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Insect defoliation impacts forest productivity worldwide, highlighting the relevance of plant–insect interactions. The larch budmoth (Zeiraphera griseana Hübner) is one of the most extensively studied defoliators, where numerous tree ring-based analyses on its host (Larix decidua Mill.) have aided in identifying outbreak dynamics over the past millennia. Yet, outbreaks have been widely absent after the early 1980s, and little is known about the in situ tree physiological responses and the allocation of carbon resources during and after defoliation. In summer 2018, we tracked an ongoing larch budmoth outbreak in a well-studied larch forest in the Swiss Alps. We performed biweekly monitoring on an affected and unaffected site using a unique combination of xylogenesis observations, measurements of non-structural carbohydrates, isotopic analysis of needle assimilates and ground-based and remote-sensed leaf trait observations. The budmoth induced a defoliation that lasted 40 days and could be detected by satellite observations. Soluble sugars significantly decreased in needles and stem phloem of the defoliated trees, while starch levels remained stable in the stem and root xylem compared to the control. Carbon and oxygen isotope ratios in needle assimilates indicated that neither photosynthetic assimilation rates nor stomatal conductance was different between sites before, during and after the outbreak. Defoliated trees ceased cell wall thickening 17 days earlier than unaffected trees, showing the earliest halt of ring formation recorded from 2007 untill 2013 and causing significant thinner cell walls, particularly in the latewood. No significant differences were found for cell enlargement rates and ring width. Our study revealed that an outbreak causes a downregulation of cell wall thickening first, while no starch is mobilized or leaf physiology is adjusted to compensate for the reduced carbon source due to defoliation. Our observations suggest that affected larch trees prioritize leaf recovery and carbon storage over wood biomass development.

X-ray microdensitometry on annually resolved tree-ring samples has gained an exceptional position in last-millennium paleoclimatology through the maximum latewood density (MXD) parameter, but also increasingly through other density parameters. For 50 years, X-ray based measurement techniques have been the de facto standard. However, studies report offsets in the mean levels for MXD measurements derived from different laboratories, indicating challenges of accuracy and precision. Moreover, reflected visible light-based techniques are becoming increasingly popular, and wood anatomical techniques are emerging as a potentially powerful pathway to extract density information at the highest resolution. Here we review the current understanding and merits of wood density for tree-ring research, associated microdensitometric techniques, and analytical measurement challenges. The review is further complemented with a careful comparison of new measurements derived at 17 laboratories, using several different techniques. The new experiment allowed us to corroborate and refresh "long-standing wisdom" but also provide new insights. Key outcomes include (i) a demonstration of the need for mass/volume-based recalibration to accurately estimate average ring density; (ii) a substantiation of systematic differences in MXD measurements that cautions for great care when combining density data sets for climate reconstructions; and (iii) insights into the relevance of analytical measurement resolution in signals derived from tree-ring density data. Finally, we provide recommendations expected to facilitate futureinter-comparability and interpretations for global change research.

Two tree-ring series, one from a high-latitude pine tree (located in northern Scandinavia) and one from a mid-latitude oak tree (located in eastern Germany) were analyzed for radiocarbon (14C) at annual resolution. The new records cover the calendar date ranges 290–460 AD and 382–486 AD, respectively, overlapping by 79 yr. The series show similar trends as IntCal13. However, some significant deviations around 400 AD are present with lower Δ14C (higher 14C ages). An average offset between the two new series and IntCal13 of about 20 years in conventional 14C age is observed. A latitudinal 14C offset between the tree sites in central and northern Europe, as would be expected due to the relatively large spatial distance, is not recorded, however. Periodic changes in the 14C records are resolved that can be attributed to the “11-year” solar cycle (Schwabe cycle) with cycle length from 9 to 11 years. The magnitude of changes in Δ14C due to the solar cycle is between 1.5 and 3‰. Since solar cyclicity is only partially synchronous between the two new series, reasons for asynchronicity are explored.

The Sun sporadically produces eruptive events leading to intense fluxes of solar energetic particles (SEPs) that dramatically disrupt the near-Earth radiation environment. Such events are directly studied for the last decades but little is known about the occurrence and magnitude of rare, extreme SEP events. Presently, a few events that produced measurable signals in cosmogenic radionuclides such as 14C, 10Be and 36Cl have been found. Analyzing annual 14C concentrations in tree-rings from Switzerland, Germany, Ireland, Russia, and the USA we discovered two spikes in atmospheric 14C corresponding to 7176 and 5259 BCE. The ~ 2% increases of atmospheric 14C recorded for both events exceed all previously known 14C peaks but after correction for the geomagnetic field, they are comparable to the largest event of this type discovered so far at 775 CE. These strong events serve as accurate time markers for the synchronization with floating tree-ring and ice core records and provide critical information on the previous occurrence of extreme solar events which threaten modern infrastructure.

