Tree-ring Network Research Articles

Dendroclimatology in the Eastern Mediterranean

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.

Summer rainfall variability in European Mediterranean mountains from the sixteenth to the twentieth century reconstructed from tree rings

Since the end of the last glacial period, European Mediterranean mountains have provided shelter for numerous species of Eurosiberian and Boreal origin. Many of these species, surviving at the southern limit of their range in Europe and surrounded by Mediterranean ones, are relatively intolerant to summer drought and are in grave danger of loss, as a result of increasingly long and frequent droughts in this region. This is the case of the Scots pine (Pinus sylvestris) and the Austrian pine (Pinus nigra ssp. salzmannii) which are found on Central Iberian Peninsula at the edge of their natural range. We used a tree ring network of these two species to reconstruct past variations in summer rainfall. The reconstruction, based upon a tree ring composite chronology of the species, dates back to 1570 (adjusted R(2) = 0.49, P < 0.000001) and captures interannual to decadal scale variability in summer precipitation. We studied the spatial representativeness of the rainfall patterns and described the occurrence rate of extremes of this precipitation. To identify associations between macroclimatic factors and tree radial growth, we employed a principal component analysis to calculate the resultant of the relationship between the growth data of both species, using this resultant as a dependent variable of a multiple regression whose independent variables are monthly mean temperature and precipitation from the average records. Spatial correlation patterns between instrumental precipitation datasets for southern Europe and reconstructed values for the 1950-1992 period indicate that the reconstruction captures the regional signal of drought variability in the study region (the origin of this precipitation is convective: thermal low pressure zones induced in the inland northeastern areas of the Iberian Peninsula). There is a clear increase in the recurrence of extreme dry events as from the beginning of twentieth century and an abrupt change to drier conditions. There appears to be a tendency toward recurrent exceptionally dry summers, which could involve a significant change for the Eurosiberian refugee species.

Increased Variability in Cold-Season Temperature since the 1930s in Subtropical China

Abstract The recent increase in the frequency of winter cold extremes has received particular attention in light of the climate's warming. Knowledge about changes in the frequency of winter cold extremes requires long-term climate data over large spatial scale. In this study, a temperature-sensitive tree-ring network consisting of 31 sampling sites collected from seven provinces in subtropical China was used to investigate the characteristics of cold-season temperature extremes during the past two centuries. The results show that the percentage of trees in a year that experienced an abnormal decrease in radial growth relative to the previous year can serve as an indicator of interannual change in January–March temperature in subtropical China. The frequency of extreme interannual decreases in cold-season temperature has increased since the 1930s. The change in cold-season temperature was significantly correlated with the intensity of the Siberian high, yet the correlation was much weaker in the period preceding the 1930s. The findings provide evidence of a frequency change in the occurrence of interannual cold-season temperature extremes in the past two centuries for subtropical China. Particularly, the pattern in the variation of cold-season temperature suggests a change in climate systems around the 1930s.

