings and by nonexplicit inclusion of mesophyll and photorespiration terms in photosynthetic discrimination model, which resulted in overestimation of iWUEiso by up to 11% and 14%, respectively. We thus extended the application of tree-ring δ13 C for iWUE estimates to high-resolution intraseasonal scale. The comparison of iWUEgas , iWUEiso , and iWUEEC provides important insights into physiological acclimation of trees across leaf, tree, and ecosystem scales under climate change and improves the upscaling of ecological models.

Estimating intra-seasonal photosynthetic discrimination and water use efficiency using δ13C of leaf sucrose in Scots pine.

Sucrose has a unique role in recording environmental and physiological signals during photosynthesis in its carbon isotope composition (δ13C) and transport of the signal to tree rings. Yet, instead of sucrose, total organic matter (TOM) or water-soluble carbohydrates (WSC) are typically analysed in studies that follow δ13C signals within trees. To study how the choice of organic material may bias the interpretation of δ13C records, we used mature field-grown Scots pine (Pinus sylvestris) to compare for the first time δ13C of different leaf carbon pools with δ13C of assimilates estimated by a chamber-Picarro system (δ13CA_Picarro), and a photosynthetic discrimination model (δ13CA_model). Compared with sucrose, the other tested carbon pools, such as TOM and WSC, poorly recorded the seasonal trends or absolute values of δ13CA_Picarro and δ13CA_model. Consequently, in comparison with the other carbon pools, sucrose δ13C was superior for reconstructing changes in intrinsic water use efficiency (iWUE), agreeing in both absolute values and intra-seasonal variations with iWUE estimated from gas exchange. Thus, deriving iWUE and environmental signals from δ13C of bulk organic matter can lead to misinterpretation. Our findings underscore the advantage of using sucrose δ13C to understand plant physiological responses in depth.

Emissions of biogenic volatile organic compounds from adjacent boreal fen and bog as impacted by vegetation composition.

Peatland ecosystems emit biogenic volatile organic compounds (BVOC), which have a net cooling impact on the climate. However, the quality and quantity of BVOC emissions, and how they are regulated by vegetation and peatland type remain poorly understood. Here we measured BVOC emissions with dynamic enclosures from two major boreal peatland types, a minerotrophic fen and an ombrotrophic bog situated in Siikaneva, southern Finland and experimentally assessed the role of vegetation by removing vascular vegetation with or without the moss layer. Our measurements from four campaigns during growing seasons in 2017 and 2018 identified emissions of 59 compounds from nine different chemical groups. Isoprene accounted for 81 % of BVOC emissions. Measurements also revealed uptake of dichloromethane. Total BVOC emissions and the emissions of isoprene, monoterpenoids, sesquiterpenes, homoterpenes, and green leaf volatiles were tightly connected to vascular plants. Isoprene and sesquiterpene emissions were associated with sedges, whereas monoterpenoids and homoterpenes were associated with shrubs. Additionally, isoprene and alkane emissions were higher in the fen than in the bog and they significantly contributed to the higher BVOC emissions from intact vegetation in the fen. During an extreme drought event in 2018, emissions of organic halides were absent. Our results indicate that climate change with an increase in shrub cover and increased frequency of extreme weather events may have a negative impact on total BVOC emissions that otherwise are predicted to increase in warmer temperatures. However, these changes also accompanied a change in BVOC emission quality. As different compounds differ in their capacity to form secondary organic aerosols, the ultimate climate impact of peatland BVOC emissions may be altered.

Tree organ growth and carbon allocation dynamics impact the magnitude and δ13C signal of stem and soil CO2 fluxes.

Incomplete knowledge of carbon (C) allocation dynamics in trees hinders accurate modeling and future predictions of tree growth. We studied C allocation dynamics in a mature Pinus sylvestris L. dominated forest with a novel analytical approach, allowing the first comparison of: (i) magnitude and δ13C of shoot, stem and soil CO2 fluxes (Ashoot, Rstem and Rsoil), (ii) concentration and δ13C of compound-specific and/or bulk non-structural carbohydrates (NSCs) in phloem and roots and (iii) growth of stem and fine roots. Results showed a significant effect of phloem NSC concentrations on tracheid growth, and both variables significantly impacted Rstem. Also, concentrations of root NSCs, especially starch, had a significant effect on fine root growth, although no effect of root NSC concentrations or root growth was detected on Rsoil. Time series analysis between δ13C of Ashoot and δ13C of Rstem or δ13C of Rsoil revealed strengthened C allocation to stem or roots under high C demands. Furthermore, we detected a significant correlation between δ13C of Rstem and δ13C of phloem sucrose and glucose, but not for starch or water-soluble carbohydrates. Our results indicate the need to include C allocation dynamics into tree growth models. We recommend using compound-specific concentration and δ13C analysis to reveal C allocation processes that may not be detected by the conventional approach that utilizes bulk organic matter.

