Search

Growth patterns of Scots pine and Siberian larch under water deficit across an intermontane valley in South Siberia depend not only on landscape physiography but on species-specific climatic sensitivity and phenology. The wide intermountain Khakass–Minusinsk Depression (KhMD) in southern Siberia presents an ideal setting for studying the potential impacts of a warming climate on forest ecosystems. The Centre of Continental Asia has one of the most intense rates of warming in the Northern Hemisphere, and the KhMD has multiple tree species of proven dendroclimatic value growing in drought-stressed environments. Investigation was aimed at spatial patterns of tree growth and its climate response across the KhMD for two main conifer species of moisture-deficient habitats, Scots pine (Pinus sylvestris L.) and Siberian larch (Larix sibirica Ledeb.). Correlation and cluster analysis were applied to a recently developed network of 15 tree-ring chronologies. Hierarchical classifications were based on the inter-chronology correlation matrix and on correlations of chronologies with monthly climate variables. Results underscore the general influence of hot-dry conditions on reducing growth and suggest a spatial grouping of chronologies governed by physiography and modified by species-dependent ecophysiological response to climate. Both applied classifications agree on the designation of geographically oriented clusters. A purely geographic grouping is broken, however, by species-specific climate dependence and phenology in deciduous Larix and evergreen Pinus. A differential ability to utilize melting snowpack in spring is advanced as a possible explanation for chronologies abandoning physiographically defined clusters. Such inter-species heterogeneity can manifest itself in the intensity of the climate change impact on vegetation, and lead to prospects of significant species composition changes in ecosystems.

The first dendroclimatic reconstruction of May 1 snow water equivalent (SWE) was developed from a Sequoiadendron giganteum regional tree-ring chronology network of 23 sites in central California for the period 90–2012 CE. The reconstruction is based on a significant relationship between May 1 SWE and tree-ring growth and shows climate variability from interannual to intercentennial time scales. A regression-based reconstruction equation explains up to 55% of the variance of SWE for 1940–2012. Split-sample validation supports our use of a reconstruction model based on the full period of reliable observational data (1940–2012). Thresholds for May 1 SWE low (15 percentile) and high (80 percentile) years were selected based on the exploratory scatterplots relationship between observed and reconstructed data for the period 1940–2012. The longest period of consecutive low-SWE years in the reconstruction is 2 years and the frequency of the lowest SWE years is highest during the period 710–809 CE. The longest high-SWE period, defined by consecutive wet years, is 3 years (558–560 CE). SWE and its reconstruction positively correlate with northeastern Pacific sea surface temperatures, the low-frequency variability of which may provide some predictive ability. Ultimately, the instrumental record and reconstruction suggest that unlike other sites in the region, twentieth century SWE variability in these Sequoia groves has remained within historical boundaries and relatively buffered from extremes and severe declines, though this is likely to change in coming decades with potentially negative effects on water availability for these trees.

Snowpack in the Sierra Nevada Mountains accounts for around one-third of California’s water supply. Melting snow provides water into dry summer months characteristic of the region’s Mediterranean climate. As climate changes, understanding patterns of snowpack, snowmelt, and biological response is critical in this region of agricultural, recreational, and ecological value. Here we investigated the relationships between tree rings of montane conifer trees ( Tsuga mertensiana, Abies magnifica, Abies concolor, Calocedrus decurrens, Juniperus occidentalis, and Pinus ponderosa) and regional climate indices with the goal of reconstructing April 1 snow-water equivalent (SWE) in the North Fork American River watershed of the Sierra Nevada. Chronologies were positively correlated with April 1 SWE of the year prior to ring formation. Temporal trends in correlation between tree-ring chronologies and climate indices indicate strengthening tree growth response to climate over time. We developed a skillful, nested reconstruction for April 1 SWE, 1661–2013. Variability of the reconstruction is within the envelope of 20th and 21st-century variability; however, the 2015 record low snowpack is unprecedented in the tree-ring record, as in results from previous studies. Future research should focus on integrating modern snow sensor data into paleoclimate research and understanding mechanistic linkages between snow and tree growth response.

Cool- and warm-season precipitation totals have been reconstructed on a gridded basis for North America using 439 tree-ring chronologies correlated with December–April totals and 547 different chronologies correlated with May–July totals. These discrete seasonal chronologies are not significantly correlated with the alternate season; the December–April reconstructions are skillful over most of the southern and western United States and north-central Mexico, and the May–July estimates have skill over most of the United States, southwestern Canada, and northeastern Mexico. Both the strong continent-wide El Niño–Southern Oscillation (ENSO) signal embedded in the cool-season reconstructions and the Arctic Oscillation signal registered by the warm-season estimates faithfully reproduce the sign, intensity, and spatial patterns of these ocean–atmospheric influences on North American precipitation as recorded with instrumental data. The reconstructions are included in the North American Seasonal Precipitation Atlas (NASPA) and provide insight into decadal droughts and pluvials. They indicate that the sixteenth-century megadrought, the most severe and sustained North American drought of the past 500 years, was the combined result of three distinct seasonal droughts, each bearing unique spatial patterns potentially associated with seasonal forcing from ENSO, the Arctic Oscillation, and the Atlantic multidecadal oscillation. Significant 200–500-yr-long trends toward increased precipitation have been detected in the cool- and warm-season reconstructions for eastern North America. These seasonal precipitation changes appear to be part of the positive moisture trend measured in other paleoclimate proxies for the eastern area that began as a result of natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

