Regional Curve Standardization Research Articles

A Python package implementing Direct Reconstruction Technique (DIRECT) for dendroclimatological studies

The DIrect REConstruction Technique (DIRECT) is a dendroclimatological method that constructs a climatic response surface to account for changes of climatic response with tree age. This surface is then used as a transfer function for climatic reconstructions. Unlike widely-used standardization methods such as the traditional curve-fitting approach, the regional curve standardization, and their signal-free modifications that perform detrending explicitly, DIRECT accounts for age-size related trend in tree-ring measurements by construction of the response surface dependent on two variables: tree-ring parameter (width, blue intensity etc.) and cambial age. The method is capable of taking into account nonlinear climatic response of trees and differing response of younger and older trees. Here we describe an application of the newly developed open-source Python package that implements DIRECT (https://github.com/Gr1Lo/direct) to one real and one theoretical dataset. The package consists of functions for reading the initial data, constructing a response surface, and for reconstructing climate variables via this surface. The functions for visual assessment of the initial data and for the estimation and selection of parameters for constructing the response surface are also presented. Also here we provide a comparison of the DIRECT method with traditional standardization-reconstruction routines.

Application of RCS and signal-free RCS to tree-ring width and maximum latewood density data

Dendroclimatic research faces the challenge of selecting appropriate detrending methods for retaining low-frequency signals in temperature reconstructions. Among the numerous methods available to dendrochronologists, regional curve standardisation (RCS) and the signal-free approach in combination with RCS (SF-RCS) are increasingly used to preserve the full spectrum of temperature variance in tree-ring data. Here, we apply RCS and SF-RCS to tree-ring width (TRW) and maximum latewood density (MXD) datasets composed of only living and combined living and relict trees from northern Scandinavia. Whereas RCS and SF-RCS produce highly similar chronologies when applied to composite (living-plus-relict) datasets, particularly for MXD, both methods fail to establish chronologies coherent with regional temperature trends when applied to living-tree datasets. Additional tests including pruning of well-replicated living-tree datasets, to approximate the heterogenous age-structure of composite datasets, reveal improved results and coherent trends in MXD. While this demonstrates the applicability of joint detrending and pruning techniques to retain meaningful low-frequency variance in living-tree MXD chronologies, similar improvements were not achieved with TRW, likely because of the much stronger age-trend inherent to this widely used proxy. Further tests with other tree species and in alpine environments are needed to verify these findings. However, such assessments require an adjustment of tree-ring sampling protocols to increase replication to 50+ trees per site including old and young individuals to facilitate data pruning.

Global warming leads to growth increase in Pinus sylvestris in the Kazakh steppe

Forest community structure and function in climate-sensitive regions exhibit uncertain changes under the influence of global warming. Especially in northern Central Asia, the response of pine growth to climate change remains unclear. Here, we identified changes in growth trends and climate response in a total of 646 trees for the abundant species Scots pine (Pinus sylvestris) at 30 sites in the Kazakh steppe. We used basal area increment, regional curve standardization and negative exponential curve detrended chronologies to comprehensively analyze trends in pine growth, and linear mixed effects and random forest models were calculated to explore growth-climate relationships. An increase in the overall growth of Scots pine under the influence of global warming, expressed in this region by increasing temperatures and precipitation, was observed during the period 1950–2018. Spatial differences in climate response between the sub-regions were identified showing that pine growth is more influenced by summer precipitation in the eastern compared to the western and central Kazakh steppe. Warm spring temperatures and more precipitation are important common climatic factors affecting pine growth in northern Central Asia. Moreover, increased drought intensity and longer drought duration negatively impact pine growth in this region. The Pacific Decadal Oscillation and North Atlantic Oscillation were identified to be the main atmospheric circulation patterns that influence the spatial differentiation in pine growth in this steppe. Albeit pine growth in northern Central Asia shows an increasing trend currently but also in future climate scenarios, the risks and challenges posed by drought should not be underestimated.

