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Tree growth in northern and upper treeline ecotones of the circumpolar boreal forest is generally limited by temperature, i.e., trees grow generally more under warm, and less under cold climatic conditions. Based on the assumption that this relationship between tree growth and climate is linear and stable through time, dendroclimatologists use tree rings as natural archives to reconstruct past temperature conditions. Such tree-ring based reconstructions, together with other natural archives (e.g., ice cores and pollen), constitute our understanding of past climatic conditions that reach beyond modern instrumental records. However, a steadily increasing amount of studies reports a recent reduction or loss of the summer temperature signal for several species and sites of the boreal forest. Such a reduction of temperature sensitivity results in temporally unstable climate-tree growth relationships, which challenges the work of dendroclimatologists by potentially leading to miscalibrations of past climatic conditions. On the upside, this shift in the trees’ climate sensitivity might point to a shift in tree growth-limiting factors and thus serve as an early indicator of climate change impacts. There is evidence that this recent reduction in temperature sensitivity might be caused by the observed strong temperature increase at high latitudes, and thus temperature-induced drought stress. Other potential drivers and amplifiers of this phenomenon are differing microsite conditions (dry vs. wet soils) and factors inherent to trees, like genetic properties or age effects. In this PhD thesis, I systematically assessed the effects of frequently discussed drivers of unstable climate-tree growth relationships (climate change, micro-site effects, genetical predisposition) on two representative species of the boreal forest, white spruce in North America and Scots pine in Eurasia, across various temporal and spatial scales. I used classical (tree-ring width) and more novel (wood density, quantitative wood anatomy) dendrochronological proxies to unravel the effects from annual to sub-monthly resolution. More precisely, in chapter I, white spruce clones were compared to non-clones at two treeline sites in Alaska to test whether their growth patterns differ, and whether white spruce clones are generally suitable for dendroclimatic assessments. Clonal reproduction is frequent at treeline due to harsh conditions, but might lead to competition among individuals due to the close proximity among each other, which in turn might obscure their climatic signal. Second, I tested the effect of warmer and drier climatic conditions on the summer temperature signal of Scots pine in Eurasia (chapter II) and on the growing season moisture signal of white spruce in North America (chapter III), respectively. Temperature-induced drought stress is expected to be the most important driver of unstable climate-growth relationships in the boreal forest. I included several sites across latitudinal (50-150 km) and longitudinal (1,000-2,200 km) gradients to cover large parts of the species’ distribution ranges. Since Scots pine covers a wide range of ecological habitats, I additionally tested the effect of dry and wet micro-site conditions on the summer temperature signal of Scots pine in chapter II. Finally, in chapter IV, a systematic literature review was carried out in order to investigate the distribution of unstable climategrowth relationships in global tree-ring studies, and the usage of such series in climate reconstructions. Furthermore, the scientific impact of these potentially inaccurate climate reconstructions was assessed. In this PhD project, warmer and drier climatic conditions led to temporally unstable climate signals in both Scots pine (chapter II) and white spruce (chapter III), as expected. Unstable climate-growth relationships were found for all tested tree-ring proxies and at all sites in North America, and at most sites in Eurasia. Micro-site effects (chapter II) and clonal growth (chapter I) had no significant effect on the climate sensitivity and high-frequency variability of the tested species, but affected absolute growth. The review (chapter IV) revealed that the phenomenon of unstable climate-growth relationships is globally widespread, and occurs independent of tree species, geographic location, and tree-ring and climate proxies. While reconstructions inferred from these unstable relationships are frequent and respective papers have a high impact, the tree-ring community seems to increasingly recognize the challenge of unstable climate-growth relationships. With these findings, this PhD project helped to shed more light on the frequency, underlying drivers, and the impact of unstable climate-growth relationships in boreal forest trees, as well as underlying reaction processes in trees. Above all, this PhD project suggests that the loss of climate sensitivity is caused by a change of growth limiting factors: temperature limitation seems to be suspended in warmer and drier years for Scots pine in Eurasia, and moisture limitation first arises under warm/dry conditions for white spruce in North America. Due to plastic growth responses in trees, the general assumption in dendroclimatology – that climategrowth relationships are stable through time – seems to be incompatible with the principle of limiting factors (one factors is always most growth limiting). To improve the validity of future climate reconstructions, statistical approaches considering synchronously or changing climatic limiting factors need to be promoted, along with attempts to select the best responding trees from a dataset. Furthermore, a better understanding of nonclimatic factors potentially affecting tree growth (e.g., age, disturbance, soil parameters) is needed. A growth reduction of mature and dominant white spruce trees sampled in this PhD project seems likely under future warming conditions, with series of wood cells being valuable early indicators of climate change effects in white spruce. However, inferences cannot be extended to the entire stand due to the applied sample design. Projected climate warming will probably lead to a further reduction of the summer temperature signal in trees of the northern boreal forest, while wider consequences for forest growth and productivity are unclear.