Dendroecology in Asia

Though centered in a core area of human settlement and agricultural activity since prehistoric times, and despite tremendous forest losses due to overexploitation and conversion processes, Asia still hosts large areas and a wide variety of ecologically contrasting forests. While the monsoon regions of lowland southeast Asia are covered by tropical and subtropical evergreen humid or seasonal rainforests, the middle altitudinal belt of the Himalayas and western Chinese mountain regions are home of evergreen broadleaved forests. Around the margins of the Tibetan Plateau, high-altitude conifer forests form the highest treelines in the northern hemisphere. Finally, steppe forests with long-living juniper species form the transition to the central Asian grasslands and deserts, accompanied by riverine forests along streams that allow the growth of trees in a dry and tree-hostile environment. These contrasting environmental and climatic conditions offer plenty of opportunity to study relationships between trees and their adaptation to local climatic conditions. The papers of this special issue were presented during the Second International Asian Dendrochronological Conference held in Xi’an, China, during 20–23 August 2011, where more than 150 tree-ring researchers, mainly from Asian countries, participated. This collection of articles exemplifies quite well the variety and recent directions of dendroecological research in Asia. Zhang et al. use a drought-sensitive network of juniper ring-width chronologies to analyse spatial and temporal patterns of moisture availability on the margin of the Asian Summer Monsoon realm on the northeastern Tibetan Plateau during the past 150 years and report about climatic teleconnections with the North Atlantic climate. Cai et al. study the drought history of the past 130 years at the arid margin of forests in the Yellow River catchment area, the eastern limit of the Asian Summer Monsoon influence. Both papers illustrate how tree-ring networks can enable the detection of shifts in current atmospheric circulation patterns. Although the number of publications about reconstructions of various climatic parameters over Asia has literally boomed during the past years, there are still huge gaps of knowledge about climate–growth relationships in different climatological realms. One important issue is the lack of local climate stations in mountainous areas and their restriction to valley sites that may cause ambiguities and obstacles to calibrate climate–growth relationships of highelevation sites. On the other hand, the floristic composition of forest communities often stays relatively stable over hundreds of meters of elevation difference, raising the question whether there do exist differences in the growthlimiting climatic factors within one tree species along altitudinal gradients. The studies of Gao et al. and He et al. on different juniper species near the western distribution limit of tree growth on the Tibetan Plateau reveal a strong coherence of climate–growth relationships over altitudinal belts. On the other hand, these relationships may vary with time under different climatic conditions, which advise some caution regarding the inconsiderate application of present onto the past, a result confirmed by the study of Zhang et al. On the other hand, Liu et al. demonstrate that an extension of the growing season length due to temperature increase has not yet been accompanied by increasing growth of high-elevation Smith fir, giving credibility that temperature-limited sites on the southern Tibetan Plateau are still reliable indicators for temperature variations. This is of great interest, since phenology and tree height growth A. Brauning (&) Erlangen, Germany e-mail: abraeuning@geographie.uni-erlangen.de

A hydroclimatic regionalization of central Mongolia as inferred from tree rings

In arid and semi-arid regions of the world, such as Mongolia, the future of water resources under a warming climate is of particular concern. The influence of increasing temperatures on precipitation is difficult to predict because precipitation trends in coming decades could have a high degree of spatial variability. In this study, we applied a rotated principal component analysis (RPCA) to a network of 20 tree-ring chronologies across central Mongolia from 1790 to 1994 to evaluate spatial hydroclimatic variability and to place recent variability in the context of the past several centuries. The RPCA results indicate that the network consists of four tree-growth anomaly regions, which were found to be relatively stable through time and space. Correlation analyses reveal spatial linkages between the tree-growth anomalies and instrumental data, where annual streamflow variability was strongly associated with tree-growth anomalies from their respective regions from 1959 to 1994 (r=0.52–0.64, p<0.05). This study highlights the extent of spatial variability in hydroclimate across central Mongolia and emphasizes the value of using tree-ring networks in locations with limited instrumental records.

Site- and species-specific responses of forest growth to climate across the European continent

Aim To evaluate the climate sensitivity of model-based forest productivity estimates using a continental-scale tree-ring network.

500 years of regional forest growth variability and links to climatic extreme events in Europe

Climatic extreme events strongly affect forest growth and thus significantly influence the inter-annual terrestrial carbon balance. As we are facing an increase in frequency and intensity of climate extremes, extensive empirical archives are required to assess continental scale impacts of temperature and precipitation anomalies. Here we divide a tree-ring network of approximately 1000 sites into fifteen groups of similar high-frequency growth variability to reconstruct regional positive and negative extreme events in different parts of Europe between 1500 and 2008. Synchronized growth maxima or minima within and among regions indicate eighteen years in the pre-instrumental period and two events in the 20th century (1948, 1976) with extensive radial growth fluctuations. Comparisons with instrumental data showed that the European tree-ring network mirrors the spatial extent of temperature and precipitation extremes, but the interpretation of pre-instrumental events is challenged by lagged responses to off-growing season climate extremes. We were able to attribute growth minima in subsequent years to unfavourable August–October conditions and to mild climate during winter months associated with respiratory carbon losses. Our results emphasize the importance of carry-over effects and species-specific growth characteristics for forest productivity. Furthermore, they promote the use of regional tree-ring chronologies in research related to climate variability and terrestrial carbon sink dynamics.

