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Basaltic fissure eruptions, which are characteristic of Icelandic volcanism, are extremely hazardous due to the large quantities of gases and aerosols they release into the atmosphere. The 1783–1784 CE Laki eruption was one of the most significant high‐latitude eruptions in the last millennium and had substantial environmental and climatic impacts. Contemporary observations recorded the presence of a sulfuric haze over Iceland and Europe, which caused famine from vegetation damage and resulted in a high occurrence of respiratory illnesses and related mortality. Historical records in northern Europe show that the summer of 1783 was anomalously warm, but regional tree‐ring maximum latewood density (MXD) data from that year are low and lead to erroneously colder reconstructed summer temperatures. Here, we measure wood anatomical characteristics of Scots pine (Pinus sylvestris) from Jämtland, Sweden in order to identify the cause of this discrepancy. We show that the presence of intraannual density fluctuations in the majority of 1783 growth rings, a sudden reduction in lumen and cell wall area, and the measurement resolution of traditional X‐ray densitometry led to the observed reduced annual MXD value. Multiple independent lines of evidence suggest these anatomical anomalies were most likely the result of direct acidic damage to trees in Northern Europe and that the normal relationship between summer temperature and MXD can be disrupted by this damage. Our study also demonstrates that quantitative wood anatomy offers a high‐resolution approach to identifying anomalous years and extreme events in the tree‐ring record. Key Points: High‐resolution tree ring density measurements provide approach for identifying extreme environmental eventsThe normal climate/MXD relationship is disrupted or obfuscated by the consequences of what is most likely the direct effects of acidic hazeScreening anatomical series for anomalies can lead to an evaluation of potentially confounding factors for climate reconstructions [ABSTRACT FROM AUTHOR]

Glacial extent and mass balance are sensitive climate proxies providing solid information on past climatic conditions. However, series of annual mass-balance measurements of more than 60 years are scarce. To our knowledge, this is the first time the latewood density data (MXD) of the Swiss stone pine (Pinus cembra L.) have been used to reconstruct the summer mass balance (Bs) of an Alpine glacier. The MXD-based Bs well correlates with a Bs reconstruction based on the May to September temperature. Winter precipitation has been used as an independent proxy to infer the winter mass balance and to obtain an annual mass-balance (Bn) estimate dating back to the glaciological year 1811/12. The reconstructed MXD/precipitation-based Bn well correlates with the data both of the Careser and of other Alpine glaciers measured by the glaciological method. A number of critical issues should be considered in both proxies, including non-linear response of glacial mass balance to temperature, bedrock topography, ice thinning and fragmentation, MXD acquisition and standardization methods, and finally the 'divergence problem' responsible for the recently reduced sensitivity of the dendrochronological data. Nevertheless, our results highlight the possibility of performing MXD-based dendroglaciological reconstructions using this stable and reliable proxy. [ABSTRACT FROM AUTHOR]

Abstract Ongoing climate change strongly affects high-elevation environments in the European Alps, influencing the cryosphere and the biosphere and causing widespread retreat of glaciers and changes in biomes. Nevertheless, high-elevation areas often lack long meteorological series, and global datasets cannot represent local variations well. Thus, proxy data, such as tree rings, provide information on past climatic variations from these remote sites. Although maximum latewood density (MXD) chronologies provide better temperature information than those based on tree-ring width (TRW), MXD series from the European Alps are lacking. To derive high-quality temperature information for the Rhaetian Alps, Pinus cembra L. trees sampled at approximately 2000 m a.s.l. were used to build one MXD chronology spanning from 1647 to 2015. The MXD data were significantly and highly correlated with seasonal May-September mean temperatures. The MXD chronology showed a generally positive trend since the middle of the 19th century, interrupted by short phases of climatic deterioration in the beginning of the 20th century and in the 1970s, conforming with the temperature trends. Our results underline the potential for using Pinus cembra L. MXD to reconstruct mean temperature variations, especially during the onset and latter part of the growing season, providing additional information on parts of the growing season not inferred from TRW. Future studies on MXD for this species will increase the availability of temporal and spatial data, allowing detailed climate reconstructions. [ABSTRACT FROM AUTHOR]