The Laacher See eruption (LSE) in Germany ranks among Europe’s largest volcanic events of the Upper Pleistocene1,2. Although tephra deposits of the LSE represent an important isochron for the synchronization of proxy archives at the Late Glacial to Early Holocene transition3, uncertainty in the age of the eruption has prevailed4. Here we present dendrochronological and radiocarbon measurements of subfossil trees that were buried by pyroclastic deposits that firmly date the LSE to 13,006 ± 9 calibrated years before present (bp; taken as ad 1950), which is more than a century earlier than previously accepted. The revised age of the LSE necessarily shifts the chronology of European varved lakes5,6 relative to the Greenland ice core record, thereby dating the onset of the Younger Dryas to 12,807 ± 12 calibrated years bp, which is around 130 years earlier than thought. Our results synchronize the onset of the Younger Dryas across the North Atlantic–European sector, preclude a direct link between the LSE and Greenland Stadial-1 cooling7, and suggest a large-scale common mechanism of a weakened Atlantic Meridional Overturning Circulation under warming conditions8–10. A revised date for the Laacher See eruption using measurements of subfossil trees shifts the chronology of European varved lakes relative to the Greenland ice core record, synchronizing the onset of the Younger Dryas across the North Atlantic–European sector.

As the worldwide standard for radiocarbon (14C) dating over the past ca. 50,000 years, the International Calibration Curve (IntCal) is continuously improving towards higher resolution and replication. Tree-ring-based 14C measurements provide absolute dating throughout most of the Holocene, although high-precision data are limited for the Younger Dryas interval and farther back in time. Here, we describe the dendrochronological characteristics of 1448 new 14C dates, between ~11,950 and 13,160 cal BP, from 13 pines that were growing in Switzerland. Significantly enhancing the ongoing IntCal update (IntCal20), this Late Glacial (LG) compilation contains more annually precise 14C dates than any other contribution during any other period of time. Thus, our results now provide unique geochronological dating into the Younger Dryas, a pivotal period of climate and environmental change at the transition from LG into Early Holocene conditions., Radiocarbon, 62 (4), ISSN:0033-8222, IntCal20: Calibration Issue

Predicting hydroclimatic changes on the Tibetan Plateau (TP) is crucial for managing water and ecosystems for the well-being of millions of people. Our understanding of the synoptic conditions on the TP is, however, still limited due to the paucity of meteorological measurements and proxy-based, high-resolution climate reconstructions. Here, we use state-of-the-art dendroclimatological techniques to investigate the paleoclimatic potential of drought-sensitive Picea likiangensis var. balfouriana forests between 4000 and 4500 m asl on the southeastern TP (SETP). The newly developed tree-ring width chronology correlates significantly with yearly changes in regional relative air humidity (RH) (r = 0.85, P

Dating of wood is a major task in historical research, archaeology and paleoclimatology. Currently, the most important dating techniques are dendrochronology and radiocarbon dating. Our approach is based on molecular decay over time under specific preservation conditions. In the models presented here, construction wood, cold soft waterlogged wood and wood from living trees are combined. Under these conditions, molecular decay as a usable clock for dating purposes takes place with comparable speed. Preservation conditions apart from those presented here are not covered by the model and cannot currently be dated with this method. For example, samples preserved in a clay matrix seem not to fit into the model. Other restrictions are discussed in the paper. One model presented covers 7,500 years with a root mean square error (RMSE) of 682 years for a single measurement. Another model reduced to the time period of the last 800 years results in a RMSE of 92 years. As multiple measurements can be performed on a single object, the total error for the whole object will be even lower.

Annually resolved and absolutely dated Maximum Latewood Density (MXD) and Blue Intensity (BI) measurements are frequently used for reconstructing summer temperature variability over the last centuries to millennia. A direct comparison of the outcome of both methods using similar material is needed due to how quickly this method is being adopted. The application of slightly different measuring systems (hardware) and analysis tools (software) in tandem with different wood samples and preparation procedures further challenges any straightforward assessment. Here we process 26 Norway spruce samples from the upper timberline in the Polish Tatra Mountains with the six most frequently used MXD and BI applications. Although offset is found in the raw MXD and BI data (0.04–0.13 g/cm3 and 0.45–1.58 dimensionless blue intensity), interannual and longer-term fluctuations are significantly (p