Prospects for dendroanatomy in paleoclimatology – a case study on <i>Picea engelmannii</i> from the Canadian Rockies

Abstract. The continuous development of new proxies as well as a refinement of existing tools are key to advances in paleoclimate research and improvements in the accuracy of existing climate reconstructions. Herein, we build on recent methodological progress in dendroanatomy, the analyses of wood anatomical parameters in dated tree rings, and introduce the longest (1585–2014 CE) dendroanatomical dataset currently developed for North America. We explore the potential of dendroanatomy of high-elevation Engelmann spruce (Picea engelmannii) as a proxy of past temperatures by measuring anatomical cell dimensions of 15 living trees from the Columbia Icefield area. X-ray maximum latewood density (MXD) and its blue intensity counterpart (MXBI) have previously been measured, allowing comparison between the different parameters. Our findings highlight anatomical MXD and maximum radial cell wall thickness as the two most promising wood anatomical proxy parameters for past temperatures, each explaining 46 % and 49 %, respectively, of detrended instrumental July–August maximum temperatures over the 1901–1994 period. While both parameters display comparable climatic imprinting at higher frequencies to X-ray derived MXD, the anatomical dataset distinguishes itself from its predecessors by providing the most temporally stable warm season temperature signal. Further studies, including samples from more diverse age cohorts and the adaptation of the regional curve standardization method, are needed to disentangle the ontogenetic and climatic components of long-term signals stored in the wood anatomical traits and to more comprehensively evaluate the potential contribution of this new dataset to paleoclimate research.

Recent unexpected decline of forest growth in North Finland: examining tree-ring, climatic and reproduction data

After a decades-long increasing trend, the recent results of the National Forest Inventory (NFI) reported a decline of forest growth in North Finland. The aim of this study was to assess climatic and reproduction influences behind the growth decline. We used tree-ring data that had been collected by NFI using systematic sampling. The tree-ring width series were detrended using the regional curve standardisation (RCS) removing age-related trends. The resulting tree-ring indices of Scots pine (Pinus sylvestris L.) showed decadal variations with low increment in the 1990s, and high increment in the 1980s and the early years of the current century. Thereafter, a prolonged growth reduction for pine started both on the mineral soil sites and peatlands. The tree-ring indices of Norway spruce (Picea abies (L.) Karst.) had less pronounced decadal variations and no trend-like reduction over the last 15 years. High spring and summer temperatures were found to enhance radial growth, but high winter temperatures were related to low growth for pine and spruce in the following summer. Temperature variation, accompanied by variables indicating years of drought and intensive flowering, accounted for 34% annual growth variance of pine and 21–44% for spruce. Thus, the results imply that climatic factors may have to some extent contributed to the recent growth reduction of pine. Due to its ecological and economic consequences growth decline needs to be further monitored and investigated. Moreover, analyses of stand and age structure, potentially affecting the growth decline, were beyond the scope of this paper, but also warrant further investigation.

Local and global environmental drivers of growth chronologies in a demersal fish in the south-eastern Pacific Ocean

Upwelling and the El Niño “Southern Oscillation” (ENSO) are recurrent climatic phenomena in the southeastern Pacific Ocean that severely affect the reproduction and growth of pelagic fish populations. However, there are not long-term growth data from demersal fish populations to test these interconections in a long-term analysis. For this reason, a first extensive growth chronology was reconstructed from the annual growth of sagittal otoliths as a proxy for somatic growth for the cardinalfish (Epigonus crassicaudus). Adult fish ranging from 35 to 40 cm in fork length and from 39 to 63 years in age were collected off Chilean waters. The master chronologies were estimated for the period from 1974 to 2014, using the regional curve standardization approach (RCS) and linear mixed models (LMMs). Growth indexes derived from both approaches followed a similar trend and were positively correlated with the Humboldt Current Index (HCI) and negatively with ENSO, Pacific Decadal Oscillation and sea surface temperature. LMMs showed that a 75% of growth variability was explained by the age of increment formation and HCI was the environmental index that most significantly affected the annual growth of cardinalfish followed by the sea surface temperature in spring. A reduced growth phase from 1974 to 1996 contrasted with a higher growth period from 1997, matching the 1997/1998 climatic regime shift, demonstrating that the enhanced growth for cardinalfish was associated with upwelling of nutrient rich water to the surface, triggering an increase of the primary and secondary productivity during the prevalence of a cold regime period in the Humboldt Current System. The consistence between RCS and LMM methods was indicative that both approaches are promising to evaluate the influence of environmental drivers on the growth condition of a demersal fish population in a highly productive marine ecosystem.

