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 recent warming trend, and associated shifts in growing season length, challenge the principle of uniformitarianism, i.e., that current relations are persistent over time, and complicates the uncritical inferences of past climate from tree-ring data. Here we conduct a comparison between tree-ring width chronologies of Pinus sylvestris L. (Scots pine), Picea abies (L.) Karst. (Norway spruce) and Betula pubescens Ehrh. (Downy birch) and phenological observations (budburst and leaf senescence) of Fagus sylvatica L. (European beech), Quercus robur L. (European oak), Betula sp. (Birch), Norway spruce and Scots pine) in Sweden to assess to what extent the tree-ring width–temperature relationship and the timing of phenological phases are affected by increased temperature. Daily meteorological observations confirm a prolongation of the thermal growing season, most consistently observed as an earlier onset of around 1–2 weeks since the beginning of the 20th century. Observations of budburst closely mimic this pattern, with budburst of the deciduous trees occurring 1–2.5 weeks earlier. In contrast to the changes seen in phenology and observational temperature data, the tree-ring width–temperature relationships remain surprisingly stable throughout the 20th century. Norway spruce, Scots pine and Downy birch all show consistently significant correlations with at least one 30 day-long window of temperature starting in late June–early July season. Norway spruce displays the largest degree of stability, with a consistent 60 day-long temperature window with significant correlation starting around Julian calendar day 150. Thus, our results suggest that the principle of uniformitarianism is not violated during the period covered by modern meteorological observations. Further research is needed to determine at what thresholds the temperature sensitivity of these species may alter or deteriorate as a consequence of the ongoing climate change.

Fennoscandia is one of the most prominent regions in the world for dendroclimatological research. Yet, millennium-long tree-ring chronologies in this region have mainly been developed from Scots pine (Pinus sylvestris L.). To explore the possibility of building long-term chronologies using other dominating tree species in the region, this paper presents the first two millennia-long Norway spruce (Picea abies (L.) Karst.) ring-width chronology from Northern Europe. The chronology is composed of living trees and subfossil wood and covers the period from BCE 115 to 2012 CE. A sufficiently replicated and robust chronology is built for the past 360 years back to 1649 CE. Further back in time, the common growth signal is reduced, and hence the reliability of the earlier section of the chronology is lower. The climate calibration results show that the spruce ring-width correlation with June-July mean temperatures over the period 1901-2012 is positive and significant (r = 0.6, p < 0.01) and representing the temperature variability of a spatial domain covering west-central Scandinavia. These results show the ability of Norway spruce to serve as a proxy for paleoclimatic research and the possibility of extending the chronology far back in time in the region, and therefore present an opportunity for carrying out new inter-and intraregional proxy analyses.

The need to understand and quantify the magnitude and frequency of past and current environmental changes increased the demand for high-resolution proxy data across spatial and temporal scales. Due to their long lifespan and global distribution, trees provide a unique and continuous record of environmental variability. More specifically, trees can be used as proxies of environmental conditions since their physical and chemical characteristics reflect the conditions in which they have been growing.

Focused on case studies from Sweden, this thesis presents a sample of applications where different tree-ring parameters were combined and compared with meteorological records, historical documents and soil profiles to provide information on natural and human-induced changes in the environment. 

Tree-ring width (TRW) measurements from living trees and subfossil wood from Jämtland, west-central Sweden, were combined to develop a two-millennia-long chronology from Norway spruce, a species that has been traditionally overlooked in dendroclimatic assessments in the region. This record enabled delivery of new perspectives on past regional climate variability and represents an important achievement for inter-and intraregional proxy analyses.

A unique maximum latewood density (MXD) chronology was developed from trees growing in drought-prone environments in the Stockholm Archipelago and used to reconstruct past precipitation variability for east-central Sweden. The results showed that MXD provides a stronger climate–growth relationship than TRW and allows a broader target seasonal average (May - July) to be reconstructed.

The resolution provided by ring width and density measurements is insufficient to understand growth responses to environmental stress at intra-annual scales. A pilot study using stem radial increment data retrieved from several dendrometers placed in the Stockholm area provided important insights into daily and seasonal growth dynamics in response to site-specific conditions. The main results show that short-term events, such as droughts, can significantly impact trees’ climate–growth relationship and their vegetative period.

Annually resolved time-series of wood elemental composition were used to investigate environmental contamination at a glassworks site in southern Sweden. The dendrochemical signals showed large variability both between and within the species in analysis, suggesting a strong control of the soil properties and species-specific uptake on trees’ elemental composition.

The findings presented in this thesis show that tree rings can be a highly suitable proxy to understand past and ongoing environmental changes and the link between ecosystems, climate and human activities. Overall, regionally developed networks of tree-ring data elucidated processes behind large-scale climate dynamics and provided new insights on past regional climate variability. In addition, locally-focused studies revealed fine-grained variations and the challenges of understanding the numerous physiological interactions between individual trees and the surrounding environment.

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.

Ring-width series are important for diverse fields of research such as the study of past climate, forest ecology, forest genetics, and the determination of origin (dendro-provenancing) or dating of archaeological objects. Recent research suggests diverging climate-growth relationships in tree-rings due to the cardinal direction of extracting the tree cores (i.e. direction-specific effect). This presents an understudied source of bias that potentially affects many data sets in tree-ring research. In this study, we investigated possible direction-specific growth variability based on an international (10 countries), multi-species (8 species) tree-ring width network encompassing 22 sites. To estimate the effect of direction-specific growth variability on climate-growth relationships, we applied a combination of three methods: An analysis of signal strength differences, a Principal Component Gradient Analysis and a test on the direction-specific differences in correlations between indexed ring-widths series and climate variables. We found no evidence for systematic direction-specific effects on tree radial growth variability in high-pass filtered ring-width series. In addition, direction-specific growth showed only marginal effects on climate-growth correlations. These findings therefore indicate that there is no consistent bias caused by coring direction in data sets used for diverse dendrochronological applications on relatively mesic sites within forests in flat terrain, as were studied here. However, in extremely dry, warm or cold environments, or on steep slopes, and for different life-forms such as shrubs, further research is advisable.