A Pinus cembra L. tree-ring record for late spring to late summer temperature in the Rhaetian Alps,Italy

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 19^th century, interrupted by short phases of climatic deterioration in the beginning of the 20^th 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.     

Dendroclimatic potential of dendroanatomy in temperature-sensitive Pinus sylvestris

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.     

Temperature sensitivity of blue intensity,maximum latewood density,and ring width data of living black spruce trees in the eastern Canadian taiga

The blue intensity (BI) technique provides opportunities to obtain surrogates to tree-ring density for reconstructing summer temperatures in high-latitude regions. In this study, we compare latewood BI (LBI) and delta BI (DBI), with the conventional X-ray maximum latewood density (MXD) and tree-ring width (TRW) data using 178 living trees of black spruce (Picea mariana (Mill.) B.S.P.), one of the most dominant species of conifers in the Northern Hemisphere, from 17 sites across the eastern Canadian taiga. The regional LBI and DBI chronologies are highly correlated to that of MXD (Pearson’s r = 0.97 and 0.92, respectively), while DBI is also similar to TRW (Pearson’s r = 0.67). Both LBI and DBI exhibit stronger responses to the May–August temperatures than TRW over larger time and spatial scales. However, only DBI is comparable to MXD data from inter-annual to decadal timescales. Low-frequency components of LBI data are likely distorted by color biases even if no obvious discoloration is present, as well as by the potentially low measurement resolution, leading to an overall weaker temperature sensitivity compared to the MXD data. Resampling experiments suggest that a minimum replication of 10 trees is needed to retain ≥90 % of the optimal temperature signal for MXD, LBI, and DBI data, and a minimum of 20 trees is required for TRW data.     

Assessing non-linearity in European temperature-sensitive tree-ring data

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.     

Stationarity of climate-growth response is only marginally influenced by the soil moisture regime in Western Siberia

Stationary (time-stable) relationships between a tree-ring proxy and climatic drivers are a prerequisite for using tree rings as paleo-climatological archives, but non-stationarity has been detected worldwide. Here we use a classical, temperature-sensitive treeline site in Western Siberia to specifically test the influence of micro-site conditions (wet versus dry) on the stationarity of climate-growth relationships in three co-existing conifer species: Larix sibirica Ledeb., Picea obovata Ledeb., and Pinus sibirica DuTour. We test two commonly used tree-ring proxies, annual tree-ring width (TRW) and maximum latewood density (MXD), using moving windows and the bootstrapped transfer function stability test (BTFS). Summer temperature is the main driver of tree growth in all three species, but non-stationarities exist in all species and both tree-ring proxies. For TRW, we found stationarity only for larch from both micro-sites, while for MXD, we found stationarity for spruce from both micro-sites, and for pine from the wet micro-site. Micro-site variability did not seem to affect stationarity in any systematic way. We highlight the necessity to systematically test the influence of different methods of stationarity tests, since BTFS was more sensitive than moving-window analysis. Taken together, our results underscore the importance of testing the assumption of stationarity for diverse micro-sites, different species and proxies at all sites prior to any tree-ring based temperature reconstruction, since even within one site results can be drastically different.     

Climate sensitivity of high- and low-elevation Larix decidua MXD chronologies from the Tatra Mountains

Maximum latewood density (MXD) measurements from high-elevation/-latitude sites are an important proxy for summer temperature reconstructions. Here, we present 201 MXD series from living larch (Larix decidua Mill.) trees that were growing at around 850 and 1450 m a.s.l. in the Slovakian Tatra Mountains, together with 56 MXD series from historical timbers of the same species and region. We explore the climate signal at the high- and low-elevation sites and assess the effects of varying temperature and precipitation regimes on MXD formation. Ranging from spring temperature to summer precipitation, the elevation-specific climate sensitivity suggests that the MXD measurements from living and relict sources should not be merged for paleoclimatic studies. This finding emphasizes the challenge of attributing a predominant climate factor that controls wood formation across a wide range of historical constructions. A better understanding of the ‘true’ climate signal requires more samples during the period of overlap between the living and historical trees.     