Xylem features detrending methods matter: A case study on earlywood vessels of Fraxinus mandshurica

Xylem anatomy can provide valuable information about tree allometry and ecophysiological performance. It has unique advantages in the study of global climate change and forest adaptation compared to traditional ring-width or density proxy parameters. Developing appropriate detrended sequences of xylem features is necessary to carry out a series of further studies. However, the differences between detrending methods and their effects for xylem features are still unclear, presenting a challenge to the choice of proper detrending methods. In this study, we used the earlywood vessel features of Fraxinus mandshurica as an example and compared the differences in four common detrending methods (Spline, CSS: cubic smoothing spline, NEP: negative exponential curve, and RCS: regional curve standardization) to highlight the importance of the detrending methods in dendroanatomical studies. The results show that detrending methods significantly affect the long-term growth trends and climate signals recorded in xylem anatomical features. Among them, NEP has good performance in terms of ring width and vessel number, which is not suitable for other vessel features because of its deterministic nature. Spline and CSS are relatively conservative methods, in which are not easy to make mistakes, but the problem of overfitting (lack of climate signals) deserves attention. Multiple RCS may be a promising detrending method with quantitative wood anatomy technology development if sufficient samples are available. Our results highlight the importance of selecting detrending methods for xylem feature identification and emphasize the importance of testing various detrending methods rather than arbitrarily choosing a standard method. The detrending method should be selected based on the purpose, such as the balance between climate signals and thorough detrended effects. We recommend applying multiple methods to test the growth trends in xylem features before selecting a detrending method. More interdisciplinary studies involving ecophysiology, wood anatomy, and dendrochronology will help to distil the valuable information in xylem features accurately.

Ensemble standardization constraints on the influence of the tree growth trends in dendroclimatology

Tree growth trends can affect the interpretation of the response of tree-ring proxies (especially tree-ring width) to climate in the low-frequency band, which in turn may limit quantitative understanding of centennial-scale climate variability. As such, it is difficult to determine if long-term trends in tree-ring measurements are caused by age-dependent growth effects or climate. Here, a trend similarity ranking method is proposed to define the range of tree growth effects on tree-ring width chronologies. This method quantifies the inner and outer boundaries of the tree growth effect following two extreme standardization methods: curve fitting standardization and regional curve standardization. The trend similarity ranking method classifies and detrends tree-ring measurements according to the ranking similarity between the regional growth curve and their long-term trends through curve fitting. This standardization process mainly affects the secular trend in tree-ring chronologies, and has no effect on their inter-annual to multi-decadal variations. Applications of this technique to the Yamal and Torneträsk tree-ring width datasets and the maximum latewood density dataset from northern Scandinavia reveals that multi-centennial and millennial-scale temperature variations in the three regions provide substantial positive contributions to the linear warming trends in the instrumental period, and that the summer warming rate during the 20th century is not unprecedented over the past two millennia in any of the three regions.

Comment on “Absence of Age‐Related Trends in Stable Oxygen Isotope Ratios From Oak Tree Rings” by Duffy et al. (2019)

AbstractDuffy et al. (2019, https://doi.org/10.1029/2019GB006195) assess age‐related trends in tree‐ring oxygen isotope (δ18O) series from the UK using two approaches. They argue that their conventional approach, which also forms the basis for the frequently used Regional Curve Standardization (RCS) method, is incapable of identifying age‐related trends and that detrending the data would distort climatic indications. However, their study disregards much of the existing dendrochronological literature essential also for their trend detection approaches. Here, we reanalyze the data from their supplementary item and show that detrending δ18O series using the RCS method does not remove long‐term trends—indicative of summertime drying in England and Wales—and that the data set used in Duffy et al. (2019) is likely too small to draw wider implications.