Dendroclimatology has focused mainly on the tree growth response to atmospheric variables. However, the roots of trees directly sense the “underground climate,” which can be expected to be no less important to tree growth. Data from two meteorological stations approximately 140 km apart in southern Siberia were applied to characterize the spatiotemporal dynamics of soil temperature and the statistical relationships of soil temperature to the aboveground climate and tree-ring width (TRW) chronologies of Larix sibirica Ledeb. from three forest–steppe stands. Correlation analysis revealed a depth-dependent delay in the maximum correlation of TRW with soil temperature. Temperatures of both the air and soil (depths 20–80 cm) were shown to have strong and temporally stable correlations between stations. The maximum air temperature is inferred to have the most substantial impact during July–September (R = −0.46–−0.64) and early winter (R = 0.39–0.52). Tree-ring indices reached a maximum correlation with soil temperature at a depth of 40 cm (R = −0.49–−0.59 at 40 cm) during April–August. High correlations are favored by similar soil characteristics at meteorological stations and tree-ring sites. Cluster analysis of climate correlations for individual trees based on the K-means revealed groupings of trees driven by microsite conditions, competition, and age. The results support a possible advantage of soil temperature over air temperature for dendroclimatic analysis of larch growth in semiarid conditions during specific seasons.

Dendroclimatology has focused mainly on the tree growth response to atmospheric variables. However, the roots of trees directly sense the “underground climate,” which can be expected to be no less important to tree growth. Data from two meteorological stations approximately 140 km apart in southern Siberia were applied to characterize the spatiotemporal dynamics of soil temperature and the statistical relationships of soil temperature to the aboveground climate and tree-ring width (TRW) chronologies of Larix sibirica Ledeb. from three forest–steppe stands. Correlation analysis revealed a depth-dependent delay in the maximum correlation of TRW with soil temperature. Temperatures of both the air and soil (depths 20–80 cm) were shown to have strong and temporally stable correlations between stations. The maximum air temperature is inferred to have the most substantial impact during July–September (R = −0.46–−0.64) and early winter (R = 0.39–0.52). Tree-ring indices reached a maximum correlation with soil temperature at a depth of 40 cm (R = −0.49–−0.59 at 40 cm) during April–August. High correlations are favored by similar soil characteristics at meteorological stations and tree-ring sites. Cluster analysis of climate correlations for individual trees based on the K-means revealed groupings of trees driven by microsite conditions, competition, and age. The results support a possible advantage of soil temperature over air temperature for dendroclimatic analysis of larch growth in semiarid conditions during specific seasons.

The year-to-year variability of precipitation has significant consequences for water management and forest health. “Whiplash” describes an extreme mode of this variability in which hydroclimate switches abruptly between wet and dry conditions. In this study, a pool of total-ring-width indices from five conifer species (Abies magnifica, Juniperus grandis, Pinus ponderosa, Pinus jeffreyi, and Tsuga mertensiana) in the Sierra Nevada is used to develop reconstructions of water-year precipitation using stepwise linear regression on lagged chronologies, and the reconstructions are analyzed for their ability to track whiplash events. A nonparametric approach is introduced to statistically classify positive and negative events, and the success of matching observed events with the reconstructions is evaluated using a hypergeometric test. Results suggest that reconstructions can effectively track whiplash events, but that tracking ability differs among species and sites. Although negative (dry-to-wet) events (1921–1989) are generally tracked more consistently than positive events, Tsuga stands out for strong tracking of positive events. Tracking ability shows no clear relationship to variance explained by reconstructions, suggesting that efforts to extend whiplash records with tree-ring data should consider optimizing reconstruction models for the whiplash signal.

Proxy records from the late Quaternary help in understanding climate variability on extended time scales. An ancient landslide deposit in Oregon U.S.A. preserved large logs from Douglas fir trees (Pseudotsuga menziesii (Mirb.) Franco) and afforded an opportunity to explore the response of tree growth to climate on annual and decadal scales. High-precision radiocarbon dating indicates an age exceeding 63 ka, i.e., the trees grew within the generally cool Marine Isotope Stage 5 (MIS 5), likely during a warmer interval optimal for Douglas fir establishment. This would include the prolonged warm MIS 5e (ca. 110–130 ka), corresponding approximately to the Eemian interglacial, which was warm like the current Holocene interglacial. A 297-year tree-ring width chronology from 12 Douglas fir logs and 227-year tree-ring δ13C and δ18O records are analyzed with spectral and wavelet analysis. Variance of the ancient rings is consistent with modern Douglas fir growth sensitive to moisture and ecological disturbances. Spectra of ancient and modern chronologies are dominated by low frequencies with significant spectral peaks appearing at high frequencies (2.1–4 years) and cyclic behavior transient over centuries. It is conceivable that the O-isotopes track moisture and that C-isotopes track temperature or sunlight. The findings illustrate the challenges in assessing the response of ancient tree-ring properties to late Quaternary climate variability.

An assemblage of tree-ring chronologies for the Nazas (NZW) in the Western Sierra Madre (WSM), Mexico was developed to determine water inflow at the Lazaro Cardenas Dam (LCD), the main source of water for surface irrigation in the Irrigation District 017 (DDR 017), Comarca Lagunera. A Principal Component Analysis of the ring-width chronologies was conducted to determine a common climate signal, and a stepwise model based on selected chronologies of the PC1 (CBA, COC) and PC2 (ARN) were used to develop a water inflow reconstruction to the Lazaro Cardenas Dam (LCD) extending from 1753 to 2003 (251 years), resulting in the following significant findings: the warm phase of the El Niño Southern Oscillation (ENSO) in the winter-spring season had a significant influence (SOI; Dec–Feb = −0.24, p < 0.01), but the North American Monsoon System (NAMS) was the most important in determining the water yield in the summer season (r = 0.48, p < 0.01). Water gauge inflow records (77 years) at the LCD used to determine the annual allocation of water for agriculture in the irrigation district 017 was an average of 1676 × 106 m3, where the maximum annual water outflow allowed of 1100 × 106 m3 for safety reasons, the dam infrastructure was released in 74% of the years and increasing to 78% when considering the reconstructed inflow. Prolonged drought episodes lasting more than 10 consecutive years were detected in the reconstructed inflow, information that could be used by decision makers to establish proper irrigation management strategies to ameliorate the economic and social impact when these extreme hydroclimatic events may occur.