Winter Arctic Oscillation signal in a European tree-ring δ18O network

In South America, the arid and semiarid subtropical regions through the Atacama Desert and north-central Chile between 19° and 32°S are currently a gap in the tree-ring chronology network. Only a short tree-ring chronology has been published for this vast region and little is known about the suitability of many woody species for tree-ring analysis and dendroclimatology. In this paper we present the first detailed analysis of the climate responses and influences of the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) on the tree-rings of three species new to dendrochronology and that typically occur at high elevations in the Andes of central and north-central Chile. Three well-replicated tree-ring chronologies of Kageneckia angustifolia, Proustia cuneifolia and Fabiana imbricata are compared with century-long regional records of precipitation and temperature, and with the N3.4 SST and PDO indices in both time and frequency-domains using correlation and wavelet analysis. The radial growth of these species is controlled by winter precipitation and is also positively correlated with temperature during most of the rainy season from April to September (autumn–spring). The regional climate as well as tree growth is strongly modulated by ENSO and ENSO-like conditions in the equatorial Pacific at both interannual and interdecadal timescales. The decadal and interdecadal variability is not correlated with PDO and appears to be related to the Interdecadal Pacific Oscillation (IPO), a Pacific-wide ENSO-like mode of climate variability. Despite their relatively short lifespans, these three new species have a high potential for dendrochronological and dendroclimatic studies in the semiarid region of Chile over the last two centuries.

A new atmospheric circulation tree-ring index (ACTI) derived from climate proxies: Procedure, results and applications

This paper introduces a new procedure to investigate the influence of weather types on spatio-temporal tree-ring growth patterns. Within the framework of the procedure a new proxy-based climate index termed as the atmospheric circulation tree-ring index (ACTI) is calculated that combines weather-type classification data with climate proxies. The new approach is tested using a Central European multi-species tree-ring width network consisting of 115 sites. The procedure used to calculate the ACTI time series includes: 1. Monte Carlo simulations, 2. multiple testing correction, and 3. selection and grouping algorithms. The tree-ring width series are grouped according to their weather-type responses, and the ACTI time series were computed for each group.The new procedure explains the climate signal preserved in tree-ring chronologies more comprehensively because the complex interaction between the different climate elements is taken into account by using weather-type classification data. Strong correlations between the ACTI and tree-ring width series (r=0.69) and between the ACTI time series and climate gridded datasets (r>0.6) were observed. Further a correlation between ACTI and northern hemisphere mean temperatures (r=0.5) were found. These results indicate that the ACTI enables the investigation of weather-type growth relationships.

Pre-monsoon precipitation signal in tree rings of timberline Betula utilis in the central Himalayas

Himalayan birch (Betula utilis D. Don) is a long-lived, broadleaf tree species native to the Himalayas. However, it has received limited attention for dendroclimatological studies. Based on 49 tree-ring cores from 41 Himalayan birch trees at two sites in the Langtang National Park, central Nepal, a 458-year chronology (back to AD 1552) was developed. To date, this is the longest for this species in the Himalayas despite a low sample depth before AD 1785. The chronology statistics show the potential of Himalayan birch for dendroclimatology, as indicated by a positive correlation with precipitation in May and March–May (p < 0.001) and an inverse relationship with temperature in May and precipitation in August (p < 0.05). The Himalayan birch ring-width chronology is thus an indicator for pre-monsoon precipitation variations in the central Himalayas. The wide distribution of Himalayan birch in High Asia presents an outstanding opportunity for developing a large-scale, single-species tree-ring network.

A long-term perspective on a modern drought in the American Southeast

The depth of the 2006–9 drought in the humid, southeastern US left several metropolitan areas with only a 60–120 day water supply. To put the region’s recent drought variability in a long-term perspective, a dense and diverse tree-ring network—including the first records throughout the Apalachicola–Chattahoochee–Flint river basin—is used to reconstruct drought from 1665 to 2010 CE. The network accounts for up to 58.1% of the annual variance in warm-season drought during the 20th century and captures wet eras during the middle to late 20th century. The reconstruction shows that the recent droughts are not unprecedented over the last 346 years. Indeed, droughts of extended duration occurred more frequently between 1696 and 1820. Our results indicate that the era in which local and state water supply decisions were developed and the period of instrumental data upon which it is based are amongst the wettest since at least 1665. Given continued growth and subsequent industrial, agricultural and metropolitan demand throughout the southeast, insights from paleohydroclimate records suggest that the threat of water-related conflict in the region has potential to grow more intense in the decades to come.