Reanalysis data show an increasing trend in Arctic precipitation over the 20th century, but changes are not homogenous across seasons or space. The observed hydroclimate changes are expected to continue and possibly accelerate in the coming century, not only affecting pan-Arctic natural ecosystems and human activities, but also lower latitudes through the atmospheric and ocean circulations. However, a lack of spatiotemporal observational data makes reliable quantification of Arctic hydroclimate change difficult, especially in a long-term context. To understand Arctic hydroclimate and its variability prior to the instrumental record, climate proxy records are needed. The purpose of this review is to summarise the current understanding of Arctic hydroclimate during the past 2000 years. First, the paper reviews the main natural archives and proxies used to infer past hydroclimate variations in this remote region and outlines the difficulty of disentangling the moisture from the temperature signal in these records. Second, a comparison of two sets of hydroclimate records covering the Common Era from two data-rich regions, North America and Fennoscan-dia, reveals inter- and intra-regional differences. Third, building on earlier work, this paper shows the potential for providing a high-resolution hydroclimate reconstruction for the Arctic and a comparison with last-millennium simulations from fully coupled climate models. In general, hydroclimate proxies and simulations indicate that the Medieval Climate Anomaly tends to have been wetter than the Little Ice Age (LIA), but there are large regional differences. However, the regional coverage of the proxy data is inadequate, with distinct data gaps in most of Eurasia and parts of North America, making robust assessments for the whole Arctic impossible at present. To fully assess pan-Arctic hydroclimate variability for the last 2 millennia, additional proxy records are required. [ABSTRACT FROM AUTHOR]

Standing dead trees (snags) play important roles in forest ecology by storing carbon as well as providing habitats for many species. Moreover, snags preserved for hundreds of years can provide useful data to extend tree-ring chronologies used for climatological and ecological studies beyond the lifespans of living trees. Here we examined the growth patterns of Scots pine snags from the central Scandinavian Mountains, in relation to still living trees. Using changes point analyses, we showed that a majority (74%) of the snags displayed significant negative growth changes prior (on average 17 years) to death. Change points around the same years were also seen in living trees, but they recovered their growth. The average growth reduction of snags showing negative growth changes before death was 46%. In most cases the final growth change points coincided with very cold summers, or to a lesser degree to period of cool summers. It was suggested that pines ending up as snags were less resilient than the trees which continued living, and thus not able to recover after cold summer events. Since the snag growth reductions prior to death were partly unrelated to climate, care should be taken when using such data in dendroclimatological studies. [ABSTRACT FROM AUTHOR]

ABSTRACT A detailed understanding of past temporal patterns and spatial expression of temperature variations is important to place recent anthropogenic climate change into a longer term context. In order to fill the current gap in our understanding of northwest European temperature variability, point-by-point principal component regression was used to reconstruct a spatial field of 0.5° temperature grids across Scotland. A sequence of reconstructions utilizing several combinations of detrending and disturbance correction procedures, and a selection of tree-ring parameters [including ring width ( RW), maximum latewood density ( MXD) and blue intensity ( BI)] was used in an evaluation of reconstruction skill. The high resolution of the reconstructed field serves also as a diagnostic tool to spatially assess the temperature reconstruction potential of local chronologies. Best reconstruction results, reaching calibration r2 = 65.8% and verification r2 = 63.7% in central Scotland over the 1901-1976 period, were achieved using disturbance-corrected and signal-free detrended RW chronologies merged with BI data after low-pass (high-pass) filtering the RW ( BI) chronologies. Calibration and verification r2 > 50% was attained for central, north and east Scotland, >40% in west and northwest, and >30% in southern Scotland with verification of nearly all grids showing some reconstruction skill. However, the full calibration potential of reconstructions outside central Scotland was reduced either due to residual disturbance trends undetected by the disturbance correction procedure or due to other climatic or non-climatic factors which may have adversely affected the strength of the climate signal. [ABSTRACT FROM AUTHOR]