Reply to Comment by S. Helama and V. V. Matskovsky on“Absence of Age‐Related Trends in Stable Oxygen Isotope Ratios From Oak Tree Rings”

AbstractWe thank Helama and Matskovsky (2019, https://doi.org/10.1177/0959683616652709) for their interest in our work. They argue that we should use Regional Curve Standardization (RCS) to identify trends in our oxygen isotope series and to detrend them prior to climate reconstruction. We disagree.

A Direct Comparison of Sympatric High-Latitude Pinus contorta and Picea albertiana Ring-Width Chronologies

An emerging trend in tree-ring research is use of multiple species for reconstructing paleoclimates, but the possible simultaneous use of boreal-cordilleran species is untested. In this study, ring-width chronologies of sympatric Pinus contorta (lodgepole pine, n = 116 series) and Picea albertiana (western white spruce, n = 348 series) were constructed to assess their temporal (dis)similarities and correlative relationships with meteorological variables. Chronology construction was based on multiple Regional Curve Standardization. Most (97.5%) Pinus chronology values occurred within ±1 SD of their Picea counterparts, but Picea values tended to be slightly less (sign-test, p < 0.001, n = 201). Pinus ring widths were more frequently (16 versus 9) and more strongly correlated (U-test, p < 0.009) with 1942–2013 meteorological variables than Picea. Both species were correlated with moisture variables, but Picea was not correlated with those of temperature. Pinus and Picea ring-width variation was best explained by summer (r = –0.434) and annual (r = –0.426) heat-moisture index values (p < 0.001, n = 72) among tested variables, respectively. Although seldom used, P. contorta appears as suitable as conventionally-used P. albertiana based on chronology similarity, but a greater diversity of significant correlation outcomes suggest Pinus is potentially more useful for boreal-cordilleran paleoclimate reconstruction.

Trees do not always act their age: size-deterministic tree ring standardization for long-term trend estimation in shade-tolerant trees

Abstract. With increasing awareness of the consequences of climate change for global ecosystems, the focus and application of tree ring research have shifted to reconstruction of long-term climate-related trends in tree growth. Contemporary methods for estimating and removing biological growth trends from tree ring series (standardization) are ill-adapted to shade-tolerant species, leading to biases in the resultant chronologies. Further, many methods, including regional curve standardization (RCS), encounter significant limitations for species in which accurate age estimation is difficult. In this study we present and test two tree ring standardization models that integrate tree size in the year of ring formation into the estimation of the biological growth trend. The first method, dubbed size-deterministic standardization (SDS), uses tree diameter as the sole predictor of the growth trend. The second method includes the combined (COMB) effects of age and diameter. We show that both the SDS and COMB methods reproduce long-term trends in simulated tree ring data better than conventional methods; this result is consistent across multiple species. Further, when applied to real tree ring data, the SDS and COMB models reproduce long-term, time-related trends as reliably as traditional RCS and more reliably than other common standardization methods (i.e. C-method, basal area increments, conservative detrending). We recommend the inclusion of tree size in the year of ring formation in future tree ring standardization models, particularly when dealing with shade-tolerant species, as it does not compromise model accuracy and allows for the inclusion of unaged trees.