Climatic response of ring width and maximum latewood density of Larix sibirica in the Altay Mountains, reveals recent warming trends

Siberian larch (Larix sibirica) is a highly climate sensitive species. Presently, the Altay Mountains is covered by widespread forests dominated by Siberian larch and thus has a great potential for dendroclimatological studies. However, tree-ring network of the Altay Mountains has not yet been well developed. The development of the new chronologies and the knowledge about the influence of climatic variables on tree growth is needed. X-ray densitometric techniques were applied to obtain ring width (RW) and maximum latewood density (MXD) of Siberian larch from two upper tree line sites in the Altay Mountains, China. Climatic responses in ring widths and maximum latewood densities from the Altay Mountains (China, Russia, and Mongolia) were investigated by simple correlation analyses. To assess the common growth forces among the individual sites of the Altay Mountains, simple correlation, principal component analyses, and spatial correlation analysis were applied over the common period of the chronologies. Ring width and maximum latewood density increases with decreasing precipitation, increasing temperature from late spring to late summer during the growing season. Based on the results of principal component analyses and spatial correlation analysis, summer temperature (June–July) is the most important forces on the Siberian larch growth of the Altay Mountains. The growth of Siberian larch in the Altay Mountains captures the current warming trend. The growth of Siberian larch did not clearly lose its sensitivity under most recent warming in our study areas. The new MXD chronologies is presently the longest, absolutely dated, tree-ring density record yet developed from China. The climate response analysis shows that the RW and MXD of Siberian larch have strong responses to temperature in the growing season. Thus, MXD and RW of Siberian larch provides the best information for climate reconstruction in the warm season. Tree-rings of Siberian larch allow detecting the recently observed warming trend and putting it into the long-term climatic context in the Altay Mountains, due to the strong growth sensitivity to temperature change.

Annual increments of juniper dwarf shrubs above the tree line on the central Tibetan Plateau: a useful climatic proxy

Dendroclimatology is playing an important role in understanding past climatic changes on the Tibetan Plateau. Forests, however, are mainly confined to the eastern Tibetan Plateau. On the central Tibetan Plateau, in contrast, shrubs and dwarf shrubs need to be studied instead of trees as a source of climate information. The objectives of this study were to check the dendrochronological potential of the dwarf shrub Wilson juniper (Juniperus pingii var. wilsonii) growing from 4740 to 4780 m a.s.l. and to identify the climatic factors controlling its radial growth. Forty-three discs from 33 stems of Wilson juniper were sampled near the north-eastern shore of the Nam Co (Heavenly Lake). Cross-dating was performed along two directions of each stem, avoiding the compression-wood side as far as possible. A ring-width chronology was developed after a negative exponential function or a straight line of any slope had been fit to the raw measurements. Then, correlations were calculated between the standard ring-width chronology and monthly climate data recorded by a weather station around 100 km away. Our study has shown high dendrochronological potential of Wilson juniper, based on its longevity (one individual was 324 years old), well-defined growth rings, reliable cross-dating between individuals and distinct climatic signals reflected by the ring-width variability. Unlike dwarf shrubs in the circum-arctic tundra ecosystem which positively responded to above-average temperature in the growing season, moisture turned out to be growth limiting for Wilson juniper, particularly the loss of moisture caused by high maximum temperatures in May-June. Because of the wide distribution of shrub and dwarf shrub species on the central Tibetan Plateau, an exciting prospect was opened up to extend the presently existing tree-ring networks far up into one of the largest tundra regions of the world.