Seasonal and synoptic climatic drivers of tree growth in the Bighorn Mountains,WY, USA (1654–1983 CE)

In the United States’ (US) Northern Rockies, synoptic pressure systems and atmospheric circulation drive interannual variation in seasonal temperature and precipitation. The radial growth of high-elevation trees in this semi-arid region captures this temperature and precipitation variability and provides long time series to contextualize instrumental-era variability in synoptic-scale climate patterns. Such variability in climate patterns can trigger extreme climate events, such as droughts, floods, and forest fires, which have a damaging impact on human and natural systems. We developed 11 tree-ring width (TRW) chronologies from multiple species and sites to investigate the seasonal climatic drivers of tree growth in the Bighorn Mountains, WY. A principal component analysis of the chronologies identified 54% of shared common variance (1894–2014). Tree growth (expressed by PC1) was driven by multiple seasonal climate variables: previous October and current July temperatures, as well as previous December and current April precipitation, had a positive influence on growth, whereas growth was limited by July precipitation. These seasonal growth-climate relationships corresponded to circulation patterns at higher atmospheric levels over the Bighorn Mountains. Tree growth was enhanced when the winter jet stream was in a northward position, which led to warmer winters, and when the spring jet stream was further south, which led to wetter springs. The second principal component, explaining 19% of the variance, clustered sites by elevation and was strongly related to summer temperature. We leverage this summer temperature signal in our TRW chronologies by combining it with an existing maximum latewood density (MXD) chronology in a nested approach. This allowed us to reconstruct Bighorn Mountains summer (June, July, and August) temperature (BMST) back to 1654, thus extending the instrumental temperature record by 250 years. Our BMST reconstruction explains 39–53% of the variance in regional summer temperature variability. The 1830s were the relatively coolest decade and the 1930s were the warmest decade over the reconstructed period (1654–1983 CE) – which excludes the most recent 3 decades. Our results contextualize recent drivers and trends of climate variability in the US Northern Rockies, which contributes to the information that managers of human and natural systems need in order to prepare for potential future variability.     

Cell wall dimensions reign supreme: cell wall composition is irrelevant for the temperature signal of latewood density/blue intensity in Scots pine

Many microdensitometric techniques are available for deriving maximum latewood density (MXD), which is the state-of-the-art proxy parameter for local to hemispheric-scale temperature reconstructions of the last millennium. Techniques based on X-ray radiation and visible light reflection, such as “blue intensity” (BI), integrate both the density/composition and the dimensions of the cell walls to derive microdensitometric data. In contrast, the dendroanatomical technique relies only on the dimensions of the cell walls. It is therefore possible to isolate cell wall variables by subtracting data derived using the dendroanatomical technique from data derived using X-ray and BI-based techniques.In this study, we explore differences in well-replicated data from parallel X-ray, BI, and dendroanatomical measurements of temperature-sensitive Pinus sylvestris trees from northern Finland. We aim to determine whether cell wall density is critical to the success of X-ray-based MXD, and whether the BI-based parameter counterpart, here termed MXBI, contains useful information about the composition of the cell wall (specifically the lignin).Our results indicate that cell wall density and cell wall BI have no relevant influence on MXD and MXBI measurements. Even in years with severely reduced lignification, identified as so-called “blue rings”, dendroanatomical MXD (aMXD) measurements do not deviate significantly from their MXD or MXBI counterparts. Moreover, derived chronologies of cell wall density and cell wall BI contain no significant climate signals when correlated with local climate. Maximum latewood density of conifers can thus be obtained without bias using the dendroanatomical technique. Because lignin content appears to play a negligible role for cell wall BI, the cell wall BI likely presents the biggest challenge when producing unbiased MXBI data. This is because BI data is notorious for cell wall color distortion across the heartwood and sapwood, and between living wood and dead wood, and may therefore distort the otherwise strong link with wood density on multidecadal scales.     

Wood density as a proxy of drought-induced forest dieback in silver fir

Drought-induced dieback is a matter of global concern. It has been widely reported and could compromise the climate warming mitigation potential of forests. That is why we need reliable early-warning proxies to achieve better forecasts of forest dieback and tree death. Tree-ring data can provide some of these needed proxies. Here I propose considering minimum (MND) and maximum (MXD) wood density values as proxies of tree vulnerability against drought. This hypothesis is evaluated in silver fir (Abies alba) forests from the western Spanish Pyrenees showing ongoing dieback processes after the severe 1985 drought. MXD increased in response to warm summer conditions in 1985 whereas MND increased in response to growing-season water deficit in 1986. The average between prior MXD (MXD_t-1) and subsequent MND (MND_t) could be used to detect severe drought impacts and forecast dieback.     