Forests are key biomes linked to biogeochemical cycles, important species reservoirs and major ecosystem services providers. The observed global climate change in the 20th century has the potential to deeply affect the conservation, functioning and structure of these ecosystems. Expressed as rising average temperatures due to the increase in atmospheric concentration of greenhouse gases such as carbon dioxide, nitrate oxide and methane, pollutants which are mostly product of burning fuel for industrial activities. These long-term changes will be heterogeneous in time and space throughout the globe. For northeastern Germany, predictions indicate that summer temperature and winter precipitation will be at a constant rise, whereas summer precipitation is expected to decrease, conditions will increase the risk of drought conditions. The changes in long-term means will be accompanied by increased frequency of weather extremes. The overall effect of climate change, both its long- and short-term components and their interaction with forest growth is uncertain. Tree species in the temperate forest are highly adapted to seasonal growth, active in late-spring and summer when temperature thresholds activate primary and secondary growth as well as leaf development, given sufficient water availability. During winter, they become dormant as an strategy to decrease damage by freezing temperatures. These adaptations ultimately determine species distributions as they occur along climate gradients within their ecological optima. Thus climate change can have a significant effect on species distribution ranges and more locally it can change species abundances. Trees being sessile organisms, possess limited dispersal capacities and rely on their adaptation potential, both genetically through selection over generations and through phenotypic plasticity (e.g. the capacity of adapting to changing conditions within a lifetime). Tree growth can be explored by dendrochronological methods, that is, by analyzing traits of annual xylem bands as produced by the vascular cambium. These traits are width, wood anatomical properties (e.g. cell wall thickness, lumen diameter), and isotopic composition. Tree-rings are integrators of environmental conditions and indicators of vitality and productivity of trees and forests. Studying these traits allows to understand the effect of climate on growth and physiological function over decadal to centennial scales in the past and by it inform about future growth performance. However, environmental information is not trivially extracted from tree-rings. Environmental signals in tree-rings are often the result of complex interactions of lagged meteorological conditions and tree-scale characteristics such as size, canopy status (i.e. social status), competition and stand density, among other factors. For this reason the monitoring of secondary growth as it unfolds, for example through dendrometer monitoring (i.e. record of the stem-radial variations at intra-annual temporal scales) and repeated sampling for the study of xylogenesis, is of major importance to understand climate-growth relationships and bridge the gap between dendroecological analysis atdifferent ecological scales (from single trees to stands to populations). Therefore this thesis contains contributions a) to the understanding of long-term climate shifts and its effect on tree growth for species in the Central European temperate forests through dendrochronological assessments and contributions b) to understanding intra-annual growth dynamics and its relationship to meteorological conditions through the analysis of monitoring records. In the retrospective analysis chapters (I-III), first an assessment was performed of the climate-growth relationships of important species of these region which indicated that deciduous species’ growth (Fagus sylvatica, Quercus robur and Q. petreae) was influenced mostly by summer water availability. For Pinus sylvestris was late spring temperature. Negative correlations between winter temperatures and growth indices of deciduous species increased over the last decades, possibly linked to less snow cover of the soil leading to root damage causing growth reductions. Scots pine presented the opposite, as positive correlations with winter temperatures became more frequent, indicating that this species’ growth rates might benefit from an elongation of the vegetation period. Afterwards the effect of stand characteristics in the climate response was explored. The climate signal of solitary oak trees growing in northeastern Germany was compared to oaks in closed stands. Solitary trees expressed higher growth rates and drought signals, which endanger its conservation as dry conditions are expected to increase in the region. As in the temperate forest crowding effects are variable throughout a tree’s lifetime, as well as other limiting factors (e.g. climate), we subsequently developed a methodology based on analysis of individual