Drought effects on volatile organic compound emissions from Scots pine stems.

Tree stems have been identified as sources of volatile organic compounds (VOCs) that play important roles in tree defence and atmospheric chemistry. Yet, we lack understanding on the magnitude and environmental drivers of stem VOC emissions in various forest ecosystems. Due to the increasing importance of extreme drought, we studied drought effects on the VOC emissions from mature Scots pine (Pinus sylvestris L.) stems. We measured monoterpenes, acetone, acetaldehyde and methanol emissions with custom-made stem chambers, online PTR-MS and adsorbent sampling in a drought-prone forest over the hot-dry summer of 2018 and compared the emission rates and dynamics between trees in naturally dry conditions and under long-term irrigation (drought release). The pine stems were significant monoterpene sources. The stem monoterpene emissions potentially originated from resin, based on their similar monoterpene spectra. The emission dynamics of all VOCs followed temperature at a daily scale, but monoterpene and acetaldehyde emission rates decreased nonlinearly with drought over the summer. Despite the dry conditions, large peaks of monoterpene, acetaldehyde and acetone emissions occurred in late summer potentially due to abiotic or biotic stressors. Our results highlight the potential importance of stem emissions in the ecosystem VOC budget, encouraging further studies in diverse environments.

Is Decreased Xylem Sap Surface Tension Associated With Embolism and Loss of Xylem Hydraulic Conductivity in Pathogen-Infected Norway Spruce Saplings?

Increased abiotic stress along with increasing temperatures, dry periods and forest disturbances may favor biotic stressors such as simultaneous invasion of bark beetle and ophiostomatoid fungi. It is not fully understood how tree desiccation is associated with colonization of sapwood by fungi. A decrease in xylem sap surface tension (σxylem) as a result of infection has been hypothesized to cause xylem embolism by lowering the threshold for air-seeding at the pits between conduits and disruptions in tree water transport. However, this hypothesis has not yet been tested. We investigated tree water relations by measuring the stem xylem hydraulic conductivity (Kstem), σxylem, stem relative water content (RWCstem), and water potential (Ψstem), and canopy conductance (gcanopy), as well as the compound composition in xylem sap in Norway spruce (Picea abies) saplings. We conducted our measurements at the later stage of Endoconidiophora polonica infection when visible symptoms had occurred in xylem. Saplings of two clones (44 trees altogether) were allocated to treatments of inoculated, wounded control and intact control trees in a greenhouse. The saplings were destructively sampled every second week during summer 2016. σxylem, Kstem and RWCstem decreased following the inoculation, which may indicate that decreased σxylem resulted in increased embolism. gcanopy did not differ between treatments indicating that stomata responded to Ψstem rather than to embolism formation. Concentrations of quinic acid, myo-inositol, sucrose and alkylphenol increased in the xylem sap of inoculated trees. Myo-inositol concentrations also correlated negatively with σxylem and Kstem. Our study is a preliminary investigation of the role of σxylem in E. polonica infected trees based on previous hypotheses. The results suggest that E. polonica infection can lead to a simultaneous decrease in xylem sap surface tension and a decline in tree hydraulic conductivity, thus hampering tree water transport.

Monitoring of ticks and tick-borne pathogens through a nationwide research station network in Finland.

In 2015 a long-term, nationwide tick and tick-borne pathogen (TBP) monitoring project was started by the Finnish Tick Project and the Finnish Research Station network (RESTAT), with the goal of producing temporally and geographically extensive data regarding exophilic ticks in Finland. In the current study, we present results from the first four years of this collaboration. Ticks were collected by cloth dragging from 11 research stations across Finland in May-September 2015-2018 (2012-2018 in Seili). Collected ticks were screened for twelve different pathogens by qPCR: Borrelia afzelii, Borrelia garinii, Borrelia valaisiana, Borrelia burgdorferi sensu stricto, Borrelia miyamotoi, Babesia spp., Anaplasma phagocytophilum, Rickettsia spp., Candidatus Neoehrlichia mikurensis, Francisella tularensis, Bartonella spp. and tick-borne encephalitis virus (TBEV). Altogether 15 067 Ixodes ricinus and 46 Ixodes persulcatus were collected during 68 km of dragging. Field collections revealed different seasonal activity patterns for the two species. The activity of I. persulcatus adults (only one nymph detected) was unimodal, with activity only in May-July, whereas Ixodes ricinus was active from May to September, with activity peaks in September (nymphs) or July-August (adults). Overall, tick densities were higher during the latter years of the study. Borrelia burgdorferi sensu lato were the most common pathogens detected, with 48.9 ±â€¯8.4% (95% Cl) of adults and 25.3 ±â€¯4.4% of nymphs carrying the bacteria. No samples positive for F. tularensis, Bartonella or TBEV were detected. This collaboration project involving the extensive Finnish Research Station network has ensured enduring and spatially extensive, long-term tick data collection to the foreseeable future.

