Open Access

Forests are ecologically important ecosystems, for example, they absorb CO2 from the atmosphere, mitigate climate change, and constitute habitats for the majority of terrestrial flora and fauna. Currently, due to increasing human pressure, forest ecosystems are increasingly subjected to changing environmental conditions, which may alter forest growth to varying degrees. However, how exactly different tree species will respond to climate change remains uncertain and requires further comprehensive studies performed at different spatial scales and using various tree-ring parameters. This dissertation aims to advance the knowledge about tree-ring densitometry and tree responses to climate variability and extremes at different spatial scales, using various tree species. More specifically, the following aims are pursued: (i) to obtain and compare wood density data using different techniques, and to assess variability among laboratories (Chapter I). (ii) To investigate microsite effects on local and regional Scots pine (Pinus sylvestris L.) responses to climate variability (Chapter II) and extremes (Chapter III), using ring width (RW) and latewood blue intensity (LBI) parameters. (iii) To give a general site- and regional-scales overview of Scots pine, pedunculate oak (Quercus robur L.), and European beach (Fagus sylvatica L.) RW responses to climate variability (Chapter IV). (iv) To discuss the challenges which may result from compiling tree ring records from different (micro)sites into large-scale networks. The study area comprises nine coastal dune sites, each represented by two contrasting microsites: dune ridge and bottom (Chapters II and III), and 310 different sites within the south Baltic Sea lowlands (Chapter IV). The dissertation confirms that sample processing and wood density measuring are very important steps, which, if not performed carefully, may result in biases in growth trends, climate-growth responses, and climate reconstructions. The performed experiment proved that the mean levels of different wood density-related parameters are never comparable due to different measurement resolutions between various techniques and laboratories. Further, the study revealed substantial biases using data measured from rings of varying width due to resolution issues, where resolution itself and wood density are lowered for narrow rings compared to wide rings (Chapter I). The (micro)site-specific investigation showed that, depending on the species, different climate variables (temperature, precipitation, or drought) constitute important factors driving tree growth across investigated locations (Chapters II and IV). However, there is evidence that the strength and/or direction of climate-growth responses differ(s) between microsite types (Chapter II) and across sites (Chapter IV). Moreover, climategrowth responses are non-stationary over time regardless of the tree species and tree-ring parameter used in the analysis (Chapters II and IV). There are also differences in RW and LBI responses to extreme events at dune ridge and bottom microsites (Chapter III). The regional-scale investigations revealed that climate-growth responses (strength and non-stationarity) are quite similar to those observed at the local scale. However, compiling RW or LBI measurements into regional networks to study tree responses to extreme events led to weakened signals (Chapter III). The findings presented in Chapters II and IV suggest that the strength, direction, and non-stationary responses are very likely caused by several climatic and non-climatic factors. The mild climate in the south Baltic Sea region presumably does not constitute a leading limiting growth factor, especially for Scots pine, whose distribution extends from southern to northern Europe. Thus, the observed climate-growth responses are usually of weak to moderate strength. In contrast, for other species reaching their distribution limit at the Baltic coast, the climatic signal can be very strong. However, the observed findings also result from the effects of microsite conditions, and potentially other factors (e.g., management, stand dynamic), which all together alter the physiological response of the tree at a local scale. Although climate at the south Baltic Sea coast is mild, extreme climate events may occur and affect tree growth. As demonstrated (Chapter III), extreme climate events affected tree growth across dune sites, however, to varying degrees. The prominent differences in tree responses to extreme climate events were significant at the local scale but averaged out at the regional scale. This is very likely associated with observed microsite differences, where each microsite experiences different drivers and dynamics of extreme growth reductions. This dissertation helped to demonstrate that integrating local tree-ring records into regional networks involves a series of challenges, which arise at different stages of research. In fact, not all possible challenges have been discussed in this dissertation. However, it can be summarized that several steps performed first at the local scale are very important for the quality and certainty of climate-growth responses, tracking tree recovery after extreme events, and potential climate reconstructions at the larger scale. Among them, identification of microsite conditions, sample preparation, and measurement, examination of growth patterns and trends, and identification of a common limiting growth factor are very important. Otherwise, the compilation of various tree-ring data into a single dataset could lead to over- or underestimation of the results and biased interpretations.

Abstract The role of future forests in global biogeochemical cycles will depend on how different tree species respond to climate. Interpreting the response of forest growth to climate change requires an understanding of the temporal and spatial patterns of seasonal climatic influences on the growth of common tree species. We constructed a new network of 310 tree‐ring width chronologies from three common tree species (Quercus robur, Pinus sylvestris and Fagus sylvatica) collected for different ecological, management and climate purposes in the south Baltic Sea region at the border of three bioclimatic zones (temperate continental, oceanic, southern boreal). The major climate factors (temperature, precipitation, drought) affecting tree growth at monthly and seasonal scales were identified. Our analysis documents that 20th century Scots pine and deciduous species growth is generally controlled by different climate parameters, and that summer moisture availability is increasingly important for the growth of deciduous species examined. We report changes in the influence of winter climate variables over the last decades, where a decreasing influence of late winter temperature on deciduous tree growth and an increasing influence of winter temperature on Scots pine growth was found. By comparing climate–growth responses for the 1943–1972 and 1973–2002 periods and characterizing site‐level growth response stability, a descriptive application of spatial segregation analysis distinguished sites with stable responses to dominant climate parameters (northeast of the study region), and sites that collectively showed unstable responses to winter climate (southeast of the study region). The findings presented here highlight the temporally unstable and nonuniform responses of tree growth to climate variability, and that there are geographical coherent regions where these changes are similar. Considering continued climate change in the future, our results provide important regional perspectives on recent broad‐scale climate–growth relationships for trees across the temperate to boreal forest transition around the south Baltic Sea.