tree-ring series rather than chronologies (site means) to disentangle these effects on heterogeneous samples and quantify them. By sampling all present crown classes in a site near Rostock (Germany), we found beech was mostly affected by water availability in the previous summer and this effect was well represented throughout the population. For oak the main climatic driver of growth was previous October temperature with a low representation throughout the obtained sample. For beech, the main trait governing the variability around the response to the main climate driver of growth was cambial age, and for oak was crown-projection/size. On the prospective analysis chapters (IV-VI), monitoring datasets from the years 2013-2019 were used for the analysis of meteorological forcing of dendrometer series, the effect of a multi-year drought event and for the development of a method to combine continuous dendrometer records with discrete histological observations from xylogenesis analysis. The analysis of meteorological forcing on stem-radial variations indicated all observed species (beech, oak, hornbeam in this case) respond similarly to atmospheric water content whereas the growth phenology displayed contrasting species differences. These findings indicate high-frequency variations in stem dynamics are similar between species as it reflects transpiration and water transport in the stem, whereas the timing of growth reflects life strategies and wood anatomical adaptations. Next we evaluated the effect of the consecutive drought years 2018-2019 using dendrometer data (beech, oak, hornbeam and sycamore maple). The increment levels after the onset of drought in 2018 were not reduced for the observed individuals, whereas in 2019 all species showed decreased growth levels, particularly beech. Most likely the water moisture reservoirs were adequately filled in winter and spring before summer 2018, which lead to increased buffer capacity to withstand the harsh conditions for radial growth. However in winter, and the spring before the summer of 2019, there was not sufficient precipitation which lead to less resistance to the second bought of the drought event. This illustrates the complex lagged meteorological effect on radial growth, which is easily obscured in retrospective dendroecological analysis and emphasizes the pivotal role of soil moisture and soil water storage in tree-growth analysis. As a final contribution, while recognizing the importance of prospective growth monitoring, we developed a software tool to visualize and combine dendrometer stem-radial variations with images of histological events, such as those obtained by microcores for xylogenesis analysis. Growth signals in dendrometers are often of smaller magnitude than variations related to stem-water dynamics. By comparing them with histological images of wood-formation it is possible to accurately assign growth phases to dendrometer series and optimize their assessment. The advancement in methodological approaches to study intra-annual tree growth data is of major importance in the context of permanent ecological monitoring plots and its role in the assessment of the consequences of climate change on forest growth and conservation. Overall the findings of this thesis indicate that climate change impacts in the temperate forest of Central Europe will be and have been varied depending on the species considered with stand, site and tree-level conditions strongly modulating its consequences and even direction. Deciduous species, particularly beech, will be at risk due to decreased water availability during summer for which beech shows a high sensitivity. While oak seems to be less prone to drought related growth reductions and it is plausible to consider changes in dominance towards drier sites, it is still at risk if vulnerability thresholds are crossed. Scots pine appears to be favored by the increased temperatures during late winter, although these are naturally found on poor sites or sites either too dry or too wet for other dominant deciduous species to establish. Nevertheless, Scots pine has been planted on a variety of site conditions and especially in northeastern Germany is among the most widespread and economically important forest trees. Furthermore, the individual variability we have found in climate responses indicates that heterogeneous stands contain resilient sub-populations that could guarantee survivorship of the species after stark changes in climate means. However, it appears that strong enough stressors such as hotter droughts can trigger wide ecosystem changes with more efficiency than shifts in climate means. Due to this intra-annual growth monitoring is particularly relevant to foretell ecosystem changes and to understand the complex relationships found in climate-growth analysis performed in dendroecological studies, as it permits to mechanistically understand how conditions outside the tree-ring formation period affects wood formation.