The rate and magnitude of temperature variability at the transition from the Last Glacial Maximum into the early Holocene represents a natural analog to current and predicted climate change. A limited number of high-resolution proxy archives, however, challenges our understanding of environmental conditions during this period. Here, we present combined dendrochronological and radiocarbon evidence from 253 newly discovered subfossil pine stumps from Zurich, Switzerland. The individual trees reveal ages of 41–506 years and were growing between the Allerod and Preboreal (∼13′900–11′300 cal BP). Together with previously collected pines from this region, this world's best preserved Late Glacial forest substantially improves the earliest part of the absolutely dated European tree-ring width chronology between 11′300 and 11′900 cal BP. Radiocarbon measurements from 65 Zurich pines between ∼12′320 and 13′950 cal BP provide a perspective to prolong the continuous European tree-ring record by another ∼2000 years into the Late Glacial era. These data will also be relevant for pinpointing the Laacher See volcanic eruption (∼12′900 cal BP) and two major Alpine earthquakes (∼13′770 and ∼11′600 cal BP). In summary, this study emphasizes the importance of dating precision and multi-proxy comparison to disentangle environmental signals from methodological noise, particularly during periods of high climate variability but low data availability, such as the Younger Dryas cold spell (∼11′700 and 12′900 cal BP).

The Younger Dryas stadial (YD) was a return to glacial-like conditions in the North Atlantic region that interrupted deglacial warming around 12900 cal BP (before 1950 AD). Terrestrial and marine records suggest this event was initiated by the interruption of deep-water formation arising from North American freshwater runoff, but the causes of the millennia-long duration remain unclear. To investigate the solar activity, a possible YD driver, we exploit the cosmic production signals of tree-ring radiocarbon (14C) and ice-core beryllium-10 (10Be). Here we present the highest temporally resolved dataset of 14C measurements (n = 1558) derived from European tree rings that have been accurately extended back to 14226 cal BP (±8, 2-σ), allowing precise alignment of ice-core records across this period. We identify a substantial increase in 14C and 10Be production starting at 12780 cal BP is comparable in magnitude to the historic Little Ice Age, being a clear sign of grand solar minima. We hypothesize the timing of the grand solar minima provides a significant amplifying factor leading to the harsh sustained glacial-like conditions seen in the YD.

Trees that were killed and buried by volcanic eruptions can be used to date an eruption with annual or even sub-annual resolution. The detection and measurement of subfossil tree-ring widths (TRW), however, often remains challenging if the material was carbonized during the eruption. Here, we show that the application of X-ray densitometry can improve the assessment of charcoal. Measuring the wood density of carbonized trees killed by the Laacher See Eruption ~13,000 years ago, facilitates the identification of the outermost rings that were formed just before the eruption. Our results suggest that anatomical techniques should be routinely applied in the assessment of historical, archaeological and subfossil wood.

Tree species differ in their ability to utilize existing soil water pools due to their root architecture, but also due to their capacity to react on spatiotemporal variations of the supply. The interplay between variations of water availability and species-specific utilization plays a crucial role in determining the water balance and cycle of ecosystems. Despite a large number of studies on the various aspects of ecosystem water relations, there exists still uncertainty regarding the plasticity of tree roots to take up water from different soil depths in relation to the mechanisms and patterns of water infiltration into the root zone.We will present results from a holistic tracer irrigation experiment in the Lötschental, Swiss Alps. A subalpine forest plot (150 m2) of Larix decidua and Picea abies was irrigated with, relative to natural soil abundance 18O and 2H depleted glacier water during 10 subsequent days in summer 2019. Water was taken from a nearby glacier river. Irrigation was conducted through a dripping system installed on the ground to increase and keep soil water content at field capacity during the experiment. Throughout the irrigation, soil moisture at three locations in the experimental as well as in a control plot was monitored in 15-minutes intervals in two soil depths. Four larch and four spruce trees per plot were selected and equipped with continuously measuring sapflow sensors. Sampling of soil and tree tissues took place on a daily basis always before noon: Soil samples were taken in close distance to the soil moisture sensors from at least three soil depths, needles and twigs from all experimental trees were sampled in the canopy of the sun-exposed crowns. Every third day xylem samples were taken from the tree stems with a 5mm increment corer. All samples were immediately cooled until the isotopic analysis. In parallel to the soil and tree sampling, physiological measurements were performed on the same trees with a Licor. In addition, also pre-dawn leaf water potentials were measured every third day. Finally, also micro cores were taken several times before, during and after the experiment for monitoring of xylem cell growth as a basis for high-resolution tree-ring isotope analysis at a later project phase. From all soil, needle, twig and stem core samples water was extracted by cryogenic vacuum distillation and d18O and d2H measured.The data of this experiment together with mechanistic modelling will elucidate the spatiotemporal pattern of soil water dynamics, water uptake by roots and tree-water relations of two species that have ecologically different life forms but are both highly representative for subalpine regions. Understanding their ability to react and capitalize on soil rewetting after dry periods will be crucial for the estimation of their survival potential and competitiveness under future dry and wet extreme events.

