Changes in surface properties of the urban environments significantly impact the local microclimate. While urban trees are known for providing important thermal regulation, the impact of urban climate on tree growth remains relatively unexplored. The present study focuses on the climate response and growth dynamics of urban Scots pine trees (P. sylvestris) in comparison to their rural counterparts. High-resolution monitoring of stem-radius variations using automatic point dendrometers was performed during the growing seasons (April–October) of 2017 and 2018 in Stockholm, Sweden. In 2018, the region experienced a severe and long-lasting summer drought. In May and July, temperatures were up to 5 °C higher relative to the reference period (1981–2010), and precipitation sums were below the reference period for the entire growing season. Our results show that the urban climate primarily impacts the daily water storage dynamics by decreasing the radius change amplitudes and delaying the time of maximum stem-water replenishment and depletion. Under standard climatic conditions, the warmer climate (1.3 °C) at the urban sites had a positive impact on radial growth increment. Drought periods significantly impact the climate–growth relationships. Stem shrinkage intensifies during the day, and lower growth rates were registered, resulting in reduced annual growth. The high-resolution monitoring provided valuable insights into daily and seasonal patterns of Scots pine stem-radius variations, showing that growth responses to increasing temperature are mainly controlled by moisture availability and site-specific conditions. 

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.

The long tradition of dendroclimatological studies in Fennoscandia is fostered by the exceptional longevity and temperature sensitivity of tree growth, as well as the existence of well-preserved subfossil wood in shallow lakes and extent peat bogs. Although some of the world’s longest ring width and density-based climate reconstructions have been developed in northern Fennoscandia, it is still unclear if differences in micro-site ecology matter, and if so, whether they have been considered sufficiently in previous studies. We developed a Fennoscandia-wide network of 44 Scots pine ring width chronologies from 22 locations between 59°–70 °N and 16°–31 °E, to assess the effects of moist lakeshores and dry inland micro-sites on tree growth. Our network reveals a strong dependency of pine growth on July temperature, which is also reflected in latitude. Differences in forest productivity between moist and dry micro-sites are likely caused by associated effects on soil temperature. While trees at moist micro-sites at western locations exhibit higher growth rates, this pattern is reversed in the continental eastern part of the network, where increased ring widths are found at drier sites. In addition to the latitudinal increase in growth sensitivity to July temperature, pines at moist sites exhibit an increased dependency of summer warmth. The highest temperature sensitivity and growth coherency, and thus greatest suitability for summer temperature reconstructions, is found in those regions where July mean temperatures range between 11.5 and 13.5 °C, and May precipitation totals do not exceed 100 mm. Our study not only provides guidance for the selection of sampling sites for tree ring-based climate reconstructions, but also reveals the effect of micro-site ecology on Fennoscandian forest growth. The manifestation of micro-site effects varies substantially over the Fennoscandian boreal forest and is predominately triggered by the geographical setting of the stand as expressed by differing abiotic site factors.

Tree-ring stable isotopes are insightful proxies providing information on pre-instrumental climate fluctuations, yet the variability of these data within a tree trunk has not been fully explored. Here, we analyze longitudinal and circumferential changes in tree-ring delta C-13 values from 1991-2010, considering seven height levels from 1 to 13 m above ground and six sampling directions (radii) separated by 60 degrees around the stem. The disk samples were taken from a 360-year old European larch (Larix decidua Mill.) that grew at 1675 m above sea level in the Simplon Valley, Switzerland. Results show that the circumferential delta C-13 variability, defined as the difference between the minimum and maximum isotope values within a single ring at a certain height, ranges from 0.5 to 2.8 parts per thousand. These differences appear substantial as they match the range of year-to-year variations retained in long tree-ring delta C-13 time series used for climate reconstruction. The assessment of longitudinal variability demonstrated a systematic change of similar to 0.1 parts per thousand m(-1) towards isotopically heavier (less negative) delta C-13 values with increasing tree height, likely reflecting a vertical gradient towards isotopically heavier needle tissue due to changing microclimatic conditions and CO2 stratification within the canopy. Calibration against regional climate data indicates no substantial signal changes in delta C-13 values within the trunk. We conclude that the longitudinal isotope gradient adds uncertainty to long delta C-13 chronologies derived from subfossil material of unknown (and changing) sampling heights. The large circumferential variability recorded in the sub-alpine larch suggests that more than two cores are needed to analyze absolute delta C-13 values representative for each tree.

Maximum latewood density (MXD) chronologies have been widely used to reconstruct summer temperature variations. Precipitation signals inferred from MXD data are, however, rather scarce. In this study, we assess the potential of using MXD data derived from Scots pine (Pinus sylvestris L.) growing in the Stockholm archipelago (Sweden) to reconstruct past precipitation variability. In this area, slow-growing pine trees emerge on flat plateaus of bedrock outcrops with thin or absent soil layers and are, therefore, sensitive to moisture variability. A 268-year-long MXD chronology was produced, and climate–growth relationships show a significant and robust correlation with May–July precipitation (P_MJJr = 0.64, p < 0.01). The MXD based May–July precipitation reconstruction covers the period 1750–2018 CE and explains 41% of the variance (r²) of the observed precipitation (1985–2018). The reconstruction suggests that the region has experienced more pluvial phases than drought conditions since the 1750s. The latter half of the 18th century was the wettest and the first half of the 19th century the driest. Climate analysis of “light rings” (LR), latewood layers of extreme low-density cells, finds their occurrence often coincides with significantly dry (<41 mm precipitation) and warmer (1–2 °C above average temperature), May–July conditions. Our analysis suggests that these extremes may be triggered by the summer North Atlantic Oscillation (SNAO).