A 400-year reconstruction of spring–summer precipitation and summer low flow from regional tree-ring chronologies in North-Eastern Austria

Atmospheric and streamflow drought back to 1600 were reconstructed with provenance-verified regional Pinus-sylvestris and Pinus-nigra chronologies of earlywood, latewood and ring width, detrended by signal-free regional curve standardization, from the Weinviertel, a dry region in Eastern Austria. Linear regression models with Weinviertel latewood indices as predictor variables were used to estimate regional April–August precipitation totals (R2 = 0.47) and seven-day minimum flow from May to October (best fit: R2 = 0.65). As a novelty, the well-known dendrochronology quality measure “Expressed Population Signal” in a modified version is used to define a scaling factor for empirically estimating the reconstructions’ time-varying reliability.For the detection of drought years in the reconstructed time series, four stages of drought of increasing intensity were defined as conditions below median, and below the 33rd, 20th, and 5th percentile—representing 2-year, 3-year, 5-year and 20-year recurrence intervals.The method for the extension of time series of low flows into the past is applicable in the Weinviertel and can be a useful source of historical hydrological and meteorological information restricted by the uncertainties coming along with proxy records. Of the analyzed period back to 1600, the 19th century is the driest one closely followed by the 18th century, sharing the overall driest period from 1775 to 1813. The phase with the longest continuous atmospheric dry period is 1807–1811. The longest hydrological drought lasted from 1832 to 1839.

Picea (spruce) growth rate (mm year−1) changes in southwest Yukon (Canada) since the mid 19th century

Analyses of maximum early tree ring growth within a regional curve standardization (RCS) framework were used to construct ring width chronologies in mm year−1 to estimate historical growth rates in southwest Yukon (Canada), without the necessity of correcting for age related changes. The analysis relied on the use of the 11th–50th year growth rings to standardize chronology comparisons. This age range represents the period of peak radial growth for Picea albertiana and P. glauca (white spruces), which are common trees on upland boreal sites in the study area. Of 1,460 available Picea ring series, 1,029 fit the criteria for inclusion in the study and they represented five commonly occurring RCS curve types. When converted to chronologies, all curve types had year-to-year fluctuations in average ring widths (RW11–50), but nine of the 12 RCS types (four types in three study areas) also had long-term growth rate increases of 15–132% between ad 1850 and 2000, based on regression modeling (R/r = 0.54–0.76, p 0.94, n > 100) with conventional tree ring width index values derived from multiple RCS curves. This indicates that the two chronology construction approaches have very similar temporal patterns despite the maximum early growth analysis technique representing ring widths in mm.

Åland churches as archives of tree-ring records sensitive to fluctuating climate

Abstract Tree-ring chronologies provide high-resolution late Quaternary palaeoclimatic data. An important aim of tree-ring research is to extend the chronologies back in time, before the period covered by old living trees. Tree-ring material from historic buildings offers an opportunity to develop long chronologies that, in some regions, may cover the period of the past millennium. Such materials have remained in conditions favourable to preservation and can be used to date the construction timber by means of dendrochronology. Apart from dating, tree-ring data may prove valuable in interpreting past climatic conditions. Here we analyse the data of 111 Scots pine (Pinus sylvestris L.) tree-ring series from the Åland Islands in south-western Finland. In so doing, we illustrate the variation of wetness and drought in the region over a historical time frame (1057–1826). Non-climatic trends were removed from these series using alternative types of detrending procedures. Tree-ring chronologies constructed from the same raw data but using different types of detrending methods agreed on annual to subcentennial scales. The chronologies produced using regional curve standardization (RCS), preferably combined with implementation of a signal-free approach, were comparable with previously published sedimentary and tree-ring evidence from the same region. While non-RCS methods are effective in removing non-climatic information from the chronology, they also resulted in removal of the long-term variation (low-frequency), which did, at least in our data, represent the palaeoclimatic signal common to different types of proxy records. These records, including our data and those of gridded reconstructions developed previously as the Old World Drought Atlas, agreed in indicating dry conditions over the pre-1250 period and around the mid-15th century. The Åland chronology is characterized by notable fluctuations in the availability of tree-ring samples; the periods with low sample replication probably pinpoint years when large construction projects were suspended on these islands.

