Search

High-resolution hydroclimate proxy records are essential for distinguishing natural hydroclimate variability from possible anthropogenically-forced changes, since instrumental precipitation observations are too short to represent the whole spectrum of natural variability. In Northern Europe, progress in this field has been hampered by a relative lack of long and truly moisture-sensitive proxy records. In this study, we provide the first assessment of the dendroclimatic potential of Blue Intensity (BI) and partial ring-width measurements (latewood and earlywood width series) from a network of cold and drought-prone Pinus sylvestris L. sites in Sweden. Our results show that all tree-ring parameters and sites share a clear and strong sensitivity to warm-season precipitation. The ΔBI parameter, in particular, shows considerable potential for hydroclimate reconstructions, here permitting a cross-validated precipitation reconstruction capable of explaining 56% (1901–2010 period) of regional-scale warm-season high-frequency precipitation variance. Using ΔBI as an alternative to ring-width improves the predictive skill with nearly a 20 percentage points increase in explained variance, reduces signal instability over time as well as allows a broader seasonal window (May–July) to be reconstructed. Additionally, we found that earlywood BI also reflect a positive late winter through early summer temperature signal. These findings emphasize that tree-rings, and in particular wood density parameters such as from BI, are capable of providing fundamental information to advance our understanding of hydroclimate variability in regions with a cool and rather humid climate regime that traditionally has been overlooked in studies of past droughts. Increasing the spatio-temporal coverage of hydroclimate records in northern Europe, and taking full advantage of the opportunities offered by the wood densitometric properties should be considered a research priority. Electronic supplementary material The online version of this article (10.1007/s00382-020-05287-2) contains supplementary material, which is available to authorized users.

A tree’s radial growth sequence can be thought of as an aggregate of different growth components such as age and size limitations, presence or absence of disturbance events, continuous impact of climate variability and variance induced by unknown origin. The potentially very complex growth patterns with prominent temporal and spatial variability imply that our understanding of climate-vegetation feedbacks essentially benefits from the expansion of large tree ring networks into data-poor regions, and our ability to disentangle growth constraints by comparing ring series at multiple scales. In this study, we analyze Central-Eastern Europe’s most substantial assemblage of primary Norway spruce forests found in the Carpathian arc. The vast data set, >10,000 tree-ring series, is stratified along a prominent gradient in climate response space over four separate landscapes. We integrated curve intervention detection and dendroclimatic standardization to decompose tree growth variance into climatic, disturbance and residual components to explore the behavior of the components over increasingly larger spatial hierarchies. We show that the residual variance of unknown origin is the most prominent variance in individual Carpathian spruce trees, but at larger spatial hierarchies, climate variance dominates. The variance induced by climate was further explored with common correlation analyses, growth response to extreme climate years and forward modeling of tree growth to identify leading modes of climate response, and potentially non-linear and mixed climate response patterns. We find that the climatic response of the different forest landscapes overall can be described as an asymptotic response to June and July temperatures, most likely intermixed with influence from winter precipitation. In the collection of landscapes, Southern Romania stands out as being the least temperature sensitive and most likely exhibiting the most complicated mixed temperature and moisture limitation.

Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794–2016 CE at 0.79 (p, Tree rings are a crucial archive for Common Era climate reconstructions, but the degree to which methodological decisions influence outcomes is not well known. Here, the authors show how different approaches taken by 15 different groups influence the ensemble temperature reconstruction from the same data.

Bulk wood density measurements are recognized for their utility in ecology, industry, and biomass estimations. In tree-ring research, microdensitometric techniques are widely used, but their ability to determine the correct central tendency has been questioned. Though rarely used, it may be possible to use bulk wood density as a tool to check the accuracy of and even correct microdensitometric measurements. Since measuring bulk wood density in parallel with X-ray densitometry is quickly and easily done, we suspect that its omission is largely due to a lack of awareness of the procedure and/or its importance. In this study, we describe a simple protocol for measuring bulk wood density tailored for tree-ring researchers and demonstrate a few possible applications. To implement real-world examples of the applications, we used a sample of existing X-ray and Blue Intensity (BI) measurements from 127 living and dead Pinus sylvestris trees from northern Sweden to produce new measurements of bulk wood density. We can confirm that the central tendency in this sample material is offset using X-ray densitometry and that the diagnosis and correction of X-ray density is easily done using bulk wood density in linear transfer functions. However, this approach was not suitable for our BI measurements due to heavy discoloration. Nevertheless, we were able to use bulk wood density to diagnose and improve the use of deltaBI (latewood BI – earlywood BI) with regard to its overall trends and multi-centennial variability in a dendroclimatological application. Moreover, we experimented with percent of latewood width, scaled with bulk wood density, as a time- and cost-effective proxy for annual ring density. Although our reconstruction only explains about half of the variation in ring density, it is most likely superior to using fixed literature values of density in allometric equations aimed at biomass estimations. With this study, we hope to raise new awareness regarding the versatility and importance of bulk wood density for dendrochronology by demonstrating its simplicity, relevance, and applicability.