The tree-ring signal for flooding along the Ob River, a large Arctic River in western Siberia, is investigated using a combination of floodplain tree-ring sites from riparian and non-riparian settings. A conceptual model is presented contrasting tree-growth responses of riparian and non-riparian trees to unusually severe flooding. A set of five riparian ( Salix and Populus) tree-ring chronologies is developed and used in combination with existing floodplain non-riparian Larix and Pinus chronologies in a binary classification tree (CT) model to classify high-flood years, defined as a Salekhard water-level gage reading in the seasonal window from May 1 to August 31 of above 470 cm for 82 or more consecutive days. Correlation and regression identifies a nonlinear relationship of riparian ring widths to discharge and flooding: higher annual discharge generally leads to higher growth, but the relationship reverses in extreme-flood years. Micrographs highlight the suppression of width and occasional distortion of cell anatomy in selected trees. CT modeling guided by cross-validation yields a CT model with a primary split on the riparian ring width and secondary split on the non-riparian ring width. The model successfully identifies four of the eight most severe high-flow years, 1934–2014. The model further identifies two years (1885 and 1914) before the start of the gaged record in 1934 as high-flow years. No appreciable difference is found in frequency of high-flow years before and after 1956, when the first major reservoirs began filling upstream of the Lower Ob. The CT modeling approach is proposed as a novel approach to dealing with nonlinearity in reconstructing flood history of Arctic rivers from tree rings.

California Department of Water Resources [4600011071]; U.S. Department of Energy, Office of Science Innovative and Novel Computational Impact on Theory and Experiment (DOE INCITE) ProgramUnited States Department of Energy (DOE); U.S. Department of Energy, Office of Biological and Environmental Research (BER)United States Department of Energy (DOE); National Oceanic and Atmospheric Administration Climate Program OfficeNational Oceanic Atmospheric Admin (NOAA) - USA

The great rivers that flow from the southern Tibetan Plateau (TP) affect billions of people in the downstream countries of Asia. Understanding of the hydrological variability of these rivers is still limited, however, because of the lack of long-term streamflow records. Tree-ring width chronologies from six sites are applied to reconstruct annual streamflow of the Salween River, the last remaining large free-flowing transboundary river draining the southern TP, and a critical water source for countries of Southeast Asia. Response function analysis shows that precipitation is the main factor limiting the radial growth of the sampled trees. Linear regression of annual (September–June) Salween River streamflow on the first principal component of tree-ring chronologies explains 53.4% of the streamflow variance, 1958–2011, and yields a reconstruction for the interval 1500–2011 CE. A tally of droughts and wet periods emphasizes the severity of droughts before the start of the gauged records, and a tendency toward wetter conditions in recent decades. Regional temperature is negatively associated with the reconstructed streamflow. Cold wet summers controlled by the Asian summer monsoon are responsible for an increasing trend in streamflow over the last decades. Reconstructed hydrological change is linked to the history of mainland Southeast Asia through the impact of water shortages on Burma society. In particular, prolonged periods of low flow of the Salween River coincide with the falls of the Toungoo Empires and the First Anglo-Burmese War. This tree-ring reconstruction provides a long-term perspective on hydrological changes in the Upper Salween River Basin that can give insight for sustainable water management on the TP and in Myanmar.

Haury Visiting Scholar Fellowship from the Laboratory of Tree-Ring Research at the University of Arizona in Tucson; NSF EAR-Hydrological Sciences Award [1445895]

The Lhasa River is the largest tributary of the Yarlung Zangbo River, provides up to 85% of the water supply for the city of Lhasa, and has a high ecologic and economic importance. Annual streamflow of the Lhasa River is reconstructed from Juniperus tibetica tree rings. The streamflow reconstruction developed for the southern Tibetan Plateau goes back to 1546 CE and has a R2 of 0.485. Spectral and wavelet analysis indicates the existence of decadal (34 and 16 years) and interannual (8.1, 5.7, 4.8, 3.8, 3.5, 3.2, 2.7, 2.3 and 2.1 years) cycles that may reflect climate forcings. Lhasa River streamflow is significantly correlated with precipitation over a vast part of the Yarlung Zangbo River basin, and represents streamflow of the upper Yarlung Zangbo River to a certain extent. Lhasa River variation is linked with large-scale climate circulation features, such as the El Nino-Southern Oscillation (ENSO) and Asian summer monsoon. Some years of abnormally high and low streamflow are related to ENSO events. Based on the reconstruction, the probability that annual streamflow does not exceed a target defined as the instrumental-period mean was lower (48% non-exceedance probability) than during the instrumental period. This study indicates that the instrumental record does not contain the full range of streamflow, especially the lowest streamflow events, and the reconstruction makes up for this shortcoming. Future water resource supply planning based on the instrumental record and streamflow reconstructions will be able to effectively reduce the risks posed by climate change.