Along with Arctic amplification, changes in Arctic hydroclimate have become increasingly apparent. Reanalysis data show increasing trends in Arctic temperature and precipitation over the 20th century, but changes are not homogenous across seasons or space. The observed hydroclimate changes are expected to continue, and possibly accelerate, in the coming century, not only affecting pan-Arctic natural ecosystems and human activities, but also lower latitudes through changes in atmospheric and oceanic circulation. However, a lack of spatiotemporal observational data makes reliable quantification of Arctic hydroclimate change difficult, especially in a long-term context. To understand hydroclimate variability and the mechanisms driving observed changes, beyond the instrumental record, climate proxies are needed. Here we bring together the current understanding of Arctic hydroclimate during the past 2000 years, as inferred from natural archives and proxies and palaeoclimate model simulations. Inadequate proxy data coverage is apparent, with distinct data gaps in most of Eurasia and parts of North America, which makes robust assessments for the whole Arctic currently impossible. Hydroclimate proxies and climate models indicate that the Medieval Climate Anomaly (MCA) was anomalously wet, while conditions were in general drier during the Little Ice Age (LIA), relative to the last 2000 years. However, it is clear that there are large regional differences, which are especially evident during the LIA. Due to the spatiotemporal differences in Arctic hydroclimate, we recommend detailed regional studies, e.g. including field reconstructions, to disentangle spatial patterns and potential forcing factors. At present, it is only possible to carry out regional syntheses for a few areas of the Arctic, e.g. Fennoscandia, Greenland and western North America. To fully assess pan-Arctic hydroclimate variability for the last two millennia additional proxy records are required. [ABSTRACT FROM AUTHOR]

Despite the emergence of new high-resolution temperature reconstructions around the world, only a few cover the Medieval Climate Anomaly (MCA). Here we present C-Scan, a new Scots pine tree-ring density-based reconstruction of warm-season (April-September) temperatures for central Scandinavia back to 850 CE, extending the previous reconstruction by 250 years. C-Scan is based on samples collected in a confined mountain region, adjusted for their differences in altitude and local environment, and standardised using the new RSFi algorithm to preserve lowfrequency signals. In C-Scan, the warm peak of MCA occurs ca. 1000-1100 CE, and the Little Ice Age (LIA) between 1550 and 1900 CE. Moreover, during the last millennium the coldest decades are found around 1600 CE, and the warmest 10 and 30 years occur in the most recent century. By comparing C-Scan with other millennium-long temperature reconstructions from Fennoscandia, regional differences in multi-decadal temperature variability, especially during the warm period of the last millennium are revealed. Although these differences could be due to methodological reasons, they may indicate asynchronous warming patterns across Fennoscandia. Further investigation of these regional differences and the reasons and mechanisms behind them are needed. [ABSTRACT FROM AUTHOR]

The recent warming trend, and associated shifts in growing season length, challenge the principle of uniformitarianism, i.e., that current relations are persistent over time, and complicates the uncritical inferences of past climate from tree-ring data. Here we conduct a comparison between tree-ring width chronologies of Pinus sylvestris L. (Scots pine), Picea abies (L.) Karst. (Norway spruce) and Betula pubescens Ehrh. (Downy birch) and phenological observations (budburst and leaf senescence) of Fagus sylvatica L. (European beech), Quercus robur L. (European oak), Betula sp. (Birch), Norway spruce and Scots pine) in Sweden to assess to what extent the tree-ring width–temperature relationship and the timing of phenological phases are affected by increased temperature. Daily meteorological observations confirm a prolongation of the thermal growing season, most consistently observed as an earlier onset of around 1–2 weeks since the beginning of the 20th century. Observations of budburst closely mimic this pattern, with budburst of the deciduous trees occurring 1–2.5 weeks earlier. In contrast to the changes seen in phenology and observational temperature data, the tree-ring width–temperature relationships remain surprisingly stable throughout the 20th century. Norway spruce, Scots pine and Downy birch all show consistently significant correlations with at least one 30 day-long window of temperature starting in late June–early July season. Norway spruce displays the largest degree of stability, with a consistent 60 day-long temperature window with significant correlation starting around Julian calendar day 150. Thus, our results suggest that the principle of uniformitarianism is not violated during the period covered by modern meteorological observations. Further research is needed to determine at what thresholds the temperature sensitivity of these species may alter or deteriorate as a consequence of the ongoing climate change. [ABSTRACT FROM AUTHOR]