Interannual variability of wood density - an important plant functional trait and environmental proxy - in conifers is poorly understood. We therefore explored the anatomical basis of density. We hypothesized that earlywood density is determined by tracheid size and latewood density by wall dimensions, reflecting their different functional tasks. To determine general patterns of variability, density parameters from 27 species and 349 sites across the Northern Hemisphere were correlated to tree-ring width parameters and local climate. We performed the same analyses with density and width derived from anatomical data comprising two species and eight sites. The contributions of tracheid size and wall dimensions to density were disentangled with sensitivity analyses. Notably, correlations between density and width shifted from negative to positive moving from earlywood to latewood. Temperature responses of density varied intraseasonally in strength and sign. The sensitivity analyses revealed tracheid size as the main determinant of earlywood density, while wall dimensions become more influential for latewood density. Our novel approach of integrating detailed anatomical data with large-scale tree-ring data allowed us to contribute to an improved understanding of interannual variations of conifer growth and to illustrate how conifers balance investments in the competing xylem functions of hydraulics and mechanical support.

We test the application of parametric, non-parametric, and semi-parametric calibration models for reconstructing summer (June–August) temperature from a set of tree-ring width and density data on the same dendro samples from 40 sites across Europe. By comparing the performance of the three calibration models on pairs” of tree-ring width (TRW) and maximum density (MXD) or maximum blue intensity (MXBI), we test whether a non-linear temperature response is more prevalent in TRW or MXD (MXBI) data, and whether it is associated with the temperature sensitivity and/or autocorrelation structure of the dendro parameters. We note that MXD (MXBI) data have a significantly stronger temperature response than TRW data as well as a lower autocorrelation that is more similar to that of the instrumental temperature data, whereas TRW exhibits a redder” variability continuum. This study shows that the use of non-parametric calibration models is more suitable for TRW data, while parametric calibration is sufficient for both MXD and MXBI data – that is, we show that TRW is by far the more non-linear proxy. publishedVersion

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought-prone areas, tree populations located at the driest and southernmost distribution limits (rear-edges) should be particularly threatened. Here we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris, and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear-edges of the continuous distributions of these tree species. We used tree-ring width data from a network of 110 forests in combination with the process-based Vaganov-Shashkin Lite growth model and climate-growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO2 emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear-edge. By contrast, growth of high-elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of –10.7% and –16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear-edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear-edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions. This article is protected by copyright. All rights reserved.

The most frequently and successfully used tree-ring parameters for the study of temperature variations are ring width and maximum latewood density (MXD). MXD is preferred over ring width due to a more prominent association with temperature. In this study we explore the dendroclimate potential of dendroanatomy based on the first truly well replicated dataset. Twenty-nine mature living Pinus sylvestris trees were sampled in North-eastern Finland at the cool and moist boreal forest zone, close to the latitudinal tree line, where ring width, X-ray MXD as well as the blue intensity counterpart MXBI were compared with dendroanatomical parameters. Maximum radial cell wall thickness as well as anatomical MXD and latewood density appeared to be the most promising parameters for temperature reconstruction. In fact, these parameters compare favorably to MXD derived from X-ray techniques as well as MXBI, in terms of shared variation and temperature correlations across frequencies and over time. The reasons for these results are thought to be the unprecedentedly high measurement resolution of the anatomical technique, which provide the optimal resolution – the cell – whereas X-ray techniques have a slightly lower resolution and BI techniques even lower. While the results of this study are encouraging, further tests on longer and multigenerational chronologies are required to more generally and fully assess the dendroclimate potential of anatomical parameters.