Meta-analysis of multidecadal biodiversity trends in Europe.

Local biodiversity trends over time are likely to be decoupled from global trends, as local processes may compensate or counteract global change. We analyze 161 long-term biological time series (15-91 years) collected across Europe, using a comprehensive dataset comprising ~6,200 marine, freshwater and terrestrial taxa. We test whether (i) local long-term biodiversity trends are consistent among biogeoregions, realms and taxonomic groups, and (ii) changes in biodiversity correlate with regional climate and local conditions. Our results reveal that local trends of abundance, richness and diversity differ among biogeoregions, realms and taxonomic groups, demonstrating that biodiversity changes at local scale are often complex and cannot be easily generalized. However, we find increases in richness and abundance with increasing temperature and naturalness as well as a clear spatial pattern in changes in community composition (i.e. temporal taxonomic turnover) in most biogeoregions of Northern and Eastern Europe.

Stem emissions of monoterpenes, acetaldehyde and methanol from Scots pine (Pinus sylvestris L.) affected by tree-water relations and cambial growth.

Tree stems are an overlooked source of volatile organic compounds (VOCs). Their contribution to ecosystem processes and total VOC fluxes is not well studied, and assessing it requires better understanding of stem emission dynamics and their driving processes. To gain more mechanistic insight into stem emission patterns, we measured monoterpene, methanol and acetaldehyde emissions from the stems of mature Scots pines (Pinus sylvestris L.) in a boreal forest over three summers. We analysed the effects of temperature, soil water content, tree water status, transpiration and growth on the VOC emissions and used generalized linear models to test their relative importance in explaining the emissions. We show that Scots pine stems are considerable sources of monoterpenes, methanol and acetaldehyde, and their emissions are strongly regulated by temperature. However, even small changes in water availability affected the emission potentials: increased soil water content increased the monoterpene emissions within a day, whereas acetaldehyde and methanol emissions responded within 2-4 days. This lag corresponded to their transport time in the xylem sap from the roots to the stem. Moreover, the emissions of monoterpenes, methanol and acetaldehyde were influenced by the cambial growth rate of the stem with 6-10-day lags.

Research questions to facilitate the future development of European long-term ecosystem research infrastructures: A horizon scanning exercise.

Distributed environmental research infrastructures are important to support assessments of the effects of global change on landscapes, ecosystems and society. These infrastructures need to provide continuity to address long-term change, yet be flexible enough to respond to rapid societal and technological developments that modify research priorities. We used a horizon scanning exercise to identify and prioritize emerging research questions for the future development of ecosystem and socio-ecological research infrastructures in Europe. Twenty research questions covered topics related to (i) ecosystem structures and processes, (ii) the impacts of anthropogenic drivers on ecosystems, (iii) ecosystem services and socio-ecological systems and (iv), methods and research infrastructures. Several key priorities for the development of research infrastructures emerged. Addressing complex environmental issues requires the adoption of a whole-system approach, achieved through integration of biotic, abiotic and socio-economic measurements. Interoperability among different research infrastructures needs to be improved by developing standard measurements, harmonizing methods, and establishing capacities and tools for data integration, processing, storage and analysis. Future research infrastructures should support a range of methodological approaches including observation, experiments and modelling. They should also have flexibility to respond to new requirements, for example by adjusting the spatio-temporal design of measurements. When new methods are introduced, compatibility with important long-term data series must be ensured. Finally, indicators, tools, and transdisciplinary approaches to identify, quantify and value ecosystem services across spatial scales and domains need to be advanced.

Decomposition of spruce wood and release of volatile organic compounds depend on decay type, fungal interactions and enzyme production patterns.