Separating the climatic signal from tree-ring width and maximum latewood density records

We propose a technique for separating the climatic signal which is contained in two tree-ring parameters widely used in dendroclimatology. The method is based on the removal of the relationship between tree-ring width (TRW) and maximum latewood density (MXD) observed for narrow tree rings from high latitudes. The new technique is tested on data from three larch stands located along the northern timberline in Eurasia. Correlations were calculated between the temperatures of pentads (five consecutive days), TRW chronologies and MXD chronologies calculated according to the standard and proposed methods. The analysis confirms the great importance of summer temperature for tree radial growth and tree-ring formation. TRW is positively correlated with the temperature of four to eight pentads (depending on the region) at the beginning of the growth season, but MXD as obtained by the standard technique is correlated with temperature over a much longer period. For maximum density series from which the relationship between MXD and TRW has been removed (MXD′), there is a clear correlation with temperatures in the second part of the growing season. These results are consistent with the known dynamics of tree-ring growth in high latitudes and mechanisms of tree-ring formation.     

Different maximum latewood density and blue intensity measurements techniques reveal similar results

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 < 0.01) positively correlated between all MXD and BI time-series. Our results emphasize the potential of faster and cheaper, as well as overall more user-friendly techniques to generate reliable MXD surrogates for high-frequency dendroclimatological studies. Although the correlations between MXD and BI were lower than within MXD and BI, the results of growth-climate response performed for both proxies show only marginal differences. The obtained level-offset further questions the suitability of joining different density surrogates for developing long-term composite chronologies to reconstruct low-frequency climate variability.     

Pre-1930 unstable relationship between climate and tree-ring width of Pinus taiwanensis hayata in southeastern China

Although it has been widely recognized that tree-ring response to climate drivers may change over recent decades, often due to anthropogenic environment changes, there are fewer reports of such changes in earlier pre-warming periods. In this paper we report on the pre-1930 unstable relationship between climate and tree-ring width (TRW) of Pinus taiwanensis Hayata in southeastern China based on reliable long-term temperature data. TRW of P. taiwanensis is strongly controlled by temperatures in two seasons, previous spring to summer (March to August, mainly June to July) and previous winter to current spring (December to March). However, TRW are insensitive to previous spring to summer temperature between 1890 and 1930. Reduced summer temperature variability, changing regimes of spring-summer temperature and precipitation, and complicated tree physiological processes behind the complex growth-climate relationship are the more likely causes of this phenomenon. This study adds to the body of knowledge that lower climate sensitivity of tree rings is not specific to the most recent decades.     

Vessel features of Quercus ilex L. growing under Mediterranean climate have a better climatic signal than tree-ring width

We investigated whether vessel time series of Holm oak (Quercus ilex L.), a diffuse to semi-ring-porous species, can record a climatic signal which differs from the signal encoded in tree-ring width (TRW). The study was conducted in ten Q. ilex trees from a coppice stand in northeast Spain. Chronologies of TRW, mean vessel area (MVA) and maximum vessel area (MAX) were developed and correlated with climate data, for the period 1985–2004 (20 years). Our results indicate that vessel features contain environmental information that is different from that stored in TRW. MAX chronologies correlate better to early spring precipitation (April–May) than TRW chronologies, and so does MVA of the largest 20–25 vessels from the first third of the ring with late spring precipitation (May–June). Also, the combination of MVA and TRW is a better predictor of summer precipitation. This explorative study clearly shows that vessel features can complement the climatic signal of TRW increasing the resolution of the climate reconstructions for the Mediterranean region.     

Multi-proxy crossdating extends the longest high-elevation tree-ring chronology from the Mediterranean

Annually precise dating is the cornerstone of dendrochronology. The accurate crossdating of relict wood is, however, frequently challenged during early chronology periods when sample replication is typically low. Here we present a multi-proxy approach in which stable carbon (δ¹³C) and radiocarbon (¹⁴C) isotope data are used to evaluate and correct dating errors in the early period of the longest high-elevation tree-ring chronology from the Mediterranean Basin. The record was initially developed using 878 tree-ring width (TRW) and 192 maximum latewood density (MXD) series from living and relict Bosnian pines (Pinus heldreichii) from Mt. Smolikas in Greece to reconstruct hydroclimate and temperature variability back to the 8th century. New annually resolved and non-pooled δ¹³C series now suggest a re-dating of first millennium relict pine samples during a period when sample replication was too low for proper TRW and/or MXD crossdating. The associated correction shifts the start of the Mt. Smolikas chronology from 575 back to 468 CE, a change independently confirmed by wiggle-matching annual ¹⁴C data along the 774/775 CE cosmic event. Our study demonstrates the importance of independent age validation for robust chronology development and shows how multi-proxy crossdating can improve dating success during periods of low sample replication.     