This work presents updated reconstructions of watershed runoff to San Francisco Estuary from tree-ring data to AD 903, coupled with models relating runoff to freshwater flow to the estuary and salinity intrusion. We characterize pre-development freshwater flow and salinity conditions in the estuary over the past millennium and compare this characterization with contemporary conditions to better understand the magnitude and seasonality of changes over this time. This work shows that the instrumented flow record spans the range of runoff patterns over the past millennium (averaged over 5, 10, 20 and 100 years), and thus serves as a reasonable basis for planning-level evaluations of historical hydrologic conditions in the estuary. Over annual timescales we show that, although median freshwater flow to the estuary has not changed significantly, it has been more variable over the past century compared to pre-development flow conditions. We further show that the contemporary period is generally associated with greater spring salinity intrusion and lesser summer–fall salinity intrusion relative to the pre-development period. Thus, salinity intrusion in summer and fall months was a common occurrence under pre-development conditions and has been moderated in the contemporary period due to the operations of upstream reservoirs, which were designed to hold winter and spring runoff for release in summer and fall. This work also confirms a dramatic decadal-scale hydrologic shift in the watershed from very wet to very dry conditions during the late 19th and early 20th centuries, while not unprecedented, these shifts have been seen only a few times in the past millennium. This shift resulted in an increase in salinity intrusion in the first three decades of the 20th century, as documented through early records. Population growth and extensive watershed modification during this period exacerbated this underlying hydrologic shift. Putting this shift in the context of other anthropogenic drivers is important in understanding the historical response of the estuary and in setting salinity targets for estuarine restoration. By characterizing the long-term behavior of San Francisco Estuary, this work supports decision-making in the State of California related to flow and salinity management for restoration of the estuarine ecosystem.

Drought in the North American Southwest is a recurring phenomenon. The knowledge of drought recurrence and severity is crucial for sustainable water resource management in the region. Tree-ring reconstructions of climate variables provide valuable indirect evidence of climate variability and elucidate the relationship between large-scale circulation anomalies and the climate in the region. Here we have developed a May–July Palmer Drought Severity Index (PDSI) reconstruction from tree-ring chronologies of Pseudotsuga menziesii from the Chiricahua Mountains (southeastern Arizona) for the period 1634–2017 CE. The driest period occurred at the beginning of the 21st Century (2000–2005), followed by 1666–1670, 1952–1956, and 1729–1734. Reconstructed PDSI of the Chiricahua Mountains shows a weak correlation with the El Nino-Southern Oscillation, with intermittent influence as previous studies have reported. The Pacific Decadal Oscillation accounts for the majority of the variability in the Chiricahua hydroclimate, except for the period from 1860–1950 when the predominant driver was the North Atlantic Oscillation.

Regional and local climate change depends on continentality, orography, and human activities. In particular, local climate modification by water reservoirs can reach far from shore and downstream. Among the possible ecological consequences are shifts in plant performance. Tree-ring width of affected trees can potentially be used as proxies for reservoir impact. Correlation analysis and t-tests were applied to climatic data and tree-ring chronologies of Pinus sylvestris L. and Larix sibirica Ledeb. from moisture-deficit habitats in the intermontane Khakass-Minusinsk Depression, to assess modification of climate and tree growth by the Krasnoyarsk and Sayano-Shushenskoe Reservoirs on the Yenisei River. Abrupt significant cooling in May–August and warming in September-March occurred after the launch of the turbines in dams, more pronounced near the Sayano-Shushenskoe dam (up to – 0.5 °C in summer and to + 3.5 °C in winter) than near the Krasnoyarsk Reservoir headwaters (– 0.3 °C and + 1.4 °C). Significant lengthening of the warm season was also found for temperature thresholds 0–8 °C. Shifts of seasonality and intensity occurred in climatic responses of all tree-ring chronologies after development of water reservoirs. Patterns of these shifts, however, depended on species-specific sensitivity to climatic modification, distance from reservoirs, and physiographic regions. Mitigation of climate continentality and extremes by reservoirs appears to have offset possible negative effects of warming on tree growth.

Annual river discharge is a critical variable for water resources planning and management. Tree rings are widely used to reconstruct annual discharge, but errors can be large when tree growth fails to respond commensurately to hydrologically important seasonal components of climate. This paper contrasts direct and indirect reconstruction as statistical approaches to discharge reconstruction for the Chemora River, in semi-arid northeastern Algeria, and explores indirect reconstruction as a diagnostic tool in reconstruction error analysis. We define direct reconstruction as predictions from regression of annual discharge on tree ring data, and indirect reconstruction as predictions from a four-stage process: (1) regression of precipitation on tree rings, (2) application of the regression model to get reconstructed precipitation for grid cells over the basin, (3) routing of reconstructed precipitation through a climatological water balance (WB) model, and (4) summing model runoff over cells to get the reconstructed discharge at a gage location. For comparative purposes, the potential predictors in both modeling approaches are the same principal components of tree ring width chronologies from a network of drought-sensitive sites of Pinus halepensis and Cedrus atlantica in northern Algeria. Results suggest that both modeling approaches can yield statistically significant reconstructions for the Chemora River. Greater accuracy and simplicity of the direct method are countered by conceptual physical advantages of the indirect method. The WB modeling inherent to the indirect method is useful as a diagnostic tool in error analysis of discharge reconstruction, points out the low and declining importance of snowmelt to the river discharge, and gives clues to the cause of severe underestimation of discharge in the outlier high-discharge year 1996. Results show that indirect reconstruction would benefit most in this basin from tree ring resolution of seasonal precipitation.