The sudden interruption of recurring larch budmoth (LBM; Zeiraphera diniana or griseana Gn.) outbreaks across the European Alps after 1982 was surprising, because populations had regularly oscillated every 8–9 years for the past 1200 years or more. Although ecophysiological evidence was limited and underlying processes remained uncertain, climate change has been indicated as a possible driver of this disruption. An unexpected, recent return of LBM population peaks in 2017 and 2018 provides insight into this insect’s climate sensitivity. Here, we combine meteorological and dendrochronological data to explore the influence of temperature variation and atmospheric circulation on cyclic LBM outbreaks since the early 1950s. Anomalous cold European winters, associated with a persistent negative phase of the North Atlantic Oscillation, coincide with four consecutive epidemics between 1953 and 1982, and any of three warming-induced mechanisms could explain the system’s failure thereafter: (1) high egg mortality, (2) asynchrony between egg hatch and foliage growth, and (3) upward shifts of outbreak epicentres. In demonstrating that LBM populations continued to oscillate every 8–9 years at sub-outbreak levels, this study emphasizes the relevance of winter temperatures on trophic interactions between insects and their host trees, as well as the importance of separating natural from anthropogenic climate forcing on population behaviour. Electronic supplementary material The online version of this article (10.1007/s00442-019-04585-9) contains supplementary material, which is available to authorized users.

Advances in accelerator mass spectrometry have resulted in an unprecedented amount of new high-precision radiocarbon (14C) -dates, some of which will redefine the international 14C calibration curves (IntCal and SHCal). Often these datasets are unaccompanied by detailed quality insurances in place at the laboratory, questioning whether the 14C structure is real, a result of a laboratory variation or measurement-scatter. A handful of intercomparison studies attempt to elucidate laboratory offsets but may fail to identify measurement-scatter and are often financially constrained. Here we introduce a protocol, called Quality Dating, implemented at ETH-Zürich to ensure reproducible and accurate high-precision 14C-dates. The protocol highlights the importance of the continuous measurements and evaluation of blanks, standards, references and replicates. This protocol is tested on an absolutely dated German Late Glacial tree-ring chronology, part of which is intercompared with the Curt Engelhorn-Center for Archaeometry, Mannheim, Germany (CEZA). The combined dataset contains 170 highly resolved, highly precise 14C-dates that supplement three decadal dates spanning 280 cal. years in IntCal, and provides detailed 14C structure for this interval., Radiocarbon, 62 (4), ISSN:0033-8222

During the growing season, trees allocate photoassimilates to increase their aboveground woody biomass in the stem (ABIstem). This ‘carbon allocation’ to structural growth is a dynamic process influenced by internal and external (e.g., climatic) drivers. While radial variability in wood formation and its resulting structure have been intensively studied, their variability along tree stems and subsequent impacts on ABIstem remain poorly understood. We collected wood cores from mature trees within a fixed plot in a well-studied temperate Fagus sylvatica L. forest. For a subset of trees, we performed regular interval sampling along the stem to elucidate axial variability in ring width (RW) and wood density (ρ), and the resulting effects on tree- and plot-level ABIstem. Moreover, we measured wood anatomical traits to understand the anatomical basis of ρ and the coupling between changes in RW and ρ during drought. We found no significant axial variability in ρ because an increase in the vessel-to-fiber ratio with smaller RW compensated for vessel tapering towards the apex. By contrast, temporal variability in RW varied significantly along the stem axis, depending on the growing conditions. Drought caused a more severe growth decrease, and wetter summers caused a disproportionate growth increase at the stem base compared with the top. Discarding this axial variability resulted in a significant overestimation of tree-level ABIstem in wetter and cooler summers, but this bias was reduced to ~2% when scaling ABIstem to the plot level. These results suggest that F. sylvatica prioritizes structural carbon sinks close to the canopy when conditions are unfavorable. The different axial variability in RW and ρ thereby indicates some independence of the processes that drive volume growth and wood structure along the stem. This refines our knowledge of carbon allocation dynamics in temperate diffuse-porous species and contributes to reducing uncertainties in determining forest carbon fixation.

Previous studies have suggested that the Late Glacial period (LG; ~ 14,600 – 11,700\ud cal BP) was characterised by abrupt and extreme climate variability over the European\ud sector of the North Atlantic. The limited number of precisely dated, high-resolution\ud proxy records, however, restricts our understanding of climate dynamics through the\ud LG. Here, we present the first annually-resolved tree-cellulose stable oxygen and\ud carbon isotope chronology (δ18Otree, δ13Ctree) covering the LG between ~14,050 –\ud 12,795 cal BP, generated from a Swiss pine trees (P. sylvestris; 27 trees, 1255 years).\ud Comparisons of δ18Otree with regional lake and ice core δ18O records reveal that LG\ud climatic changes over the North Atlantic (as recorded by Greenland Stadials and Inter-\ud Stadials) were not all experienced to the same degree in the Swiss trees. Possible\ud explanations include: (1) LG climate oscillations may be less extreme during the\ud summer in Switzerland, (2) tree-ring δ18O may capture local precipitation and humidity\ud changes and/or (3) decayed cellulose and various micro-site conditions may overprint\ud large-scale temperature trends found in other δ18O records. Despite these challenges,\ud our study emphasises the potential to investigate hydroclimate conditions using\ud subfossil pine stable isotopes.

