Trees and the environment : Possibilities and challenges in tree-ring research across spatial and temporal scales based on case studies in Sweden

Abstract: 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a sample of applications where different tree-ring parameters were combined and compared with meteorological records, historical documents and soil profiles to provide information on natural and human-induced changes in the environment.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.