The potential of dendrochemistry as a tool for tracing anthropogenic contamination at a glassworks site in southeastern Sweden was investigated through a multidisciplinary approach combining continuous high-resolution time series of tree rings and sediment profiles. Tree cores from Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and European aspen (Populus tremula) were analysed for their elemental composition using an energy dispersive X-ray fluorescence (ED-XRF) technique. Sediment cores were sampled along a transect extending from the pollution point source to unpolluted areas and analysed using core-scanning-XRF (CS-XRF). High contaminant concentrations in the soil were found for As (approximate to 2000 ppm), Pb (>5000 ppm), Ba (approximate to 1000 ppm) and Cd (approximate to 150 ppm). The concentrations decreased with depth and distance from the pollution source. The dendrochemical analyses revealed alterations in the Barium, Chlorine and Manganese profiles, allowing the identification of seven potential asynchronous releases from the glassworks. Our results suggest that differences in the response of tree species to elemental uptake together with soil chemical properties dictate the success of dendrochemistry as an environmental monitoring tool.

Although quantitative isotope data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which to achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to using the speleothem data for data-model comparisons. Here, we illustrate this using 456 globally distributed speleothem delta O-18 records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates the process of procuring large numbers of records if data-model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotope values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model's ability to simulate spatial isotopic trends. Our analyses provide a protocol for using speleothem isotope data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on delta O-18 values, the optimum period for the modern observational baseline and the selection of an appropriate time window for creating means of the isotope data for palaeo-time-slices.

Continental records with absolute dates of the timing and progression of climatic conditions during the Last Interglacial (LIG) from northern Europe are rare. Speleothems from northern Europe have a large potential as archives for LIG environmental conditions since they were formed in sheltered environments and may be preserved beneath ice sheets. Here, we present delta C-13 and delta O-18 values from speleothem Kf-21, from Korallgrottan in Jamtland (northwest Sweden). Kf-21 is dated with five MC-ICPMS U-Th dates with errors smaller than similar to 1 ka. Kf-21 started forming at similar to 130.2 ka and the main growth phase with relatively constant growth rates lasted from 127.3 ka to 124.4 ka, after which calcite formation ceased. Both delta C-13 and delta O-18 show rapid shifts but also trends, with a range of values within their Holocene counterparts from Korallgrottan. Our results indicate an early onset of the LIG in northern Europe with ice-free conditions at similar to 130 ka. Higher growth rates combined with more negative delta O-18 values between similar to 127.3 and 126.8 ka, interpreted here as warmer and more humid conditions, as well as indications of a millennial-scale cold spell centered at 126.2 ka, resemble findings from speleothem records from other parts of Europe, highlighting that these were regional scale climatic patterns.

Pinus sylvestris tree-ring delta C-13 and delta O-18 records from locally moist sites in central and northern Sweden contain consistently stronger climate signals than their dry site counterparts. We produced twentieth century stable isotope data from Pinus sylvestris trees near lakeshores and inland sites in northern Sweden (near Kiruna) and central Sweden (near Stockholm) to evaluate the influence of changing microsite conditions on the climate sensitivity of tree-ring delta C-13 and delta O-18. The data reveal a latitudinal trend towards lower C and O isotope values near the Arctic tree line (-0.8 parts per thousand for delta C-13 and - 2.4 parts per thousand for delta O-18 relative to central Sweden) reflecting widely recognized atmospheric changes. At the microsite scale, delta C-13 decreases from the dry inland to the moist lakeshore sites (- 0.7 parts per thousand in Kiruna and - 1.2 parts per thousand in Stockholm), evidence of the importance of groundwater access to this proxy. While all isotope records from northern and central Sweden correlate significantly against temperature, precipitation, cloud cover and/or drought data, climate signals in the records from moist microsites are consistently stronger, which emphasizes the importance of site selection when producing stable isotope chronologies. Overall strongest correlations are found with summer temperature, except for delta O-18 from Stockholm correlating best with instrumental drought indices. These findings are complemented by significant positive correlations with temperature-sensitive ring width data in Kiruna, and inverse (or absent) correlations with precipitation-sensitive ring width data in Stockholm. A conclusive differentiation between leading and co-varying forcings is challenging based on only the calibration against often defective instrumental climate data, and would require an improved understanding of the physiological processes that control isotope fractionation at varying microsites and joined application of forward modelling.

A lake sediment record from the north-eastern European Russian Arctic was examined using palaeolimnological methods, including subfossil chironomid and diatom analysis. The objective of this study is to disentangle environmental history of the lake and climate variability during the past 2000 years. The sediment profile was divided into two main sections following changes in the lithology, separating the limno-telmatic phase between similar to 2000 and 1200 cal. yr BP and the lacustrine phase between similar to 1200 cal. yr BP and the present. Owing to the large proportion of semi-terrestrial chironomids and poor modern analogues, a reliable chironomid-based temperature reconstruction for the limno-telmatic phase was not possible. However, the lacustrine phase showed gradually cooling climate conditions from similar to 1200 cal. yr BP until similar to 700 cal. yr BP. The increase in stream chironomids within this sediment section indicates that this period may also have had increased precipitation that caused the adjacent river to overflow, subsequently transporting chironomids to the lacustrine basin. After a short-lived warm phase at similar to 700 cal. yr BP, the climate again cooled, and a progressive climate warming trend was evident from the most recent sediment samples, where the biological assemblages seem to have experienced an eutrophication-like response to climate warming. The temperature reconstruction showed more similarities with the climate development in the Siberian side of the Urals than with northern Europe. This study provides a characteristic archive of arctic lake ontogeny and a valuable temperature record from a remote climate-sensitive area of northern Russia.