Radiocarbon measurements in tree rings can be used to estimate atmospheric 14C concentration and thereby used to create a 14C calibration curve. When wood is discovered in construction sites, rivers, buildings, and lake sediments, it is unclear if the wood could fill gaps in the 14C calibration curve or if the wood is of historical interest until the age is determined by dendrochronology or 14C dating. However, dendrochronological dating is subjected to many requirements and 14C dating is costly and time consuming, both of which can be frivolous endeavors if the samples are not in the age range of interest. A simplified 14C dating technique, called Speed Dating, was thus developed. It can be used to quickly obtain 14C ages as wood samples are neither chemically treated nor graphitized. Instead, wood is combusted in an elemental analyzer (EA) and the CO2 produced is carried into an accelerator mass spectrometer (AMS) with a gas ion source. Within a day, 75 samples can be measured with uncertainties between 0.5–2% depending on the age, preservation, and contaminants on the material and Speed Dating costs about one-third of conventional AMS dates.

This technical note and its corresponding video aim to show how to sample trees for small basic dendroecological studies. Video

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.

Successful cross-dating of subfossil wood, ideally in combination with precise information on germination and dieback, requires the accurate detection of tree-ring width (TRW) boundaries along continuous measurement tracks from pith to bark. However, wood decay and the mechanical deformation of cells often challenge the dendrochronological analysis and subsequent paleoclimatic and environmental interpretations. Here, we show that wood anatomical techniques can improve the assessment of heavily degraded and/or deformed material. We apply state-of-the-art sample preparation, thin sectioning and double-staining to a unique collection of Late Glacial pines that were growing ∼13,000 years ago in the vicinity of Zurich, Switzerland. Highly resolved anatomical observations not only reveal detailed insights into the quality of each cell, but also allow the extent of wood deformation to be identified. By improving the detection and cross-dating of particularly narrow rings, TRW series might be extended towards the pith and bark, where decomposition and distortion is usually most severe, thus adding to a better understanding of possible germination and dieback processes, respectively. Moreover, thin sectioning has the potential to manually reconstruct original TRWs by correcting for post-mortem deformed ring structures. Our results suggest that anatomical techniques should be routinely applied in the assessment of historical, archaeological and subfossil wood.

Summary The effects of climate change on Arctic ecosystems can range between various spatiotemporal scales and may include shifts in population distribution, community composition, plant phenology, primary productivity and species biodiversity. The growth rates and age structure of tundra vegetation as well as its response to temperature variation, however, remain poorly understood because high-resolution data are limited in space and time. Anatomical and morphological stem characteristics were recorded to assess the growth behaviour and age structure of 871 dwarf shrubs from 10 species at 30 sites in coastal East Greenland at ˜70°N. Recruitment pulses were linked with changes in mean annual and summer temperature back to the 19th century, and a literature review was conducted to place our findings in a pan-Arctic context. Low cambial activity translates into estimated average/maximum plant ages of 59/204 years, suggesting relatively small turnover rates and stable community composition. Decade-long changes in the recruitment intensity were found to lag temperature variability by 2 and 6 years during warmer and colder periods, respectively (r = 0.851961–2000 and 1881–1920). Synthesis. Our results reveal a strong temperature dependency of Arctic dwarf shrub reproduction, a high vulnerability of circumpolar tundra ecosystems to climatic changes, and the ability of evaluating historical vegetation dynamics well beyond the northern treeline. The combined wood anatomical and plant ecological approach, considering insights from micro-sections to community assemblages, indicates that model predictions of rapid tundra expansion (i.e. shrub growth) following intense warming might underestimate plant longevity and persistence but overestimate the sensitivity and reaction time of Arctic vegetation.