Dendrochronology, the science of tree-rings is a tool which has been widely used for many years for understanding changes in the environment, as trees react to environmental changes over time. In the contemporary situation, where climate warming in the Arctic is unequivocal and its effects on the Alpine and tundra ecosystems are seen pronouncedly in the past decade, the role of dendro-studies and the use of trees and shrubs alike as proxies of change has become critical. Studies clearly indicate that warming in the Arctic and Alpine tundra has resulted in increased vegetation in recent years. Shrubs, in these sensitive ecosystems, have proven to be highly instrumental as they likely benefit from this warming and hence are good indicators and auditees of this change. Therefore, in this study, we investigate the potential of shrubs in the evolving field of dendro-ecology/climatology. Studies from classical dendrochronology used annual rings from trees. Further, because of shrub sensitivity to contemporary change, shrub-based dendrochronological research has increased at a notable scale in the last decade and will likely continue. This is because shrubs grow even beyond the tree line and promise environmental records from areas where tree growth is very limited or absent. However, a common limitation noted by most shrub studies is the very hard cross-dating due to asynchronous growth patterns. This limitation poses a major hurdle in shrub-based dendrochronological studies, as it renders weak detection of common signals in growth patterns in population stands. This common signal is traced by using a ‘site-chronology’. In this dissertation, I studied shrub growth through various resolutions, starting from understanding radial growth within individuals along the length of the stem, to comparison of radial growth responses among male and female shrubs, to comparing growth responses among trees and shrubs to investigation of biome-wide functional trait responses to current warming. Apart from Chapter 4 and Chapter 6, I largely used Juniperus communis sp. for investigations as it is the most widely distributed woody dioecious species often used in dendro-ecological investigations in the Northern Hemisphere. Primarily, we investigated radial growth patterns within shrubs to better understand growth within individuals by comparing different stem-disks from different stem heights within individuals. We found significant differences in radial growth from different stem-disks with respect to stem heights from same individuals. Furthermore, we found that these differences depending on the choice of the stem-disk affect the resulting site-chronology and hence climate-sensitivity to a substantial extent and that the choice of a stem-disk is a crucial precursor which affects climate-growth relationships. Secondly, we investigated if gender difference – often reported causing differential radial growth in dioecious trees – is an influential factor for heterogeneous growth. We found that at least in case of Juniperus communis. L and Juniperus communis ssp nana. WILLD there is no substantial gender biased difference in radial growth which might affect the site-chronology. We did find moderate differences between sexes in an overall analysis and attribute this to reproductive effort in females. In our study to test the potential of shrubs for reconstruction, we used a test case of Alnus viridis ssp crispa. We found a strong correlation between ring-width indices and summer temperature. Initially, the model failed the stability tests when we tested the stability of this relation using a response function model. However, using wood-anatomical analysis we discovered that this was because of abnormal cell-wall formation resulting in very thin rings in the year 2004. Pointer year analysis revealed that the thin rings were caused because of a moth larval outbreak and when corrected for these rings the model passed all stability tests. Furthermore, to see if trees and shrubs growing in same biomes react to environmental changes similarly, a network analysis with sites ranging from the Mediterranean biome to the Ural Mountains in Russia was carried out. We found that shrubs react better to the current climate warming and have a decoupled divergent temperature response as compared to coexisting trees. This outcome reiterated the importance of shrub studies in relation to contemporary climate change. Even though trees and shrubs are woody forms producing annual rings, they have very different growth patterns and need different methods for analysis and data treatment. Finally, in a domain-wide network analysis from plant-community vegetation survey, we investigated functional relationships between plant traits (leaf area, plant height, leaf nitrogen content, specific leaf area (SLA), and leaf dry matter content (LDMC)) and abiotic factors viz. temperature and soil moisture. We found a strong relation between summer temperature and community height, SLA and LDMC on a spatial scale. Contrarily, the temporal-analysis revealed SLA and LDMC lagged and did not respond to temperature over the last decade. We realized that there are complex interactions between intra-specific and inter-specific plant traits which differ spatially and temporally impacting Arctic ecosystems in terms of carbon turn over, surface albedo, water balance and heat-energy fluxes. We found that ecosystem functions in the Arctic are closely linked with plant height and will be indicative of warming in the short term future becoming key factors in modelling ecosystem projections.