Effect of three wood-decaying fungi on decomposition of spruce wood was studied in solid-state cultivation conditions for a period of three months. Two white rot species (Trichaptum abietinum and Phlebia radiata) were challenged by a brown rot species (Fomitopsis pinicola) in varying combinations. Wood decomposition patterns as determined by mass loss, carbon to nitrogen ratio, accumulation of dissolved sugars and release of volatile organic compounds (VOCs) were observed to depend on both fungal combinations and growth time. Similar dependence of fungal species combination, either white or brown rot dominated, was observed for secreted enzyme activities on spruce wood. Fenton chemistry suggesting reduction of Fe3+ to Fe2+ was detected in the presence of F. pinicola, even in co-cultures, together with substantial degradation of wood carbohydrates and accumulation of oxalic acid. Significant correlation was perceived with two enzyme activity patterns (oxidoreductases produced by white rot fungi; hydrolytic enzymes produced by the brown rot fungus) and wood degradation efficiency. Moreover, emission of four signature VOCs clearly grouped the fungal combinations. Our results indicate that fungal decay type, either brown or white rot, determines the loss of wood mass and decomposition of polysaccharides as well as the pattern of VOCs released upon fungal growth on spruce wood.

Do all chlorophyll fluorescence emission wavelengths capture the spring recovery of photosynthesis in boreal evergreen foliage?

Chlorophyll a fluorescence (ChlF) is closely related to photosynthesis and can be measured remotely using multiple spectral features as solar-induced fluorescence (SIF). In boreal regions, SIF shows particular promise as an indicator of photosynthesis, in part because of the limited variation of seasonal light absorption in these ecosystems. Seasonal spectral changes in ChlF could yield new information on processes such as sustained nonphotochemical quenching (NPQS ) but also disrupt the relationship between SIF and photosynthesis. We followed ChlF and functional and biochemical properties of Pinus sylvestris needles during the photosynthetic spring recovery period to answer the following: (a) How ChlF spectra change over seasonal timescales? (b) How pigments, NPQS , and total photosynthetically active radiation (PAR) absorption drive changes of ChlF spectra? (c) Do all ChlF wavelengths track photosynthetic seasonality? We found seasonal ChlF variation in the red and far-red wavelengths, which was strongly correlated with NPQS , carotenoid content, and photosynthesis (enhanced in the red), but not with PAR absorption. Furthermore, a rapid decrease in red/far-red ChlF ratio occurred in response to a cold spell, potentially relating to the structural reorganization of the photosystems. We conclude that all current SIF retrieval features can track seasonal photosynthetic dynamics in boreal evergreens, but the full SIF spectra provides additional insight.

Interannual and Seasonal Dynamics of Volatile Organic Compound Fluxes From the Boreal Forest Floor.

In the northern hemisphere, boreal forests are a major source of biogenic volatile organic compounds (BVOCs), which drive atmospheric processes and lead to cloud formation and changes in the Earth's radiation budget. Although forest vegetation is known to be a significant source of BVOCs, the role of soil and the forest floor, and especially interannual variations in fluxes, remains largely unknown due to a lack of long-term measurements. Our aim was to determine the interannual, seasonal and diurnal dynamics of boreal forest floor volatile organic compound (VOC) fluxes and to estimate how much they contribute to ecosystem VOC fluxes. We present here an 8-year data set of forest floor VOC fluxes, measured with three automated chambers connected to the quadrupole proton transfer reaction mass spectrometer (quadrupole PTR-MS). The exceptionally long data set shows that forest floor fluxes were dominated by monoterpenes and methanol, with relatively comparable emission rates between the years. Weekly mean monoterpene fluxes from the forest floor were highest in spring and in autumn (maximum 59 and 86 µg m-2 h-1, respectively), whereas the oxygenated VOC fluxes such as methanol had highest weekly mean fluxes in spring and summer (maximum 24 and 79 µg m-2 h-1, respectively). Although the chamber locations differed from each other in emission rates, the inter-annual dynamics were very similar and systematic. Accounting for this chamber location dependent variability, temperature and relative humidity, a mixed effects linear model was able to explain 79-88% of monoterpene, methanol, acetone, and acetaldehyde fluxes from the boreal forest floor. The boreal forest floor was a significant contributor in the forest stand fluxes, but its importance varies between seasons, being most important in autumn. The forest floor emitted 2-93% of monoterpene fluxes in spring and autumn and 1-72% of methanol fluxes in spring and early summer. The forest floor covered only a few percent of the forest stand fluxes in summer.

Newtonian boreal forest ecology: The Scots pine ecosystem as an example.

