Plant-based data from southern Finland were used to reconstruct late Holocene warm-season temperature variability on inter-annual to longer scales. Temperature-sensitive records representing maximum latewood density of Pinus sylvestris tree rings (since AD 760) and phenological stages of several plant species (since AD 1750) explained ~60% and ~70% of instrumentally observed temperature variance, respectively. The value of a multi-proxy approach was demonstrated by statistical models including both variables, which explained ~80% of the temperature variance. Temperatures from the CRUTEM5 and Berkeley datasets had slight variations in their correlativity with proxy data, possibly resulting from their differing spatial representativeness over the proxy sites. Temperature history inferred from maximum latewood densities extended over the past millennium and correlated with previously published data from similar proxy records in Fennoscandia and adjacent areas. These data indicate that the region cooled since the Medieval Climate Anomaly and warmed markedly since the Little Ice Age/Maunder Minimum. In the study region, the magnitude of this long-term warming was 2.1 °C and 2.8 °C, calculated between the coldest and warmest 100-year and 30-year intervals, respectively. Collectively, our results display the potential of plant-based data from low-lying and mild boreal sites to extend our understanding of preindustrial and recent climatic changes.

Heavy metal pollution from untreated or poorly managed mining waste is a major environmental concern, leading to the leaching of contaminants into surrounding ecosystems. Traditional monitoring methods are costly and limited in their ability to reconstruct historical contamination trends. Dendrochemical methods offer a promising alternative for assessing long-term pollution dynamics. This study investigates temporal patterns of copper (Cu), nickel (Ni), and zinc (Zn) accumulation in tree rings from 22 European aspens (Populus tremula) growing near an abandoned copper mining field in southeastern Sweden. Tree rings were analyzed using energy-dispersive X-ray fluorescence (ED-XRF), while corresponding soil samples were examined for heavy metal concentrations and pH. Considerable heterogeneity in Cu and pH was observed, with elevated Cu levels across the site. Although no significant correlations were found between soil and tree-ring metal concentrations, increased accumulation of Cu, Ni, and Zn was detected in the trees. Temperature showed a more consistent influence on metal uptake than precipitation. Anomalous uptake increased under warmer and wetter conditions but was absent during severe drought. This study emphasizes the potential of dendrochemical methods for monitoring historical pollution. It highlights the need to refine sampling strategies to better account for localized soil and climatic factors, thereby improving the reliability of dendrochemical methods in assessing pollution trends.

X-ray based microdensitometry is conventionally used to produce climate-related tree-ring records. Micro X-ray fluorescence (µXRF) applications represent another growing area of interest and opportunities in dendrochronology. This paper demonstrates a method to correctly juxtapose and precisely synchronise the densitometry and µXRF profiles. Among µXRF variables, full fluorescence spectrum (FFS) corresponds distinctly well with the microdensitometry-based wood density variations. Accordingly, the FFS provides the most applicable variable to integrate the µXRF and density profiles. The method proposed here can be used to demonstrate the strength and sign of µXRF variables and wood density relations. Moreover, the µXRF based records can be readily compared to density variables, such as the latewood maximum density, which is demonstrated in this paper.

The jet stream is an important dynamic driver of climate variability in the Northern Hemisphere mid-latitudes1–3. Modern variability in the position of summer jet stream latitude in the North Atlantic–European sector (EU JSL) promotes dipole patterns in air pressure, temperature, precipitation and drought between northwestern and southeastern Europe. EU JSL variability and its impacts on regional climatic extremes and societal events are poorly understood, particularly before anthropogenic warming. Based on three temperature-sensitive European tree-ring records, we develop a reconstruction of interannual summer EU JSL variability over the period 1300–2004 ce (R2 = 38.5%) and compare it to independent historical documented climatic and societal records, such as grape harvest, grain prices, plagues and human mortality. Here we show contrasting summer climate extremes associated with EU JSL variability back to 1300 ce as well as biophysical, economic and human demographic impacts, including wildfires and epidemics. In light of projections for altered jet stream behaviour and intensified climate extremes, our findings underscore the importance of considering EU JSL variability when evaluating amplified future climate risk.

Middelalderfiske, fellefangst og fraflytting: En dendrokronologisk undersøkelse av et 1300-talls stasjonært fiskeanlegg i Nord-Mesna, Sørøst-Norge. (Medieval fishing and abandonment: A dendrochronological investigation of a 14th-century stationary fishing trap in Lake Nord-Mesna, in the interior of Scandinavia.) Fornvännen 119. Stockholm. Fishing in rivers and lakes has traditionally been a longstanding and dependable means of sustenance for the people of inland Scandinavia. However, our understanding of pre-modern fishing traditions has been hampered by a lack of written sources, as well as a scarcity of archaeological data. However, a recent excavation and comprehensive dendrochronological analysis of a fish trapping enclosure system in Lake Nord-Mesna (520 masl.), in the boreal forests of inland Norway, has provided unique insight into freshwater fishing traditions, techniques and organization. The excavated structure, believed to be a fish weir with lath screen traps set into shallow water, was established in the late 1200s. In the following years it was regularly maintained in the spring/early summer. The last documented repair was in 1343. Its abandonment is interpreted as resulting from a recession likely induced by factors such as plague and climatic deterioration, that affected inland areas of Scandinavia in the 1300s. These results provide archaeological evidence of medieval utilization of effective enclosure traps in the region. This contradicts the prevailing notion that this form of fishing was later introduced by Forest Finns who migrated to the area in the 17th century. In addition, the findings give new and significant information about the organization and practice of medieval fishing in inland Scandinavia.