The inexpensive Blue Intensity proxy has been considered a complement or surrogate to maximum latewood density (MXD), but is associated with biases from differential staining between sapwood and heartwood and also between deadwood samples and living-wood samples that compromise centennial-scale information. Here, we show that, with some minor adjustments, ΔBlue Intensity (ΔBI) is comparable with MXD or ΔDensity (Δ = the difference or contrast between latewood and earlywood density) in dendroclimatological reconstructions of summer temperatures in the Central Scandinavian region, using Pinus sylvestris L. (Scots pine), on annual and multi-centennial timescales. By using ΔBI, this bias is significantly reduced, but the contrast between earlywood and latewood in BI is altered with degree of staining, while for density it is not. Darker deadwood samples have a reduced contrast compared with the lighter living-wood samples that make ΔBI and ΔDensity chronologies diverge. Here, we quantify this behaviour in BI and offer an adjustment that can reduce this bias. The adjustment can be derived on independent samples, so in future work on BI, parallel density measurements are not necessary. We apply this methodology to two Central Scandinavian Scots pine chronologies that averaged into a composite is able to reconstruct summer temperatures with an explained variance in excess of 60% in each verification period using a split sample calibration verification procedure. Although the amount of data used to derive this contrast adjustment produces desirable results, more tests are needed to confirm its performance, and we suggest that future work on the BI proxy should aim for a small subset of parallel BI and density measurements while the bulk of the data is only measured with the BI technique. This is to ensure that the adjustment is continuously updated with new data and that the conclusions derived here are robust. [ABSTRACT FROM PUBLISHER]

Blue intensity (BI) has the potential to provide information on past summer temperatures of a similar quality to maximum latewood density (MXD), but at a substantially reduced cost. This paper provides a methodological guide to the generation of BI data using a new and affordable BI measurement system; CooRecorder. Focussing on four sites in the Scottish Highlands from a wider network of 42 sites developed for the Scottish Pine Project, BI and MXD data from Scots pine ( Pinus sylvestris L.) were used to facilitate a direct comparison between these parameters. A series of experiments aimed at identifying and addressing the limitations of BI suggest that while some potential limitations exist, these can be minimised by adhering to appropriate BI generation protocols. The comparison of BI data produced using different resin-extraction methods (acetone vs. ethanol) and measurement systems (CooRecorder vs. WinDendro) indicates that comparable results can be achieved. Using samples from the same trees, a comparison of both BI and MXD with instrumental climate data revealed that overall, BI performs as well as, if not better than, MXD in reconstructing past summer temperatures (BI r 2 = 0.38–0.46; MXD r 2 = 0.34–0.35). Although reconstructions developed using BI and MXD data appeared equally robust, BI chronologies were more sensitive to the choice of detrending method due to differences in the relative trends of non-detrended raw BI and MXD data. This observation suggests that the heartwood–sapwood colour difference is not entirely removed using either acetone or ethanol chemical treatment, which may ultimately pose a potential limitation for extracting centennial and longer timescale information when using BI data from tree species that exhibit a distinct heartwood–sapwood colour difference. Additional research is required in order to develop new methods to overcome this potential limitation. However, the ease with which BI data can be produced should help justify and recognise the role of this parameter as a potential alternative to MXD, particularly when MXD generation may be impractical or unfeasible for financial or other reasons. [ABSTRACT FROM AUTHOR]