To assess past climate variability in west-central Scandinavia, a new 972-year-long temperature reconstruction, based on adjusted delta blue intensity (ΔBIadj), was created. Presently, it is the longest blue intensity chronology in Fennoscandia and the third longest in the northern hemisphere. Measurements were obtained from 119 tree line Scots pine (Pinus sylvestris L.) samples from Rogen, in the central Scandinavian Mountains, Sweden. Early and latewood blue intensity absorption data were used to create ΔBIadj. The data were detrended using a signal-free regional curve standardization method (RSFi) to minimize biological noise and maximize low-frequency climate information. The Rogen ΔBIadj chronology has a substantially stronger temperature signal at inter-annual timescales than the corresponding tree-ring width (RW) chronology, and it displays good spatial representation for the south-central parts of Scandinavia. The ΔBIadj summer (June through August) temperature reconstruction, extending back to 1038 CE, exhibits three warm periods in 1040–1190 CE, 1370–1570 CE and the 20th century and one extended cold period between 1570 and 1920 CE. Regional summer temperature anomalies are associated with a Scandinavian–Greenland dipole sea-level pressure pattern, which has been stable for the past several centuries. Major volcanic eruptions produce distinct anomalies of ΔBIadj indices indicating cooling of summer temperatures in the subsequent years. Our results show that ΔBIadj from Pinus sylvestris in Scandinavia is a suitable proxy providing opportunities to explore past temperature variability at various frequencies, atmospheric dynamics and variability in external forcing. Nevertheless, long-term trend differences with RW imply that further research is needed to fully understand the application of this technique in dendroclimatology.

A contribution to the 2k Network. ABSTRACT: Despite the spatially homogenous summer temperature pattern in Fennoscandia there are large spreads among the many existing reconstructions resulting in an uncertainty in the timing and amplitude of past changes. Also there has been a general bias towards northernmost Fennoscandia. In an attempt to provide a more spatially coherent view of summer (June–August JJA) temperature variability within the last millennium we utilized seven density and three blue intensity Scots pine (Pinus sylvestris L.) chronologies collected from the altitudinal (Scandinavian Mountains) and latitudinal (northernmost part) treeline. To attain a JJA temperature signal as strong as possible as well as preserving multicentury scale variability we used a new tree ring parameter where the earlywood information is removed from the maximum density and blue intensity and a modified signal free standardization method. Two skilful reconstructions for the period 1100–2006 CE were made one regional reconstruction based on an average of the chronologies and one field (gridded) reconstruction. The new reconstructions were shown to have much improved spatial representations compared to those based on data from only northern sites thus making it more valid for the whole region. An examination of some of the forcings of JJA mean temperatures in the region shows an association with sea surface temperature over the eastern North Atlantic but also the subpolar and subtropical gyres. Moreover using Superposed Epoch Analysis a significant cooling in the year following a volcanic eruption was noted and for the largest explosive eruptions the effect could remain for up to 4 years. This new improved reconstruction provides a mean to reinforce our understanding of forcings on summer temperatures in the North European sector.

The demand for large-scale and long-term information on tree growth is increasing rapidly as environmental change research strives to quantify and forecast the impacts of continued warming on forest ecosystems. This demand, combined with the now quasi-global availability of tree-ring observations, has inspired researchers to compile large tree-ring networks to address continental or even global-scale research questions. However, these emergent spatial objectives contrast with paleo-oriented research ideas that have guided the development of many existing records. A series of challenges related to how, where, and when samples have been collected is complicating the transition of tree rings from a local to a global resource on the question of tree growth. Herein, we review possibilities to scale tree-ring data (A) from the sample to the whole tree, (B) from the tree to the site, and (C) from the site to larger spatial domains. Representative tree-ring sampling supported by creative statistical approaches is thereby key to robustly capture the heterogeneity of climate-growth responses across forested landscapes. We highlight the benefits of combining the temporal information embedded in tree rings with the spatial information offered by forest inventories and earth observations to quantify tree growth and its drivers. In addition, we show how the continued development of mechanistic tree-ring models can help address some of the non-linearities and feedbacks that complicate making inference from tree-ring data. By embracing scaling issues, the discipline of dendrochronology will greatly increase its contributions to assessing climate impacts on forests and support the development of adaptation strategies.