Isaac Newton's approach to developing theories in his book Principia Mathematica proceeds in four steps. First, he defines various concepts, second, he formulates axioms utilising the concepts, third, he mathematically analyses the behaviour of the system defined by the concepts and axioms obtaining predictions and fourth, he tests the predictions with measurements. In this study, we formulated our theory of boreal forest ecosystems, called NewtonForest, following the four steps introduced by Newton. The forest ecosystem is a complicated entity and hence we needed altogether 27 concepts to describe the material and energy flows in the metabolism of trees, ground vegetation and microbes in the soil, and to describe the regularities in tree structure. Thirtyfour axioms described the most important features in the behaviour of the forest ecosystem. We utilised numerical simulations in the analysis of the behaviour of the system resulting in clear predictions that could be tested with field data. We collected retrospective time series of diameters and heights for test material from 6 stands in southern Finland and five stands in Estonia. The numerical simulations succeeded to predict the measured diameters and heights, providing clear corroboration with our theory.

Solar eclipse demonstrating the importance of photochemistry in new particle formation.

Solar eclipses provide unique possibilities to investigate atmospheric processes, such as new particle formation (NPF), important to the global aerosol load and radiative balance. The temporary absence of solar radiation gives particular insight into different oxidation and clustering processes leading to NPF. This is crucial because our mechanistic understanding on how NPF is related to photochemistry is still rather limited. During a partial solar eclipse over Finland in 2015, we found that this phenomenon had prominent effects on atmospheric on-going NPF. During the eclipse, the sources of aerosol precursor gases, such as sulphuric acid and nitrogen- containing highly oxidised organic compounds, decreased considerably, which was followed by a reduced formation of small clusters and nanoparticles and thus termination of NPF. After the eclipse, aerosol precursor molecule concentrations recovered and re-initiated NPF. Our results provide direct evidence on the key role of the photochemical production of sulphuric acid and highly oxidized organic compounds in maintaining atmospheric NPF. Our results also explain the rare occurrence of this phenomenon under dark conditions, as well as its seemingly weak connection with atmospheric ions.

Enhanced Volatile Organic Compounds emissions and organic aerosol mass increase the oligomer content of atmospheric aerosols.

Secondary organic aerosol (SOA) accounts for a dominant fraction of the submicron atmospheric particle mass, but knowledge of the formation, composition and climate effects of SOA is incomplete and limits our understanding of overall aerosol effects in the atmosphere. Organic oligomers were discovered as dominant components in SOA over a decade ago in laboratory experiments and have since been proposed to play a dominant role in many aerosol processes. However, it remains unclear whether oligomers are relevant under ambient atmospheric conditions because they are often not clearly observed in field samples. Here we resolve this long-standing discrepancy by showing that elevated SOA mass is one of the key drivers of oligomer formation in the ambient atmosphere and laboratory experiments. We show for the first time that a specific organic compound class in aerosols, oligomers, is strongly correlated with cloud condensation nuclei (CCN) activities of SOA particles. These findings might have important implications for future climate scenarios where increased temperatures cause higher biogenic volatile organic compound (VOC) emissions, which in turn lead to higher SOA mass formation and significant changes in SOA composition. Such processes would need to be considered in climate models for a realistic representation of future aerosol-climate-biosphere feedbacks.

(137)Cs distributions in soil and trees in forest ecosystems after the radioactive fallout - Comparison study between southern Finland and Fukushima, Japan.

The nuclear accidents at Chernobyl and Fukushima released large amounts of (137)Cs radionuclides into the atmosphere which spread over large forest areas. We compared the (137)Cs concentration distribution in different parts of two coniferous forest ecosystems (needle litter, stems and at different depths in the soil) over short and long term periods in Finland and Japan. We also estimated the change in (137)Cs activity concentrations in needle and soil between 1995 and 2013 in Southern Finland based on the back-calculated (137)Cs activity concentrations. We hypothesized that if the (137)Cs activity concentrations measured in 1995 and 2013 showed a similar decline in concentration, the (137)Cs activity concentration in the ecosystem was already stable in 1995. But if not, the (137)Cs activity concentrations were still changing in 2013. Our results showed that the vertical distribution of the (137)Cs fallout in the soil was similar in Hyytiälä and Fukushima. The highest (137)Cs concentrations were observed in the uppermost surface layers of the soil, and they decreased exponentially deeper in the soil. We also observed that (137)Cs activity concentrations estimated from the samples in 1995 and 2013 in Finland showed different behavior in the surface soil layers compared to the deep soil layer. These results suggested that the (137)Cs nuclei were still mobile in the surface soil layers 27 years after the accident. Our results further indicated that, in the aboveground parts of the trees, the (137)Cs concentrations were much closer to steady-state when compared to those of the surface soil layers based on the estimated declining rates of (137)Cs concentration activity in needles which were similar in 1995 and 2013. Despite its mobility and active role in the metabolism of trees, the (137)Cs remains in the structure of the trees for decades, and there is not much exchange of (137)Cs between the heartwood and surface layers of the stem.