Characteristics of extreme droughts inferred from tree-ring data in the Qilian Mountains, 1700−2005

Severe droughts were evaluated using a tree-ring network of 12 chronologies (1700−2005) from the Qilian Mountains on the northeastern Tibetan Plateau. A total of 98 drought years, including 78 moderate, 15 severe, and 5 extreme events, were identified for the past 306 yr. Following the starting year of a drought phase, drought commonly continued for 1 to 2 yr. The spatial patterns of moisture conditions in central and eastern China corresponding to 19 major drought years during the period 1700−2000 were grouped into 3 clusters, with one particular pattern showing droughts of nation-wide impact (i.e. 1721, 1928 and 1966). Drought events became more frequent during the 20th century. Moreover, the multi-year event during the late 1920s and early 1930s was the longest in the past 306 yr. As shown by the tree-ring data, this extreme drought event occurred first in the western Qilian Mountains, and then progressed gradually toward the central and eastern parts of the area, culminating in 1928. Comparison with historical documents demonstrates that the climatic information imbedded in the tree-ring network can provide quantitative measures of drought severity and fill the gaps where historical documents are lacking and/or incomplete.

Regional-scale winter-spring temperature variability and chilling damage dynamics over the past two centuries in southeastern China

Winter-spring cold extreme is a kind of serious natural disaster for southeastern China. As such events are recorded in discrete documents, long and continuous records are required to understand their characteristics and driving forces. Here we report a regional-scale winter-spring (January–April) temperature reconstruction based on a tree-ring network of pine trees (Pinus massoniana) from five sampling sites over a large spatial scale (25–29°N, 111–115°E) in southeastern China. The regional tree-ring chronology explains 48.6% of the instrumental temperature variance during the period 1957–2008. The reconstruction shows six relatively warm intervals (i.e., ~1849–1855, ~1871–1888, ~1909–1920, ~1939–1944, ~1958–1968, 1997–2007) and five cold intervals (i.e., ~1860–1870, ~1893–1908, ~1925–1934, ~1945–1957, ~1982–1996) during 1849–2008. The last decade and the 1930s were the warmest and coldest decades, respectively, in the past 160 years. The composite analysis of 500-hPa geopotential height fields reveals that distinctly different circulation patterns occurred in the instrumental and pre-instrumental periods. The winter-spring cold extremes in southeastern China are associated with Ural-High ridge pattern for the instrumental period (1957–2008), whereas the cold extremes in pre-instrumental period (1871–1956) are associated with North circulation pattern.

Geographically structured and temporally unstable growth responses of Juniperus thurifera to recent climate variability in the Iberian Peninsula

Geographically structured tree-ring networks are needed to fully understand the spatiotemporal variability in climatic sensitiveness of trees and to study their future responses to global warming. We aim to identify the spatially constrained structure of radial-growth patterns of the Spanish juniper (Juniperus thurifera L.) and to assess whether their climate–growth responses were unstable during the late twentieth century. Tree–ring width chronologies were built for 13 J. thurifera stands in Spain using dendrochronological methods and related to monthly climatic data. Sites were grouped according to their growth patterns using hierarchical cluster analysis. The relationships among geographical, climatic and stand features and their influence on radial growth were evaluated using redundancy analysis. The climate–growth relationships and their temporal stability were assessed using Pearson’s and moving bootstrapped correlations, respectively. Stands formed three geographical groups according to their high-frequency growth variation: North West and Centre, North East and South East. We found that J. thurifera radial-growth patterns depended on geographical and climatic factors, but not on the stand structure, and responded to a northwest–southeast gradient of decreasing rainfall and influence of Atlantic Westerlies and Mediterranean cyclonic activity. The positive response to June precipitation was unstable during the late twentieth century and started earlier in populations from western mesic sites than in eastern xeric sites. This pattern may be related to either decreasing water availability in western than in eastern sites or the resilience of J. thurifera growth from xeric sites in response to the increasing summer aridity.