The streamflow regimes of eastern boreal Canada are snow-melt and ice-melt driven with the highest flows occurring in spring. Over the last few decades, a positive streamflow trend has been observed, with increasing severity and frequency of spring flooding. Further changes in flood dynamics are projected as a consequence of global climate change. The validity of projections is restricted by the lack of long and spatially well-replicated observations. High-resolution proxy records are needed to better understand the natural range of variability in spring runoff and associated atmospheric controls.Recent research has shown that riparian black ash trees (Fraxinus nigra Marsh.) exposed to periodic submersion produce “flood rings” whose earlywood cross-sectional vessel area is linearly associated with the severity of flooding. Twelve continuous chronologies of ring width and earlywood vessel anatomy were developed for Lake Duparquet to extend the record of Harricana River mean spring flow. A visually determined index of flood rings was also developed to determine i) the spatial coherency of the spring flood signal and ii) the coherency of the flood signal among natural, regulated and unflooded rivers.The reconstruction spans the period 1770-2016 and captures more than 65% of the variance of Harricana river spring flow. Trend analysis indicates an increase in both magnitude and frequency of the major floods starting at the end of the Little Ice Age (LIA, 1850-1890), with highest peaks after 1950. Time-frequency analysis shows non-stationarity: a stable 30-year periodicity during the LIA is replaced by a decadal pattern starting around 1850, and evolves into a more high-frequency pattern after 1930. The signal is strongly coherent between watersheds for natural rivers and weaker for regulated basins. Field correlations with gridded climate data indicate the broad spatially coherent pattern of spring high flows across much of central/eastern north Canada is positively associated with April-May precipitation and snow cover, and negatively associated with March-April maximum temperature.These large-scale associations support atmospheric forcing of inter-annual hydroclimatic variability. While the Artic and North Atlantic Oscillations have previously been found to influence winter and spring climate conditions in eastern Quebec, our results contrast with a significant negative association with El-Niño Southern Oscillation from January to May, and the Pacific Decadal Oscillation from December to February. In Lake Duparquet, warm and wet air from Pacific-South Ocean (El-Niño) are associated with early spring and small floods, while cold and dry air masses (La-Niña) correlate to late thaw and high floods in spring. The association with sea surface temperature and 200mb geopotential field heights reveal a clear atmospheric connection between eastern north boreal Canada and the tropical Pacific Ocean.The novel application of wood-cell anatomy to hydroclimatology underscores an increase in flood frequency and severity since the end of the 18th century in northeastern Canada. More broadly, the application highlights how analysis of tree rings from riparian trees can be used to extend the flood history of boreal rivers.

Long highly-resolved proxies for runoff are in high demand for hydrological forecasts and water management in arid Central Asia. An accurate (R2 = 0.53) reconstruction of October-September discharge of the Ili River in Kazakhstan, 1779–2015, is developed from moisture-sensitive tree rings of spruce sampled in the Tian Shan Mountains. The fivefold extension of the gauged discharge record represents the variability of runoff in the Lake Balkhash Basin for the last 235 years. The reconstruction shows a 40 year long interval of low discharge preceded a recent high peak in the first decade of the 2000s followed by a decline to more recent levels of discharge not seen since the start of the gauged record. Most reconstructed flow extremes (± 2σ) occur outside the instrumental record (1936–2015) and predate the start of large dam construction (1969). Decadal variability of the Ili discharge corresponds well with hydrological records of other Eurasian internal drainages modeled with tree rings. Spectral analysis identifies variance peaks (highest near 42 year) consistent with main hemispheric oscillations of the Eurasian climatic system. Seasonal comparison of the Ili discharge with sea-level-pressure and geopotential height data suggests periods of high flow likely result from the increased contribution of snow to runoff associated with the interaction of Arctic air circulation with the Siberian High-Pressure System and North Atlantic Oscillation.

This study analyses dynamics of the hydrological regime of Bele-Shira closed basin and evaluates the potential for using radial growth of Siberian larch (Larix sibirica) for its assessment. We investigated the relationships between different characteristics of the water level variation of Lake Shira, precipitation amount and long-term regional chronologies developed from 56 living trees and 32 dead trees on three sites across this basin. Graphical and correlation analysis indicate that the interannual change (June minus previous June) of the water level of Lake Shira is strongly positively related to the annual sum of precipitation from July to June and the radial growth of larch. It was shown that this hydrological characteristic integrates the current dynamics of the regional precipitation and moisture regime as a whole of the Bele-Shira closed basin on interannual and decadal scales. The water level of Lake Shira fluctuates on a multi-year timescale in synchrony with the cumulative sum of the tree-ring chronology and also has strong positive long-term trend, probably driven by the continual groundwater inflow from neighboring Lake Itkul. Delayed relationships of precipitation and radial growth with the Lake Shira level change are interpreted with reference to a water balance model of the closed basin. Results offer the possibility of reconstructing interannual and decadal variation of the hydrological regime during the last few centuries through regression models using tree-ring chronologies or the dynamics of climatic variables recovered from them.

There is a lack of knowledge on how tree species respond to climatic constraints like water shortages and related atmospheric patterns across broad spatial and temporal scales. These assessments are needed to project which populations will better tolerate or respond to global warming across the tree species distribution range. Warmer and drier conditions have been forecasted for the Mediterranean Basin, where Aleppo pine (Pinus halepensis Mill.) is the most widely distributed conifer in dry sites. This species shows plastic growth responses to climate, being particularly sensitive to drought. We evaluated how 32 Aleppo pine forests responded to climate and atmospheric patterns during the second half of the 20th century by using dendrochronology. Climatic constraints of radial growth were inferred by fitting the Vaganov‒Shashkin (VS-Lite) growth model. Aleppo pine growth decreased and showed the highest common coherence among trees in dry, continental sites, such as those situated in southeastern and eastern inland Spain and Algeria. Growth increased in wetter sites. Tree growth was enhanced by prior wet winter conditions and cool and wet spring conditions, whilst warm summers were associated with decreased growth. The relationships between site ring-width chronologies were spatially structured, which explains why Aleppo pine growth was distinctly linked to indices describing atmospheric circulation patterns, showing a stronger influence in western sites (Western Mediterranean Oscillation and Northern Atlantic Oscillation). The climatic constraints of growth and their biogeographical variability were captured by the VS-Lite model, with better fits in dry and continental sites, showing strong growth coherence between trees and climatic limitations of growth. Further research using similar broad-scale approaches to climate&ndash, growth relationships and in drought-prone regions deserves more attention.