Forest ecosystems around the world and especially boreal forests, are facing drastically changing climatic conditions. It is known that these changes could challenge their functionality and vitality. Still, the exact impact is not fully understood, as tree growth is a complex process and depends on countless environmental and genetic factors. To estimate the effects of climate change on tree growth and forest development precisely, we must learn more about tree growth itself. A comprehensive approach is needed where trees and forests are investigated on different scales and levels of detail, ranging from global studies to studies on single individuals. In this dissertation, I follow such a comprehensive approach, using the North American conifer white spruce as an example. I present three papers in the form of three chapters in which my co-authors and I studied the growth and anatomy of white spruce (Picea glauca [Moench] Voss) and how it is influenced by environmental, climatic, and genetic factors. We used diverse approaches and methods on different spatial scales, ranging from investigations on the landscape to the local scale. We established three paired plots with forest and treeline sites (two cold-limited and one drought-limited). as well as one additional forest site. In the first chapter, we concentrated on the genetic diversity of white spruce within and between populations at all study sites throughout Alaska. The genetic investigations were combined with analyses on the individual growth response of trees to climatic conditions to find whether genetic similarities or spatial proximity caused similarities in growth and climatic sensitivity. In the second chapter, we studied the direct and indirect effects of environmental conditions on the xylem tissue of white spruce. We analyzed the impact of precipitation, temperature, and tree height on four xylem anatomical traits in trees growing at the three treelines. The investigated traits represented the main functions of xylem tissue (i.e., water transport and structural support). In the third chapter, we investigated similar xylem anatomical traits at one cold-limited treeline. We compared xylem anatomy and annual increment between genetic groups and individuals and between spatial groups to investigate whether spatial or genetic grouping influenced the anatomy and growth of white spruce. We found an overall high gene flow and high genetic diversity in white spruce. However, the sensitivity of the growth and anatomical traits of white spruce was driven mainly by spatial rather than genetic effects and differed between study sites. Trees from the drought-limited site were more sensitive towards precipitation and a moisture index, while trees from the cold-limited sites were more sensitive towards temperature. A strong direct effect of tem- perature was primarily found in latewood traits related to the structural sup- port of the tree. Earlywood traits related to water transport, however, were influenced mainly by tree height. Tree height itself was potentially affected by diverse abiotic and biotic factors (e.g., (micro)climate, soil conditions, and competition). Thus, traits related to water transport were indirectly influenced by environmental conditions. Genetic effects in xylem anatomical traits were found in the earlywood hydraulic diameter and latewood den- sity, whereas in general, primarily spatial rather than genetic grouping was influencing the anatomy of white spruce. Overall, white spruce showed to be a genetically diverse species with a high gene flow. The effects of spatial proximity and spatial grouping on the sensitivity and anatomy of white spruce indicate high phenotypic plastic- ity. This high phenotypic plasticity combined with the vast genetic diversity translates into an immense potential for the species to adjust (phenotypically) and possibly adapt (genetically) to changing conditions. Thus, in terms of climate change, white spruce may be a rather persistent species that manages to cope with the drastic changes. Though additional work might be needed to draw a more solid conclusion, the presented work shows how a comprehensive study approach can help to interpret and understand the growth and ecology of a tree species. It may be an inspiration for future studies to broaden their approaches and to use comprehensive methods on different levels of detail to not only observe trees but to explore and understand them.