A 1556 year-long early summer moisture reconstruction for the Hexi Corridor, Northwestern China

We report a 1556 year-long tree-ring width chronology for the Hexi Corridor, in the arid Northwestern China, established by applying the signal-free regional curve standardization method to 416 juniper ring-width series. We found that drought in early summer (May–June) is the primary controlling factor for tree growth in this area. We then developed an early summer moisture (i.e., scPDSI) reconstruction from 455 CE to present. Our reconstruction captures multi-centennial scale moisture variations, showing two long-term dry periods during 800–950 CE and 1000–1200 CE, and two long-term wet periods during 1200–1450 CE and 1510–1620 CE. We found strong similarities between hydroclimatic changes in the Hexi Corridor and Qaidam Basin from interannual to centennial timescales; however, at multi-centennial (> 300 years) timescales, hydroclimatic variations in the two regions showed significant regional differences. The Hexi Corridor witnessed a generally dry Medieval Climate Anomaly (MCA, here 800–1200 CE) and the drying 20th century, whereas the Qaidam Basin experienced high-precipitation periods during the MCA and 20th century. The different correlation pattern with Northern Hemisphere temperature suggest that the Qaidam Basin will receive more precipitation under global warming, whereas the Hexi Corridor will become dryer in the future.

A 7.5 ka chronology of stable carbon isotopes from tree rings with implications for their use in palaeo-cloud reconstruction

Abstract Tree-ring stable isotope chronologies provide very high-resolution palaeoclimatic data, and the number of records is increasing rapidly worldwide. To extend the chronologies back in time, before the period covered by the old living trees, the use of subfossil wood samples is required. Typically, the longest continuous subfossil chronologies consist of regionally collected tree-ring materials, rather than tree rings from a single site, and are likely more sensitive to data heterogeneity. Yet, the characteristics of such datasets remain hitherto unexplored. Here we produce a continuous, decadally resolved chronology of 13C/12C ratio (δ13C) from Finnish Lapland over the past 7.5 ka (5500 BCE to 2010 CE) for which there is replication of at least five Pinus sylvestris trees. Less negative δ13C values were observed as trees age and for western sites (higher in elevation and further from the cold oceanic air flow). The age-related trends in living tree δ13C data were expressed mainly over the first fifty years mimicking the “juvenile effect” whereas the subfossil data showed trend over the trees' lifespan. These findings demonstrated the need to detrend the individual δ13C series before averaging them into the mean chronology. The corresponding biases were removed from the isotopic data using the methods frequently applied for tree-ring width and density proxies, the Regional Curve Standardization (RCS) combined with signal-free approach. While the RCS procedures commonly preserve the long-term variations in the resulting chronology, not all types of them did so as demonstrated for chronologies produced using separate RCS models for the δ13C series with relatively high and low isotopic level (offset from the grand mean). It was shown that these δ13C levels (i.e. the relative isotopic enrichment) result both from the low-frequency climate signal and biogeographical aspects (the site longitude/altitudes). The non-climatic biases were removed from the δ13C series by using separate RCS models for the subsets (western and eastern) of isotopic series. Similar to previous investigations using annually resolved δ13C data from Lapland, our chronology had strong negative correlations to variations in cloud cover. Here, a bootstrapping experiment was used to verify this dendroclimatic association. The resulting palaeo-cloud reconstruction portrayed decadal to multi-millennial variations with centennial anomalies coinciding with the mid and late Holocene events of climate transitions, highlighting the value of subfossil isotope chronologies from tree rings in synthesising climate dynamics from several proxy sources over the present interglacial.