Ancient DNA from historical and subfossil wood has a great potential to provide new insights into the history of tree populations. However, its extraction and analysis have not become routine, mainly because contamination of the wood with modern plant material can complicate the verification of genetic information. Here, we used sapwood tissue from 22 subfossil pines that were growing c. 13 000 yr bp in Zurich, Switzerland. We developed and evaluated protocols to eliminate surface contamination, and we tested ancient DNA authenticity based on plastid DNA metabarcoding and the assessment of post-mortem DNA damage. A novel approach using laser irradiation coupled with bleaching and surface removal was most efficient in eliminating contaminating DNA. DNA metabarcoding confirmed which ancient DNA samples repeatedly amplified pine DNA and were free of exogenous plant taxa. Pine DNA sequences of these samples showed a high degree of cytosine to thymine mismatches, typical of post-mortem damage. Stringent decontamination of wood surfaces combined with DNA metabarcoding and assessment of post-mortem DNA damage allowed us to authenticate ancient DNA retrieved from the oldest Late Glacial pine forest. These techniques can be applied to any subfossil wood and are likely to improve the accessibility of relict wood for genome-scale ancient DNA studies.

Highlights • Previous age estimates of the Laacher See Eruptions (LSE) around 12,900 years are still diverging and imprecise. • The combination of dendrochronology, wood anatomy, and 14C measurements holds the potential to establish a precise LSE date. • An absolute calendric date of the LSE would improve the synchronization of European Late Glacial to Holocene archives. Abstract The precise date of the Laacher See eruption (LSE), central Europe’s largest Late Pleistocene volcanic event that occurred around 13,000 years ago, is still unknown. Here, we outline the potential of combined high-resolution dendrochronological, wood anatomical and radiocarbon (14C) measurements, to refine the age of this major Plinian eruption. Based on excavated, subfossil trees that were killed during the explosive LSE and buried under its pyroclastic deposits, we describe how a firm date of the eruption might be achieved, and how the resulting temporal precision would further advance our understanding of the environmental and societal impacts of this event. Moreover, we discuss the relevance of an accurate LSE date for improving the synchronization of European terrestrial and lacustrine Late Glacial to Holocene archives.

Direct effects of climate change on animal physiology, and indirect impacts from disruption of seasonal synchrony and breakdown of trophic interactions are particularly severe in Arctic and Alpine ecosystems. Unravelling biotic from abiotic drivers, however, remains challenging because high-resolution animal population data are often limited in space and time. Here, we show that variation in annual horn growth (an indirect proxy for individual performance) of 8043 male Alpine ibex (Capra ibex) over the past four decades is well synchronised among eight disjunct colonies in the eastern Swiss Alps. Elevated March to May temperatures, causing premature melting of Alpine snowcover, earlier plant phenology and subsequent improvement of ibex food resources, fuelled annual horn growth. These results reveal dependency of local trophic interactions on large-scale climate dynamics, and provide evidence that declining herbivore performance is not a universal response to global warming even for high-altitude populations that are also harvested.

International audience; Despite an increasing demand for Burgundy truffles (Tuber aestivum), gaps remain in our understanding of the fungus’ overall lifecycle and ecology. Here, we compile evidence from three independent surveys in Hungary and Switzerland. First, we measured the weight and maturity of 2,656 T. aestivum fruit bodies from a three-day harvest in August 2014 in a highly productive orchard in Hungary. All specimens ranging between 2 and 755 g were almost evenly distributed through five maturation classes. Then, we measured the weight and maturity of another 4,795 T. aestivum fruit bodies harvested on four occasions between June and October 2015 in the same truffière. Again, different maturation stages occurred at varying fruit body size and during the entire fruiting season. Finally, the predominantly unrelated weight and maturity of 81 T. aestivum fruit bodies from four fruiting seasons between 2010 and 2013 in Switzerland confirmed the Hungarian results. The spatiotemporal coexistence of 7,532 small-ripe and large-unripe T. aestivum, which accumulate to ~182 kg, differs from species-specific associations between the size and ripeness that have been reported for other mushrooms. Although size-independent truffle maturation stages may possibly relate to the perpetual belowground environment, the role of mycelial connectivity, soil property, microclimatology, as well as other abiotic factors and a combination thereof, is still unclear. Despite its massive sample size and proof of concept, this study, together with existing literature, suggests consideration of a wider ecological and biogeographical range, as well as the complex symbiotic fungus-host interaction, to further illuminate the hidden development of belowground truffle fruit bodies.

a b s t r a c t Three tree-ring maximum latewood density chronologies were developed from high elevation Picea schrenkiana sites in the western Tien Shan Mountains using different detrending methods. The new chro- nologies extend back to the early 16th and late 17th centuries, and contain significant late spring and summer temperature signals, respectively. An assessment of varying detrending methods and band-pass filtering the chronologies revealed only slightly differing low frequency trends retained in the maximum latewood densities. The distance between sampling sites and the varying seasonality of limiting climatic factors are identified as key drivers affecting the correlation among the maximum latewood density records in the study region. The new chronologies represent reliable proxies of high elevation late spring and summer temperature variability in an area underrepresented by such data, and are ready-to-use for network analyses addressing longer-term climate variations in eastern central Asia.