Recent studies have raised concerns regarding the reconstruction of glacier mass balance using tree-ring data. The method relies on a stable relationship between both variables and summer (June-August) or melt season (May-September) temperature. However, with recent anthropogenic climate change the stability of this relationship is challenged by both, a divergence between tree-ring and temperature, as well as mass balance and temperature data. Establishing to what extent this divergence influences the reconstruction of mass balance using tree-ring data is important to assess the future viability and applicability of the method. In this paper we analyze the relationship between the Tornetrask tree-ring and Storglaciaren mass balance records, their response to climate change, and investigate changes in the relationship. We provide evidence for a sensitivity loss in the Tornetrask record and quantify its impact on the reconstruction of summer mass balance of Storglaciaren. We find that by including years post 1980, the amplitude of reconstructed variability is reduced, but it remains possible to explain the variance of the record significantly. Our results suggest that for glaciers without an extensive mass balance record the applicability of the method is challenged.

For ancient types of barley at sites in the Scottish Isles, Faroes, and Iceland, we calculated minimum temperature requirements for grain production (grain production threshold, GPT) as accumulated degree days over the cropping season. Site suitability for barley from AD 1200 to 2000 was investigated by comparing these thresholds with reconstructions of annual cropping season degree days (CSDD) using temperature and tree-ring data. In Iceland, between AD 1200 and 1500, reconstructed CSDD were more favorable in the southwest (Reykjavik), with fewer years below the GPT, than in the North, East and West, but there were two periods (1340–1389 and 1426–1475) with low average CSDD and several years below the GPT which possibly influenced the abandonment of barley cultivation around this time. Reconstructed CSDD for the Faroes (Tórshavn) had only one year below the GPT, but 15 periods of four or more consecutive years with low CSDD which would have challenged barley cultivation, especially in the thirteenth century. Reconstructed CSDD were highest for the Scottish Isles, allowing a more prominent role of barley in the farming system and economy. Nevertheless, years with poor harvests or famines were common and about half were associated with low CSDD, resulting in a significant temperature link but also demonstrating the important contribution of other factors. Despite frequent unfavorable years in both the Faroes and Scottish Isles, resilient production systems, well-adapted barley strains and socio-economic factors allowed barley cultivation to continue, and some ancient types to survive to the present day. 

Earth system models and various climate proxy sources indicate global warming is unprecedented during at least the Common Era. However, tree-ring proxies often estimate temperatures during the Medieval Climate Anomaly (950–1250 CE) that are similar to, or exceed, those recorded for the past century, in contrast to simulation experiments at regional scales. This not only calls into question the reliability of models and proxies but also contributes to uncertainty in future climate projections. Here we show that the current climate of the Fennoscandian Peninsula is substantially warmer than that of the medieval period. This highlights the dominant role of anthropogenic forcing in climate warming even at the regional scale, thereby reconciling inconsistencies between reconstructions and model simulations. We used an annually resolved 1,170-year-long tree-ring record that relies exclusively on tracheid anatomical measurements from Pinus sylvestris trees, providing high-fidelity measurements of instrumental temperature variability during the warm season. We therefore call for the construction of more such millennia-long records to further improve our understanding and reduce uncertainties around historical and future climate change at inter-regional and eventually global scales. 

In micro-densitometry of wood it is standard procedure to extract resin and other soluble compounds before X-ray analysis to eliminate the influence of these extractives on wood-density. Dendrochemical studies using X-ray fluorescence analysis on the other hand are commonly conducted without previous extraction. However, it is well known that translocation processes of elements during heartwood formation in trees or (temporal) differences in sap content of wood samples can influence dendrochemical element profiles. This might bias environmental signals stored in time series of element concentrations in wood proxies. We hypothesize that metals tightly bound to cell walls show a more robust proxy potential for environmental conditions than easily translocated ones. To eliminate the noise of these soluble substances in wood elemental time series, their extraction prior to analysis might be necessary. In our study we tested the effect of different solvents (water, alcohol, and acetone) and different extraction times on elemental time series of three tree species with differing wood structure (Pinus sylvestris; Quercus robur and Populus tremula). Micro-XRF analysis was conducted on nine replicates per species using an ITRAX-Multiscanner. A set of elements commonly detected in wood (S, Cl, K, Ca, Ti, Mn, Fe, and Ni) was analysed at high resolution before and after several extraction runs. Besides lowering their levels, extraction did not significantly change the temporal trends for most elements. However, for some elements, e.g., Potassium, Chlorine or Manganese, especially the water extraction led to significant decreases in concentrations and altered temporal trends. Apparently the dipole effect of water produced the strongest extraction power of all three solvents. In addition we observed a dependency of extraction intensity from wood density which differed between wood types. Our results help in interpreting and evaluating element profiles and mark a step forward in establishing dendrochemistry as a robust proxy in dendro-environmental research.

Tree-ring records constitute excellent high-resolution data and provide valuable information for climate science and paleoclimatology. Tree-ring reconstructions of past temperature variations agree to show evidence for annual-to-centennial anomalies in past climate and place the industrial-era warming in the context of the late Holocene climate patterns and regimes. Despite their wide use in paleoclimate research, however, tree rings have also been deemed unsuitable as low-frequency indicators of past climate. The arising debate concerns whether the millennia-long tree-ring records show signals of orbital forcing due to the Milankovitch cycles. Here, we produce a summer-temperature reconstruction from tree-ring chronology running through mid- and late-Holocene times (since 5486 BCE) comprising minimum blue channel light intensity (BI). The BI reconstruction correlates with existing and new tree-ring chronologies built from maximum latewood density (MXD) and, unlike the MXD data, shows temperature trends on Milankovitch scales comparable to various types of sedimentary proxy across the circumpolar Arctic. Our results demonstrate an unrevealed potential of novel, unconventional tree-ring variables to contribute to geoscience and climate research by their capability to provide paleoclimate estimates from inter-annual scales up to those relevant to orbital forcing.