Open Access

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.

Treeline ecosystems are of great scientific interest to study the effects of limiting environmental conditions on tree growth. However, tree growth is multidimensional, with complex interactions between height and radial growth. In this study, we aimed to disentangle effects of height and climate on xylem anatomy of white spruce [Picea glauca (Moench) Voss] at three treeline sites in Alaska; i.e., one warm and drought-limited, and two cold, temperature-limited. To analyze general growth differences between trees from different sites, we used data on annual ring width, diameter at breast height (DBH), and tree height. A representative subset of the samples was used to investigate xylem anatomical traits. We then used linear mixed-effects models to estimate the effects of height and climatic variables on our study traits. Our study showed that xylem anatomical traits in white spruce can be directly and indirectly controlled by environmental conditions: hydraulic-related traits seem to be mainly influenced by tree height, especially in the earlywood. Thus, they are indirectly driven by environmental conditions, through the environment’s effects on tree height. Traits related to mechanical support show a direct response to environmental conditions, mainly temperature, especially in the latewood. These results highlight the importance of assessing tree growth in a multidimensional way by considering both direct and indirect effects of environmental forcing to better understand the complexity of tree growth responses to the environment.