Supplementing broader-scale dendroecological approaches with high-resolution wood anatomical analyses constitutes a useful technique to assess spatiotemporal patterns of climate-induced growth responses in circumpolar tundra vegetation. A systematic evaluation of dendrochronological and wood anatomical features in arctic dwarf shrubs is, however, still missing. Here, we report on nearly thousand samples from ten major dwarf shrub species that were collected at 30 plot-sites around 70°N and 22°W in coastal East Greenland. Morphological root and stem characteristics, together with intra-annual anatomical variations are outlined and the potential and limitation of ring counting is stressed. This study further demonstrates the possibility to gain annually resolved insight on past dry matter production and carbon allocation in arctic (and alpine) environments well beyond northern (and upper) treelines, where vegetation growth is particularly sensitive to environmental change.

Climate affects the timing, rate and dynamics of tree growth, over time scales ranging from seconds to centuries. Monitoring how a tree's stem radius varies over these time scales can provide insight into intra-annual stem dynamics and improve our understanding of climate impacts on tree physiology and growth processes. Here, we quantify the response of radial conifer stem size to environmental fluctuations via a novel assessment of tree circadian cycles. We analyze four years of sub-hourly data collected from 56 larch and spruce trees growing along a natural temperature gradient of ∼6 °C in the central Swiss Alps. During the growing season, tree stem diameters were greatest at mid-morning and smallest in the late evening, reflecting the daily cycle of water uptake and loss. Along the gradient, amplitudes calculated from the stem radius cycle were ∼50% smaller at the upper site (∼2200 m a.s.l.) relative to the lower site (∼800 m a.s.l.). We show changes in precipitation, temperature and cloud cover have a substantial effect on typical growing season diurnal cycles; amplitudes were nine times smaller on rainy days (>10 mm), and daily amplitudes are approximately 40% larger when the mean daily temperature is 15–20 °C than when it is 5–10 °C. We find that over the growing season in the sub-alpine forests, spruce show greater daily stem water movement than larch. However, under projected future warming, larch could experience up to 50% greater stem water use, which may severely affect future growth on already dry sites. Our data further indicate that because of the confounding influences of radial growth and short-term water dynamics on stem size, conventional methodology probably overstates the effect of water-linked meteorological variables (i.e. precipitation and relative humidity) on intra-annual tree growth. We suggest future studies use intra-seasonal measurements of cell development and consider whether climatic factors produce reversible changes in stem diameter. These study design elements may help researchers more accurately quantify and attribute changes in forest productivity in response to future warming.

Based on an analysis of maximum latewood density data from northern Scandinavia, along with published dendrochronological records, this study finds evidence that previous tree-ring-reliant reconstructions of large-scale near-surface air temperature underestimated long-term pre-industrial warmth during Medieval and Roman times. Solar insolation changes, resulting from long-term oscillations of orbital configurations1, are an important driver of Holocene climate2,3. The forcing is substantial over the past 2,000 years, up to four times as large as the 1.6 W m−2 net anthropogenic forcing since 1750 (ref. 4), but the trend varies considerably over time, space and with season5. Using numerous high-latitude proxy records, slow orbital changes have recently been shown6 to gradually force boreal summer temperature cooling over the common era. Here, we present new evidence based on maximum latewood density data from northern Scandinavia, indicating that this cooling trend was stronger (−0.31 °C per 1,000 years, ±0.03 °C) than previously reported, and demonstrate that this signature is missing in published tree-ring proxy records. The long-term trend now revealed in maximum latewood density data is in line with coupled general circulation models7,8 indicating albedo-driven feedback mechanisms and substantial summer cooling over the past two millennia in northern boreal and Arctic latitudes. These findings, together with the missing orbital signature in published dendrochronological records, suggest that large-scale near-surface air-temperature reconstructions9,10,11,12,13 relying on tree-ring data may underestimate pre-instrumental temperatures including warmth during Medieval and Roman times.

A microtome designed for the surface preparation of entire increment cores allows cutting plane surfaces on cores up to a length of 40 cm. Compared to the common sanding procedure, the wood cells of the annual rings remain open, not filled with swarf, and the cell walls are smooth and hence clearly visible. This article aims at describing the functionality of the microtome and the procedures needed for an accurate surface preparation to achieve a good contrast for subsequent image analysis. Possible applications for a more detailed analysis of variations in the tracheid structure of conifers and vessel sizes of oak are presented, which can be included in time series analyses.