In eastern boreal Canada, variability in river discharge is poorly understood at the multi-century scale due to short instrumental records. In recent decades, increased magnitude and frequency of spring floods have raised concerns about the potential effects of climate change on flood risk. Unlike tree-ring width, flood rings have a demonstrated dendrochronological utility for reconstructing high discharge in boreal environments. In this study, twelve chronologies of earlywood vessel cross-sectional area (a new hydrological proxy) and ring width were developed from riparian Fraxinus nigra trees periodically flooded in spring. These chronologies were used as predictors of Harricana River spring discharge, which was reconstructed for the period 1771–2016. The reconstruction captured 69% of the variance over a 102-year calibration period. The reconstruction indicates that the magnitude and frequency of spring high discharge has increased since the end of the Little Ice Age (1850–1870 CE) and since 1950. The change from a multi-decadal frequency in the late 19th century to a decadal and then interannual frequency in the late 20th century is associated with an increase in snow cover over much of central-eastern Canada. The association between the reconstructed spring discharge and spring atmospheric circulation indices NINO3.4, AMO, NAO may also have changed in these periods and further work is needed to assess the stability of these associations. The correlation between reconstructed and instrumental spring discharge at the regional scale, as well as the shared features in reconstructed discharge and other paleorecords from subarctic Quebec suggest a common hydrological signal across the study area and for the early 20th to 21st centuries. The unprecedently low and high spring discharge in recent decades compared to the historical natural variability of the last 250 years also suggests that the increase in flood frequency and magnitude originates from climate change.

CRDF Global Research Partnerships program [RUC1-7075-EK-12, FSCX-15-61824-0]; Russian Foundation for Basic Research [13-04-01964, 05-04-48298, 00-05-65041]

Recent Mediterranean droughts have highlighted concerns that climate change may be contributing to observed drying trends, but natural climate variability in the region is still poorly understood. We analyze 900 years (1100–2012) of Mediterranean drought variability in the Old World Drought Atlas (OWDA), a spatiotemporal tree-ring reconstruction of the June–July–August self calibrating Palmer Drought Severity Index. In the Mediterranean, the OWDA is highly correlated with spring precipitation (April–June), the North Atlantic Oscillation (January–April), the Scandinavian Pattern (January–March), and the East Atlantic Pattern (April–June). Drought variability displays significant east-west coherence across the basin on multi-decadal to centennial time scales and north-south anti-phasing in the eastern Mediterranean, with a tendency for wet anomalies in the Black Sea region (e.g., Greece, Anatolia, the Balkans, etc) when coastal Libya, the southern Levant, and the Middle East are dry, possibly related to the North Atlantic Oscillation. Recent droughts are centered in the Western Mediterranean, Greece, and the Levant. Events of similar magnitude in the Western Mediterranean and Greece occur in the OWDA, but the recent 15-year drought in the Levant (1998–2012) is the driest in the record. Estimating uncertainties using a resampling approach, we conclude there is an 89% likelihood this drought is drier than any comparable period of the last 900 years and a 98% likelihood it is drier than the last 500 years. These results confirm the exceptional nature of this drought relative to natural variability in recent centuries, consistent with studies that have found evidence for anthropogenically forced drying in the region.

Summary Recent streamflow droughts in south coastal British Columbia have had major socioeconomic and ecological impacts. Increasing drought severity under projected climate change poses serious water management challenges, particularly in the small coastal watersheds that serve as primary water sources for most communities in the region. A 332-year dendrohydrological record of regionalized mean summer streamflow for four watersheds is analyzed to place recent drought magnitudes in a long-term perspective. We present a novel approach for optimizing tree-ring based reconstructions in small watersheds in temperate environments, combining winter snow depth and summer drought sensitive proxies as model predictors. The reconstruction model, estimated by regression of observed flows on Tsuga mertensiana ring-width variables and a tree-ring derived paleorecord of the Palmer Drought Severity Index, explains 64% of the regionalized streamflow variance. The model is particularly accurate at estimating lowest flow events, and provides the strongest annually resolved paleohydrological record in British Columbia. The extended record suggests that since 1658 sixteen natural droughts have occurred that were more extreme than any within the instrumental period. Flow-duration curves show more severe worst-case scenario droughts and a higher probability of those droughts in the long-term reconstruction than in the hydrometric data. Such curves also highlight the value of dendrohydrology for probabilistic drought assessment. Our results suggest current water management strategies based on worst-case scenarios from historical gauge data likely underestimate the potential magnitudes of natural droughts. If the low-flow magnitudes anticipated under climate change co-occur with lowest possible natural flows, streamflow drought severities in small watersheds in south coastal British Columbia could exceed any of those experienced in the past ∼350 years.

Expansion of agricultural practices from the Fertile Crescent to China during the mid and late Holocene are believed to have shaped the early network of Silk Road routes and possibly regulated the dynamics of trade and exchange in the urban oases along the Silk Road throughout its existence. While the impacts of climate change on the Silk Road are more or less documented for the medieval period, they remain poorly understood for early history of the Silk Road, especially in Central Asia. We analyze hydroclimatic proxies derived from fluvial stratigraphy, geochronology, and tree-ring records that acted on various time scales in the Lake Balkhash Basin to learn how changes in water supply could have influenced the early farmers in the Semirechye region of southern Kazakhstan. Our approach aims to identify short-term and long-term variability of regional runoff and to compare the hydrological data with cultural dynamics coupled with the archaeological settlement pattern and agricultural production. The reconstructed runoff variability underscore the contribution of winter precipitation driven by the interaction between the Arctic oscillation and the Siberian High-Pressure System, to Central Asian river discharge. We show that Saka people of the Iron Age employed extensive ravine agriculture on the alluvial fans of the Tian Shan piedmont, where floodwater farming peaked between 400 BC and 200 BC. The early Silk Road farmers on the alluvial fans favored periods of reduced flood flows, river stability and glacier retreat in the Tian Shan Mountains. Moreover, they were able to apply simple flow control structures to lead water across the fan surface. It is very unlikely that changes in water supply ever significantly constricted agricultural expansion in this region.