We undertake the first comprehensive effort to integrate North Atlantic marine climate records for the last millennium, highlighting some key components common within this system at a range of temporal and spatial scales. In such an approach, careful consideration needs to be given to the complexities inherent to the marine system. Composites therefore need to be hydrographically constrained and sensitive to both surface water mass variability and three-dimensional ocean dynamics. This study focuses on the northeast (NE) North Atlantic Ocean, particularly sites influenced by the North Atlantic Current. A composite plus regression approach is used to create an inter-regional NE North Atlantic reconstruction of sea surface temperature (SST) for the last 1000 years. We highlight the loss of spatial information associated with large-scale composite reconstructions of the marine environment. Regional reconstructions of SSTs off the Norwegian and Icelandic margins are presented, along with a larger-scale reconstruction spanning the NE North Atlantic. The latter indicates that the ‘Medieval Climate Anomaly’ warming was most pronounced before ad 1200, with a long-term cooling trend apparent after ad 1250. This trend persisted until the early 20th century, while in recent decades temperatures have been similar to those inferred for the ‘Medieval Climate Anomaly’. The reconstructions are consistent with other independent records of sea-surface and surface air temperatures from the region, indicating that they are adequately capturing the climate dynamics of the last millennium. Consequently, this method could potentially be used to develop large-scale reconstructions of SSTs for other hydrographically constrained regions. [ABSTRACT FROM AUTHOR]

Combining nine tree growth proxies from four sites, from the west coast of Norway to the Kola Peninsula of NW Russia, provides a well replicated (> 100 annual measurements per year) mean index of tree growth over the last 1200 years that represents the growth of much of the northern pine timberline forests of northern Fennoscandia. The simple mean of the nine series, z-scored over their common period, correlates strongly with mean June to August temperature averaged over this region (r = 0.81), allowing reconstructions of summer temperature based on regression and variance scaling. The reconstructions correlate significantly with gridded summer temperatures across the whole of Fennoscandia, extending north across Svalbard and south into Denmark. Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above recent levels during the 10th century. The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly different from the 11th century. The coldest century was the 17th. The impact of volcanic eruptions is clear, and a delayed recovery from pairs or multiple eruptions suggests the presence of some positive feedback mechanism. There is no clear and consistent link between northern Fennoscandian summer temperatures and solar forcing. [ABSTRACT FROM AUTHOR]

Measurements of tree ring width and relative density have contributed significantly to many of the large-scale reconstructions of past climatic change, but to extract the climate signal it is first necessary to remove any nonclimatic age-related trends. This detrending can limit the lower-frequency climate information that may be extracted from the archive (the "segment length curse"). This paper uses a data set of ring widths, maximum latewood density and stable carbon and oxygen isotopes from 28 annually resolved series of known-age Pinus sylvestris L. trees in northwestern Norway to test whether stable isotopes in tree rings require an equivalent statistical detrending. Results indicate that stable oxygen and carbon isotope ratios from tree rings whose cambial age exceeds c.50 years exhibit no significant age trends and thus may be used to reconstruct environmental variability and physiological processes at this site without the potential loss of low-frequency information associated with detrending. [ABSTRACT FROM AUTHOR]

Abstract: Scots pine tree-ring width (TRW) data from Jämtland in the Central Scandinavian Mountains has been used to reconstruct summer temperatures back to 1630 BC. However, it was recently shown that this reconstruction was of limited spatial importance. In this paper, we aim to explain this limitation in the TRW data as a temperature proxy, as well as assess the temperature information from new maximum latewood density (MXD) data. Furthermore, the effect of two standardization methods is evaluated: regional curve standardization (RCS) and a more traditional standardization, termed “non-RCS” standardization. Three TRW and two MXD sites were analyzed. Our results showed that despite the proximity to the Norwegian Sea, the MXD data is a powerful temperature proxy. Difference among sites in TRW data, especially on decadal timescales, together with a lower temperature association, suggests that other factors, such as changes in the local climate regimes, weakens the temperature signal. In general the RCS method overestimates pine growth trends in the latter half of the twentieth century, a feature not seen when using “non-RCS” standardization. This is likely due to an age-bias of older trees in most recent parts of the tree-ring chronologies. This effect will have consequences when reconstructing climate with tree-ring data. To overcome this problem, all age-classes should be represented throughout a chronology. If this is not possible, the use of “non-RCS” standardization is recommended, although this method results in a loss of low-frequency variability. [ABSTRACT FROM AUTHOR]