The temperature sensitivity along elevational gradients is more stable in maximum latewood density than tree-ring width

Tree ring-based temperature reconstructions are preferably derived from maximum latewood density (MXD) compared to tree-ring width (TRW). Although temperature signals in MXD are less dependent on site ecology, systematic analyses of the effects of elevation and slope aspect on ring formation are still lacking. Here, we assess the climate sensitivity of MXD and TRW chronologies from six larch (Larix decidua Mill.) sites across the Simplon valley in the southwestern Swiss Alps, representing elevations from 1400 to 2150 m asl on both north- and south-facing slopes. We find decreasing temperature signals with decreasing elevation in MXD and TRW, though correlation coefficients are generally higher for MXD and on the warmer and dryer south exposed slopes. While the greatest temperature signals are found for MJJA at highest elevations with r = 0.71 for MXD and r = 0.57 for TRW (both p < 0.05 and for the 1928–2009 common period), MXD still correlates significantly positive at the lowest elevation site that is ~750 m below the treeline. Our findings indicate the suitability of MXD over TRW for temperature reconstructions when using historical wood sources of unknown origin.     

Tree ring width and wood density as the indicators of climatic factors and insect outbreaks affecting spruce growth

Tree ring width (TRW), maximum (MXD), mean (MED) and minimum (MID) wood density were investigated in samples from the vicinity of the Tuchola Forest Biosphere Reserve (Northern Poland) in an attempt to distinguish the relative importance of climate and insect attack on the growth of Norway spruce. Selected climate parameters were used for a multiple regression to predict tree-ring width during insect outbreaks. This also used AICc for model selection. Additionally, k-means clustering was then used to group the yearly data of TRW, MXD, MID and the data of insect outbreaks. The respective climate data and data on insect outbreaks during the years 1962–1996 revealed a strong influence of May precipitation on TRW and insect outbreaks on MID. Missing tree rings or narrow rings and lower MXD together with higher MID might indicate increased insect activity.     

Different climate response of three tree ring proxies of Pinus sylvestris from the Eastern Carpathians,Romania

The aim of this study was to compare the climatic responses of three tree rings proxies: tree ring width (TRW), maximum latewood density (MXD), and blue intensity (BI). For this study, 20 cores of Pinus sylvestris covering the period 1886–2015 were extracted from living non-damaged trees from the Eastern Carpathian Mountains (Romania). Each chronology was compared to monthly and daily climate data. All tree ring proxies had a stronger correlation with the daily climate data compared to monthly data. The highest correlation coefficient was obtained between the MXD chronology and daily maximum temperature over the period beginning with the end of July and ending in the middle of September (r = 0.64). The optimal intervals for the temperature signature were 01 Aug – 24 Sept for the MXD chronology, 05 Aug – 25 Aug for the BI chronology, and both 16 Nov of the previous year – 16 March of the current year and 15 Apr – 05 May for the TRW chronology. The results from our study indicate that MXD can be used as a proxy indicator for summer maximum temperature, while TRW can be used as a proxy indicator for just March maximum temperature. The weak and unstable relationship between BI and maximum temperature indicates that BI is not a good proxy indicator for climate reconstructions over the analysed region.     

用西伯利亚落叶松年轮最大密度重建和布克赛尔5—8月份平均最高温度

利用采自和布克塞尔铁布克山二号沟的西伯利亚落叶松树轮样本,研制出树轮最大密度年表(MXD)和年轮宽度年表(TRW),分析了其年表特征和气候响应特点。结果表明,该样点的落叶松树轮最大密度年表与和布克塞尔气象站5-8月份平均温度和平均最高温度度均具有很好的正相关关系,最高单相关系数为0.660。用铁布克山二号沟的最大密度差值年表序列,可较好地重建和布克塞尔地区1795-2007年来春夏季平均最高温度距平,47 a重建值对实测值的解释方差达43.5%,且方程稳定。重建结果揭示,在和布克赛尔地区,20世纪平均最高温度距平要高于20世纪以前的平均最高温度距平,20世纪前中期平均最高温度距平出现了明显的上升,并且在重建的时段的末期,5-8月份平均最高温度距平表现出上升趋势。     

Blue intensity for dendroclimatology: Should we have the blues? Experiments from Scotland

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 r² = 0.38–0.46; MXD r² = 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.