In the original publication of the article Fig. 1 was incorrect. The correct Fig. 1 is given below.

Drought is one of the main natural factors in declining tree-ring growth and the production of agricultural crops in Algeria. This paper addresses the variability of growing conditions for wheat in Algeria with climatic data and a tree-ring reconstruction of January–June precipitation from ten Pinus halepensis tree-ring chronologies. A regression-based reconstruction equation explains up to 74% of the variance of precipitation in the 1970–2011 calibration period and cross validates well. Classification of dry years by the 30% percentile of observed precipitation (131 mm) yields a maximum length of drought of five years (1877–1881) and increasing frequency of dry years in the late 20th and early 21st centuries. A correlation-based sensitivity analysis shows a similar pattern of dependence of tree-growth and wheat production on monthly and seasonal precipitation, but contrasting patterns of dependence on temperature. The patterns are interpreted by reference to phenology, growth phases, and – for wheat – agricultural practices.

Old, multi-aged populations of riparian trees provide an opportunity to improve reconstructions of streamflow. Here, ring widths of 394 plains cottonwood ( Populus deltoides, ssp. monilifera) trees in the North Unit of Theodore Roosevelt National Park, North Dakota, are used to reconstruct streamflow along the Little Missouri River (LMR), North Dakota, US. Different versions of the cottonwood chronology are developed by (1) age-curve standardization (ACS), using age-stratified samples and a single estimated curve of ring width against estimated ring age, and (2) time-curve standardization (TCS), using a subset of longer ring-width series individually detrended with cubic smoothing splines of width against year. The cottonwood chronologies are combined with the first principal component of four upland conifer chronologies developed by conventional methods to investigate the possible value of riparian tree-ring chronologies for streamflow reconstruction of the LMR. Regression modeling indicates that the statistical signal for flow is stronger in the riparian cottonwood than in the upland chronologies. The flow signal from cottonwood complements rather than repeats the signal from upland conifers and is especially strong in young trees (e.g. 5–35 years). Reconstructions using a combination of cottonwoods and upland conifers are found to explain more than 50% of the variance of LMR flow over a 1935–1990 calibration period and to yield reconstruction of flow to 1658. The low-frequency component of reconstructed flow is sensitive to the choice of standardization method for the cottonwood. In contrast to the TCS version, the ACS reconstruction features persistent low flows in the 19th century. Results demonstrate the value to streamflow reconstruction of riparian cottonwood and suggest that more studies are needed to exploit the low-frequency streamflow signal in densely sampled age-stratified stands of riparian trees.

The goal of this research report is to describe annual precipitation reconstruction from Pinus sylvestris trees on three sites in the Abakan region, located in the Minusinsk Depression, at the confluence of the Yenisei and Abakan Rivers, Russia. The study was performed during the 4th annual international summer course “Tree Rings, Climate, Natural Resources and Human Interaction” held in Abakan, 5-19 August 2013. The reconstruction, for the 12-month total precipitation ending in July of the growth year, is based on a reliable and replicable statistical relationship between precipitation and tree-ring growth, and shows climate variability on both interannual and interdecadal time scales. The regional tree-ring chronology accounts for 56% of the variance of observed annual precipitation in a linear regression model, with the strongest monthly precipitation signal concentrated in May and June of the current growing season. Composite 500 mb height-anomaly maps suggest that the tree-ring data from thi...

Tree-ring carbon and oxygen isotope ratios have been used to understand past dynamics in forest carbon and water cycling. Recently, this has been possible for different parts of single growing seasons by isolating anatomical sections within individual annual rings. Uncertainties in this approach are associated with correlated climate legacies that can occur at a higher frequency, such as across successive seasons, or a lower frequency, such as across years. The objective of this study was to gain insight into how legacies affect cross-correlation in the δ13 C and δ18 O isotope ratios in the earlywood (EW) and latewood (LW) fractions of Pinus ponderosa trees at thirteen sites across a latitudinal gradient influenced by the North American Monsoon (NAM) climate system. We observed that δ13 C from EW and LW has significant positive cross-correlations at most sites, whereas EW and LW δ18 O values were cross-correlated at about half the sites. Using combined statistical and mechanistic models, we show that cross-correlations in both δ13 C and δ18 O can be largely explained by a low-frequency (multiple-year) mode that may be associated with long-term climate change. We isolated, and statistically removed, the low-frequency correlation, which resulted in greater geographical differentiation of the EW and LW isotope signals. The remaining higher-frequency (seasonal) cross-correlations between EW and LW isotope ratios were explored using a mechanistic isotope fractionation-climate model. This showed that lower atmospheric vapor pressure deficits associated with monsoon rain increase the EW-LW differentiation for both δ13 C and δ18 O at southern sites, compared to northern sites. Our results support the hypothesis that dominantly unimodal precipitation regimes, such as near the northern boundary of the NAM, are more likely to foster cross-correlations in the isotope signals of EW and LW, potentially due to greater sharing of common carbohydrate and soil water resource pools, compared to southerly sites with bimodal precipitation regimes.