The temporal variations in distribution pattern of Scots pines (Pinus sylvestris L.) have been used as an annual resolution record of past lake-level changes. Logs, preserved for thousands of years in bog and lakes (subfossil wood), were sampled from small lakes of the Scandinavian Mountains in west-central Sweden to construct a tree-ring chronology from an area where pines are sensitive to growth season (ie, summer) temperature. The chronology spans from AD 2006 to 4868 BC, with two minor gaps at 1600 BC and AD 900 and one larger gap at 2900 BC. The dendrochronological approach can provide a high quality long-term perspective on lake level fluctuations, which possibly can be coupled to changes in humidity. Submerged trunks were found in situ, embedded in sediments, and because trees from the earliest periods were not necessarily found at the deepest levels, this shows clearly that lake levels must have been lower than present at the time of tree-growth. The lake levels must have fluctuated, creating alternating conditions of pine regeneration and mortality. The fluctuating lake levels recorded are suggested to be a result of regional humidity increases and decreases, mainly governed by precipitation changes. Periods of lower lake levels were inferred at 4900-4800 BC, 3800-3600 BC, 3400-3250 BC, 2400-2200 BC, 2100-1800 BC, 1500-1100 BC, 900-800 BC, 400-100 BC, AD 50-300, AD 400-600, AD 900-1100, AD 1350-1500 and AD 1700-1800. Periods of higher lake levels are tentatively encountered at 3600-3400 BC, 3200-2900 BC, 2200-2100 BC, 1700-1500 BC, 1100-900 BC, 800-400 BC, 100 BC-AD 50, AD 300-400, AD 750-900, AD 1100-1250 and AD 1550-1700. [ABSTRACT FROM AUTHOR]

A Scots pine (Pinus sylvestris L.) tree-ring width chronology from Jämtland, in the central Scandinavian Mountains, built from living and sub-fossil wood, covering the period 1632 BC to AD 2002, with a minor gap during AD 887–907, is presented. This is the first multi-millennial tree-ring chronology from the central parts of Fennoscandia. Pine growth in this tree line environment is mainly limited by summer temperatures, and hence the record can be viewed as a temperature proxy. Using the regional curve standardization (RCS) technique, pine-growth variability on short and long time scales was retained and subsequently summer (June–August) temperatures were reconstructed yielding information on temperature variability during the last 3600 years. Several periods with anomalously warm or cold summers were found: 450–550 BC (warm), AD 300–400 (cold), AD 900–1000 (the Medieval Warm Period, warm) and AD 1550–1900 (Little Ice Age, cold). The coldest period was encountered in the fourth century AD and the warmest period 450 to 550 BC. However, the magnitude of these anomalies is uncertain since the replication of trees in the Jämtland record is low during those periods. The twentieth century warming does not stand out as an anomalous feature in the last 3600 years. Two multimillennial tree-ring chronologies from Swedish and Finnish Lapland, which have previously been used as summer temperature proxies, agree well with the Jämtland record, indicating that the latter is a good proxy of local, but also regional, summer temperature variability. [ABSTRACT FROM AUTHOR]

Not only do high-latitude tree-ring data reveal minor and major Holocene climatic variation, but the distribution in time of subfossil trees provides information about former tree-line fluctuation. Over 152 samples of Scots pine (Pinus sylvestris) were collected and measured from lake Lilla Rörtjärnen, situated close to the present treeline. Five floating chronologies were built spanning 498 B.C.-A.D. 19, A.D. 50-390, A.D. 431-884, A.D. 946-1256, and A.D. 1337-1865. The floating chronologies were crossdated with a dendrochronology from Torneträsk. No trees were dated from 20-49 A.D., 391-430 A.D., 885-945 A.D., and 1257-1336, A.D. The temporal distribution of pines in the lake suggests periods of intensive germination, with each phase occurring within 80 to 100 yr from the beginning of each floating chronology. At the end of each phase higher lake levels drowned trees close to the shore. Both the existence and the preservation of the dead pines is likely to have been controlled by changes in lake level. [ABSTRACT FROM AUTHOR]