Large-Scale Precipitation Variability over Northwest China Inferred from Tree Rings

Abstract A preliminary study of a point-by-point spatial precipitation reconstruction for northwestern (NW) China is explored, based on a tree-ring network of 132 chronologies. Precipitation variations during the past ~200–400 yr (the common reconstruction period is from 1802 to 1990) are reconstructed for 26 stations in NW China from a nationwide 160-station dataset. The authors introduce a “search spatial correlation contour” method to locate candidate tree-ring predictors for the reconstruction data of a given climate station. Calibration and verification results indicate that most precipitation reconstruction models are acceptable, except for a few reconstructions (stations Hetian, Hami, Jiuquan, and Wuwei) with degraded quality. Additionally, the authors compare four spatial precipitation factors in the instrumental records and reconstructions derived from a rotated principal component analysis (RPCA). The northern and southern Xinjiang factors from the instrumental and reconstructed data agree well with each other. However, differences in spatial patterns between the instrumentation and reconstruction data are also found for the other two factors, which probably result from the relatively poor quality of a few stations. Major drought events documented in previous studies—for example, from the 1920s through the 1930s for the eastern part of NW China—are reconstructed in this study.

Climate/growth relationships in a Pinus cembra high-elevation network in the Southern French Alps

In the context of climate change, assessing climate–growth relationships is of high importance in order to understand how forest ecosystems evolve and to test climate models at regional scale. This study aims to identify the climate variables that explain most of the variation in Pinus cembra radial growth at different spatial–temporal scales, response functions and moving response functions were processed on chronologies from a dense tree ring network. This original network is centred on the French Alps influenced by the Atlantic, Continental, and Mediterranean synoptic systems. A spatial clustering pattern matching the latitudinal climatic gradient was observed in tree ring indexed chronologies and climate responses. The stationary response functions computed for each cluster-averaged population evidenced a thermo-dependent northern cluster (Cl1), a rainfall-dependent southern cluster (Cl3), and an alternative response for the intermediate cluster in between (Cl2). Since the late nineteenth century, the moving response functions indicate an increasing influence of winter precipitation for the northern and the intermediate clusters. Considering the increase in temperature and reduction both of summer precipitation predicted by general circulation models and of the snow cover and duration in the French Alps, stone pine is likely to be exposed to stressful conditions during the twenty-first century.

Reconstructing drought variability for Mongolia based on a large‐scale tree ring network: 1520–1993

Previous tree ring based hydrologic studies in Mongolia have been regional in scale. Here, we present a large‐scale summer drought reconstruction for Mongolia that reveals the main summer moisture patterns of the past. This reconstruction is based on a network of tree ring chronologies that span the country. The resulting drought model explains 61% of the variance and is extended to cover 1520–1993 by using a nested approach to modeling. Severe droughts and harsh winter conditions occurred in Mongolia from ∼1999–2002 and contributed to massive livestock mortality and economic loss. These droughts were extreme in the context of the past several hundred years. Significant periodicities are found at 40 (90%), 19.3–25 (99%), 11.6 (95%), 6.4–7.2 (95%), and 2.8 years (99%). This high‐resolution drought reconstruction and associated tree ring chronologies supplement and extend the sparse meteorological data in Mongolia and can be used to better understand climate variability and potential forcings of climate.

Regional extreme climate events on the northeastern Tibetan Plateau since AD 1450 inferred from tree rings

Qilian juniper ( Juniperus przewalskii Kom.) is a widely distributed tree species growing on south-facing slopes in the northeastern Tibetan Plateau in arid northwestern China. We established a tree-ring width network based on two new chronologies and four previously published chronologies. Correlation and response function analyses demonstrate that precipitation positively influences radial growth. Despite of minor differences in local climate–growth relations, precipitation for the annual window between previous July and current June shows consistent positive correlations with ring width at all study sites. Similar to the so called ‘pointer year’ approach, ‘anomalous’ growth years were defined to extract extreme climate events for the period AD 1450–2006. We defined a dryness–wetness grade series with five grades of climate events inferred from anomalous year analysis. During the last 50 years, the frequency of wet events increased and that of drought events decreased noticeably, implying that the probability of occurrence of dry years in the northeastern Tibetan Plateau will further decrease in the future if regional warming continues. Combining our proxy records with a historical dryness–wetness record from eastern China, we mapped dryness–wetness patterns over large parts of China. By analyzing the atmospheric pressure patterns at the 850 hPa level over China for selected extreme event years, we found that the confluence of cold and hot air is a precondition for a flood event in the northeastern Tibetan Plateau. Thus, a counter-clockwise atmospheric circulation centered in south of Lake Baikal only occurs in flood event years.