Annually resolved summer temperatures for the European Alps are described. The reconstruction covers the a.d. 755–2004 period and is based on 180 recent and historic larch [Larix decidua Mill.] density series. The regional curve standardization method was applied to preserve interannual to multicentennial variations in this high-elevation proxy dataset. Instrumental measurements from high- (low-) elevation grid boxes back to 1818 (1760) reveal strongest growth response to current-year June–September mean temperatures. The reconstruction correlates at 0.7 with high-elevation temperatures back to 1818, with a greater signal in the higher-frequency domain (r = 0.8). Low-elevation instrumental data back to 1760 agree with the reconstruction’s interannual variation, although a decoupling between (warmer) instrumental and (cooler) proxy data before ∼1840 is noted. This offset is larger than during any period of overlap with more recent high-elevation instrumental data, even though the proxy time series always contains some unexplained variance. The reconstruction indicates positive temperatures in the tenth and thirteenth century that resemble twentieth-century conditions, and are separated by a prolonged cooling from ∼1350 to 1700. Six of the 10 warmest decades over the 755–2004 period are recorded in the twentieth century. Maximum temperature amplitude over the past 1250 yr is estimated to be 3.1°C between the warmest (1940s) and coldest (1810s) decades. This estimate is, however, affected by the calibration with instrumental temperature data. Warm summers seem to coincide with periods of high solar activity, and cold summers vice versa. The record captures the full range of past European temperature variability, that is, the extreme years 1816 and 2003, warmth during medieval and recent times, and cold in between. Comparison with regional- and large-scale reconstructions reveals similar decadal to longer-term variability.

Dendroecological research uses information stored in tree rings to understand how single trees and even entire forest ecosystems responded to environmental changes and to finally reconstruct such changes. This is done by analyzing growth variations back in time and correlating various plant-specific parameters to (for example) temperature records. Integrating wood anatomical parameters in these analyses would strengthen reconstructions, even down to intra-annual resolution. We therefore present a protocol on how to sample, prepare, and analyze wooden specimen for common macroscopic analyses, but also for subsequent microscopic analyses. Furthermore we introduce a potential solution for analyzing digital images generated from common small and large specimens to support time-series analyses. The protocol presents the basic steps as they currently can be used. Beyond this, there is an ongoing need for the improvement of existing techniques, and development of new techniques, to record and quantify past and ongoing environmental processes. Traditional wood anatomical research needs to be expanded to include ecological information to this field of research. This would support dendro-scientists who intend to analyze new parameters and develop new methodologies to understand the short and long term effects of specific environmental factors on the anatomy of woody plants.

Annually resolved and absolutely dated tree-ring chronologies are the most important proxy archives to reconstruct climate variability over centuries to millennia. However, the suitability of tree-ring chronologies to reflect the “true” spectral properties of past changes in temperature and hydroclimate has recently been debated. At issue is the accurate quantification of temperature differences between early nineteenth-century cooling and recent warming. In this regard, central Europe (CEU) offers the unique opportunity to compare evidence from instrumental measurements, paleomodel simulations, and proxy reconstructions covering both the exceptionally hot summer of 2003 and the year without summer in 1816. This study uses 565 Swiss stone pine (Pinus cembra) ring width samples from high-elevation sites in the Slovakian Tatra Mountains and Austrian Alps to reconstruct CEU summer temperatures over the past three centuries. This new temperature history is compared to different sets of instrumental measurements and state-of-the-art climate model simulations. All records independently reveal the coolest conditions in the 1810s and warmest after 1996, but the ring width–based reconstruction overestimates the intensity and duration of the early nineteenth-century summer cooling by approximately 1.5°C at decadal scales. This proxy-specific deviation is most likely triggered by inflated biological memory in response to reduced warm season temperature, together with changes in radiation and precipitation following the Tambora eruption in April 1815. While suggesting there exists a specific limitation in ring width chronologies to capture abrupt climate perturbations with increased climate system inertia, the results underline the importance of alternative dendrochronological and wood anatomical parameters, including stable isotopes and maximum density, to assess the frequency and severity of climatic extremes.

The long-term history ofZeiraphera dinianaGn. (the larch budmoth, LBM) outbreaks was reconstructed from tree rings of host subalpine larch in the European Alps. This record was derived from 47 513 maximum latewood density measurements, and highlights the impact of contemporary climate change on ecological disturbance regimes. With over 1000 generations represented, this is the longest annually resolved record of herbivore population dynamics, and our analysis demonstrates that remarkably regular LBM fluctuations persisted over the past 1173 years with population peaks averaging every 9.3 years. These regular abundance oscillations recurred until 1981, with the absence of peak events during recent decades. Comparison with an annually resolved, millennium-long temperature reconstruction representative for the European Alps (r=0.72, correlation with instrumental data) demonstrates that regular insect population cycles continued despite major climatic changes related to warming during medieval times and cooling during the Little Ice Age. The late twentieth century absence of LBM outbreaks, however, corresponds to a period of regional warmth that is exceptional with respect to the last 1000+ years, suggesting vulnerability of an otherwise stable ecological system in a warming environment.