The Arctic is one of the regions where the effect of global change is most evident. Associated with warming are changes in snow, sea ice and hydroclimate, all which have significant impacts on environments and society. However, due to short observational records, it is difficult to set the current climate in a long-term context. Arctic driftwood (DW), available throughout the Holocene, is a paleoclimate resource that may shed information on past sea-ice, ocean current and atmospheric conditions because it is transported by sea ice across the Arctic. Moreover, DW tree-ring data can be used to interpret climate in the boreal forests where the trees grew. Here we present a study of 380 DW samples collected on eastern and south-western Svalbard. Combining species identification and dendrochronology, it was found that the DW mainly consisted of Pinus sylvestris , Picea sp. and Larix sp. (87% of all samples), mainly originating from northern Russia. In total, 60% of the DW could be dated and their provenance determined, and four tree-ring width chronologies representing Yenisei and Dvina-Pechora were constructed, facilitating extension and improvement of the existing chronologies representing those regions. Moreover, DW from relict beaches that can be subjected to dendrochronological analyses, provides possibilities to extend pan-Arctic tree-ring data even further back in time. Because there are several processes governing the temporal patterns of wood deposition in the Arctic, using DW as an indicator of sea-ice variations needs further investigation. [ABSTRACT FROM AUTHOR]

The relationship between climate and forest is critical to understanding the influence of future climate change on terrestrial ecosystems. Research on trees at high elevations has uncovered the relationship in the Hengduan Mountains region, a critical biodiversity hotspot area in southwestern China. The relationship for the area at low elevations below 2800 m a.s.l. in the region remains unclear. In this study, we developed tree ring width chronologies of Pinus yunnanensis Franch. at five sites with elevations of 1170–1725 m in this area. Monthly precipitation, relative humidity, maximum/mean/minimum air temperature and the standardized precipitation evapotranspiration index (SPEI), a drought indicator with a multi-timescale, were used to investigate the radial growth-climate relationship. Results show that the growth of P. yunnanensis at different sites has a similar response pattern to climate variation. Relative humidity, precipitation, and air temperature in the dry season, especially in its last month (May), are critical to the radial growth of trees. Supplemental precipitation amounts and reduced mean or maximum air temperature can promote tree growth. The high correlations between chronologies and SPEI indicate that the radial growth of trees at the low elevations of the region is significantly limited by the moisture availability. Precipitation in the last month of the previous wet season determines the drought regime in the following dry seasons. In spite of some differences in the magnitudes of correlations in the low-elevation area of the Hengduan Mountains region, chronologies generally matched well with each other at different elevations, and the differences are not evident with the change in elevation. [ABSTRACT FROM AUTHOR]