Recent growth trends of black pine ( Pinus nigra J.F. Arnold) in the eastern mediterranean

Abstract Past and present environmental changes cause significant changes in tree growth in many parts of the world, where both decreasing and increasing growth trends have been detected over the last decades. The Mediterranean basin is especially sensitive to climate change and subsequent tree growth declines. In this article, we present the first study on recent tree growth trends in Turkey. Pinus nigra is a drought-sensitive species and one of the most common and economically important native conifers to Turkey. Tree-ring cores were taken from 61 Pinus nigra plots spread over the entire Lakes District (Southwest Turkey), near the species’ southern range limit. The samples cover the 1839–2013 time period. We apply the Regional Curve Standardization technique and statistical modeling to the tree ring width data to investigate long-term growth trends. Both methods show remarkably similar results: a slowly increasing growth trend until the 1970s, followed by a decreasing trend. This recent negative trend is highly correlated with increased temperature and drought in summer, which suggests that it is likely caused by climate change.

Establishment of a 4650-year-long eigenvalue chronology based on tree-ring cores from Qilian junipers (Juniperus przewalskii Kom.) in Western China

This paper describes a tree-ring width chronology that spans the past 4650 years, established using the recently developed eigenanalysis technique. The aim is to show whether this eigenanalysis method enables the extraction of long-term tree-growth variations that are due to climatic changes, from a large dataset comprising 1263 tree-ring width records sampled from the highlands of Western China. In order to exclude the so-called growth rate/life span association effect, tree-ring width records were sorted into six subsets, based on the life spans of the trees sampled: 200–400, 400–600, 600–800, 800–1000, 1000–1500 years old, and trees older than 1500 years. Some partial chronologies were created, by pairing the set of all tree samples (living, dead, archaeological remains) with the living trees belonging to each subset. We computed the contours of tree-growth variations (on both 100-year and longer time scales) for each subset, ending with six pairs of these partial subset chronologies. Two sums of all these partial chronologies thus yielded a record of precipitation variations over a period ranging from 2627 BCE up to 2012 CE. It was found that this record shows a high degree of similarity to the existing chronology produced using the regional curve standardization (RCS) method applied to the same dataset, indicating that the eigenvalue chronology is capable of faithfully extracting long-term climatic variations. This also confirms that the first eigenvector represents the growth pattern that is characteristic of each biologically unique tree as well as the micro-environment of each tree stand. The variations observed over the last millennium seem to be connected to a cycle of solar activity with a period of ∼200 years. However, a clear lack of correspondence between solar activity and tree growth prior to 1000 CE indicates that the recent consistency may be coincidental. We believe that the eigenanalysis technique is readily applicable to other kinds of tree-ring datasets from different parts of the world.

Species-specific tree growth and intrinsic water-use efficiency of Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica) growing in a boreal permafrost region of the Greater Hinggan Mountains, Northeastern China

Increasing air temperature and atmospheric CO2 concentrations (Ca) can profoundly affect photosynthesis and intrinsic water-use efficiency (iWUE). However, the response of trees in boreal permafrost regions to rapid warming and Ca increases is poorly constrained by prior research. Here, we evaluated long-term changes in growth (using regional curve standardization [RCS]) and iWUE of Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica) in the boreal permafrost region of northeastern China and species-specific responses to increasing Ca and temperature. From 1930–2010, RCS growth of Dahurian larch and Mongolian pine decreased, while iWUE increased by 25.5 and 21.1%, respectively. RCS growth of both species was negatively correlated with winter temperatures, but Mongolian pine depended most strongly on previous December to current February temperatures and Dahurian larch depended most strongly on March temperatures. Moisture conditions only weakly influenced growth.We found similar long-term changes of tree-ring δ13C in the two species. Carbon isotopic discrimination of Dahurian larch and Mongolian pine was determined mainly by growing season temperature (positive) and moisture (negative), but with different signal strengths, suggesting that stomatal conductance influenced tree-ring δ13C. Commonality analysis showed that RCS growth was affected mainly by temperature, but also by the combined effect (interaction) of iWUE and temperature. However, the contribution of iWUE alone was lower for Mongolian pine. Our results suggest that the increased iWUE caused by increasing Ca will not improve tree growth sufficiently to compensate for temperature-induced water stress. The rate of temperature increase has slowed around 1990, which would have stabilized the degree of temperature-induced water stress, and this could be helpful to tree growth recovery in the permafrost region of northeastern China.