The first large-scale network of tree-ring chronologies from the western Mediterranean (WM; 32°N–43°N, 10°W–17°E) is described and analyzed to identify the seasonal climatic signal in indices of annual ring width. Correlation and rotated empirical orthogonal function analyses are applied to 85 tree-ring series and corresponding gridded climate data to assess the climate signal embedded in the network. Chronologies range in length from 80 to 1129 years. Monthly correlations and partial correlations show overall positive associations for Pinus halepensis (PIHA) and Cedrus atlantica (CDAT) with winter (December–February) and spring (March–May) precipitation across this network. In both seasons, the precipitation correlation with PIHA is stronger, while CDAT chronologies tend to be longer. A combination of positive correlations between growth and winter–summer precipitation and negative partial correlations with growing season temperatures suggests that chronologies in at least part of the network reflect soil moisture and the integrated effects of precipitation and evapotranspiration signal. The range of climate response observed across this network reflects a combination of both species and geographic influences. Western Moroccan chronologies have the strongest association with the North Atlantic Oscillation.

Projected changes in global rainfall patterns will likely alter water supplies and ecosystems in semiarid regions during the coming century. Instrumental and paleoclimate data indicate that natural hydroclimate fluctuations tend to be more energetic at low (multidecadal to multicentury) than at high (interannual) frequencies. State-of-the-art global climate models do not capture this characteristic of hydroclimate variability, suggesting that the models underestimate the risk of future persistent droughts. Methods are developed here for assessing the risk of such events in the coming century using climate model projections as well as observational (paleoclimate) information. Where instrumental and paleoclimate data are reliable, these methods may provide a more complete view of prolonged drought risk. In the U.S. Southwest, for instance, state-of-the-art climate model projections suggest the risk of a decade-scale megadrought in the coming century is less than 50%; the analysis herein suggests that the risk is at least 80%, and may be higher than 90% in certain areas. The likelihood of longer-lived events (>35 yr) is between 20% and 50%, and the risk of an unprecedented 50-yr megadrought is nonnegligible under the most severe warming scenario (5%–10%). These findings are important to consider as adaptation and mitigation strategies are developed to cope with regional impacts of climate change, where population growth is high and multidecadal megadrought—worse than anything seen during the last 2000 years—would pose unprecedented challenges to water resources in the region.

A May–July precipitation nested reconstruction for the period AD 1415–2010 was developed from multi-century tree-ring records of Pinus nigra, Pinus brutia, and Cedrus brevifolia for Cyprus. Calibration and verification statistics for the period 1917–2010 show a good level of skill, and split-sample validation over 1917–2010 supports temporal stability of the tree-ring signal for precipitation. Smoothed annual time series of reconstructed precipitation and a tally of drought events in a moving time window indicate that the calibration period is not representative of the full range of drought variability. While convective precipitation in the warm season may be driven strongly by local factors, composite maps of geopotential height anomaly for dry years and wet years support large-scale atmospheric-flow influence related to height anomalies over the broader region of northeast Africa and the eastern Mediterranean. Emerging positive trend in reconstruction residuals may be an early sign of exacerbation of drought stress on trees by recent warming in May–July. Future warming expected from increases in greenhouse gases poses a threat to forest resources in Cyprus and elsewhere in the Mediterranean.

This study reports two multi-century regional reconstructions of annual precipitation based on Pinus ponderosa and P. edulis from four sites in central northern Arizona. It compares standard regional and time-nested methods to generate reconstructions from 1581–2016 C.E. and 1529–2016 C.E., respectively. The strongest climate relationship is a positive correlation between total ring width and 12-month total precipitation ending in July of the growth year. The chronologies account for 50% of the variance of observed annual precipitation in the regional model and 59%, 60%, and 47% and 35% in the nested models. The two reconstructions are highly correlated (Pearson's correlation r > 0.97, p < 0.001) demonstrating that the reconstructions are highly similar over the period common to both reconstructions, with the nested-models’ advantage of extending the range of the reconstruction. The precipitation reconstructions are significantly correlated (r = 0.66, p < 0.001) with the North American Drought Atlas (NADA).

The first large-scale network of 79 tree-ring chronologies in the Eastern Mediterranean and Near East (EMNE; 33°N–42°N, 21°E–43°E) is described and analyzed to identify the seasonal climatic signal in indices of annual ring width. Correlation analysis and cluster analysis are applied to tree-ring data and gridded climate data to assess the climate signal embedded in the network in preparation for climate field reconstructions and formal proxy/model intercomparison experiments. The lengths of the 79 combined chronologies range from 89 to 990 years. The monthly correlations and partial correlations reveal a pervasive positive association with May, June, and sometimes July precipitation, positive correlations with winter and spring (December through April) temperatures, and negative relationships with May through July temperature, although as expected, there are site-to-site exceptions to these general patterns. Cluster analysis suggests three groups of sites based on their association with climate. The chronologies for the EMNE have coherent seasonal precipitation and temperature signals across a fairly broad geographical domain. The predominant signal is a positive growth response to May–June precipitation. Collectively, the findings suggest that the network can be exploited to develop season-specific field reconstructions of precipitation and drought history in the EMNE.

Dendroclimatology in the Eastern Mediterranean (EM) region has made important contributions to the understanding of climate variability on timescales of decades to centuries. These contributions, beginning in the mid-20th century, have value for resource management, archaeology, and climatology. A gradually expanding tree-ring network developed by the first author over the past 15 years has been the framework for some of the most important recent advances in EM dendroclimatology. The network, now consisting of 79 sites, has been widely applied in large-scale climatic reconstruction and in helping to identify drivers of climatic variation on regional to global spatial scales. This article reviews EM dendroclimatology and highlights contributions on the national and international scale. DOI: 10.2458/azu_rc.56.18321 ( Radiocarbon ) DOI: 10.3959/1536-1098-70.3.61 ( Tree-Ring Research )