Maximum latewood density (MXD) chronologies have been widely used to reconstruct summer temperature variations. Precipitation signals inferred from MXD data are, however, rather scarce. In this study, we assess the potential of using MXD data derived from Scots pine (Pinus sylvestris L.) growing in the Stockholm archipelago (Sweden) to reconstruct past precipitation variability. In this area, slow-growing pine trees emerge on flat plateaus of bedrock outcrops with thin or absent soil layers and are, therefore, sensitive to moisture variability. A 268-year-long MXD chronology was produced, and climate–growth relationships show a significant and robust correlation with May–July precipitation (PMJJr = 0.64, p < 0.01). The MXD based May–July precipitation reconstruction covers the period 1750–2018 CE and explains 41% of the variance (r2) of the observed precipitation (1985–2018). The reconstruction suggests that the region has experienced more pluvial phases than drought conditions since the 1750s. The latter half of the 18th century was the wettest and the first half of the 19th century the driest. Climate analysis of "light rings" (LR), latewood layers of extreme low-density cells, finds their occurrence often coincides with significantly dry (<41 mm precipitation) and warmer (1–2 °C above average temperature), May–July conditions. Our analysis suggests that these extremes may be triggered by the summer North Atlantic Oscillation (SNAO). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Today few high-quality tree-ring based temperature reconstructions extending over the past millennium exist, and those have, in general, low replication in their early parts. Here we present a new and updated maximum latewood density (MXD) chronology extending over the last 1200 years, built from local Scots pine wood sources (living trees, drywood preserved the ground, and subfossil wood extracted from lakes) all collected within 20 km in the Scandinavian Mountains in Jämtland. The MXD data was used to reconstruct April-September mean temperatures, where 60% of the variance in observed temperatures could be explained. The reconstruction exhibited distinct multidecadal variability, with the coldest periods centred on ca. 900, 1450, 1600 and 1900 CE, and the warmest periods on ca. 1160, 1250, 1500, 1660 CE. The last part of the 20th – early part of 21st century was the warmest period throughout the whole record, and the reconstruction suggests that, on average, the Medieval Climate Anomaly (MCA, 950–1250 CE) was only slightly warmer than the Little Ice Age (LIA, 1450–1900 CE). In fact, compared to earlier reconstructions from the region, the new reconstruction suggested lower MCA warm-season temperatures. However, despite sufficient replication during that period, high inhomogeneity among the MXD series makes this period slightly uncertain. The unique drywood on which the chronology was built, displayed a distinct regeneration pattern, where changes in Scots pine establishment was interpreted as responses to changes in forest fire activity and climate throughout the past millennium. [ABSTRACT FROM AUTHOR]

Climate field reconstructions from networks of tree-ring proxy data can be used to characterize regional-scale climate changes, reveal spatial anomaly patterns associated with atmospheric circulation changes, radiative forcing, and large-scale modes of ocean-atmosphere variability, and provide spatiotemporal targets for climate model comparison and evaluation. Here we use a multiproxy network of tree-ring chronologies to reconstruct spatially resolved warm season (May–August) mean temperatures across the extratropical Northern Hemisphere (40-90°N) using Point-by-Point Regression (PPR). The resulting annual maps of temperature anomalies (750–1988 CE) reveal a consistent imprint of volcanism, with 96% of reconstructed grid points experiencing colder conditions following eruptions. Solar influences are detected at the bicentennial (de Vries) frequency, although at other time scales the influence of insolation variability is weak. Approximately 90% of reconstructed grid points show warmer temperatures during the Medieval Climate Anomaly when compared to the Little Ice Age, although the magnitude varies spatially across the hemisphere. Estimates of field reconstruction skill through time and over space can guide future temporal extension and spatial expansion of the proxy network. [ABSTRACT FROM AUTHOR]

Dendroarchaeology almost exclusively uses ring-width (RW) data for dating historical structures and artefacts. Such data can be used to date tree-ring sequences when regional climate dominates RW variability. However, the signal in RW data can be obscured due to site specific ecological influences (natural and anthropogenic) that impact crossdating success. In this paper, using data from Scotland, we introduce a novel tree-ring parameter (Blue Intensity – BI) and explore its utility for facilitating dendro-historical dating of conifer samples. BI is similar to latewood density as they both reflect the combined hemicellulose, cellulose and lignin content in the latewood cell walls of conifer species and the amount of these compounds is strongly controlled, at least for trees growing in temperature limited locations, by late summer temperatures. BI not only expresses a strong climate signal, but is also less impacted by site specific ecological influences. It can be concurrently produced with RW data from images of finely sanded conifer samples but at a significantly reduced cost compared to traditional latewood density. Our study shows that the probability of successfully crossdating historical samples is greatly increased using BI compared to RW. Furthermore, due to the large spatial extent of the summer temperature signal expressed by such data, a sparse multi-species conifer network of long BI chronologies across Europe could be used to date and loosely provenance imported material. [ABSTRACT FROM AUTHOR]