Tree Growth In Regions Research Articles

Unrevealing the water-use strategies for typical ecological restoration plants and cash crops in the Eastern Chinese Loess Plateau region

Study regionFour geomorphic types in the eastern Loess Plateau (ECLP). Study focusquantitatively study the water-use strategies of typical plants (ecological restoration plants, food crops) based on stable isotope technology. New hydrological insights for the regionSignificant spatial-temporal heterogeneity appeared in regional soil water content and stable isotopes, with more significant fluctuations in soil water isotope values appearing in the rocky mountain region. Regional tree growth (including natural and plantation forests) was mainly influenced by precipitation (35±14 %) and surface layer (0–40 cm depth) soil water (25±11 %) during April and June, whereas deep soil water gradually became the main water source (> 20 %) in autumn and winter. A. Lavandulifolia mainly uses surface soil water (> 23 %), and the proportion of deep soil water utilization has increased after September. Surface soil water (0–40 cm) was the primary water source for T. aestivum during the overwintering (> 16 %) and reviving periods (> 26 %), and Z. mays utilized the largest proportion of soil water from 0 to 20 cm depth layer during the seeding (> 60 %) and maturation stages (> 35 %). Collectively, the results of this study have important implications for sustainable vegetation protection and the optimal allocation of water resources in the arid region.

Monitoring of Monthly Height Growth of Individual Trees in a Subtropical Mixed Plantation Using UAV Data

The assessment of changes in the height growth of trees can serve as an accurate basis for the simulation of various ecological processes. However, most studies conducted on changes in the height growth of trees are on an annual scale. This makes it difficult to obtain basic data for correcting time differences in the height growth estimates of trees within a year. In this study, the digital elevation models (DEMs) were produced based on stereo images and light detection and ranging (LiDAR) data obtained by unmanned aerial vehicles (UAVs). Individual tree crowns were segmented by employing the watershed segmentation algorithm and the maximum value within each crown was extracted as the height of each tree. Subsequently, the height growth of each tree on a monthly-scale time series was extracted to simulate the time difference correction of regional tree height estimates within a year. This was used to verify the feasibility of the time difference correction method on a monthly scale. It is evident from the results that the DEM based on UAV stereo images was closely related to the DEM based on UAV LiDAR, with correlation coefficients of R2 = 0.96 and RMSE = 0.28 m. There was a close correlation between the tree height extracted from canopy height models (CHMs) based on UAV images and the measured tree height, with correlation coefficients of R2 = 0.99, and RMSE = 0.36 m. Regardless of the tree species, the total height growth in each month throughout the year was 46.53 cm. The most significant changes in the height growth of trees occurred in May (14.26 cm) and June (14.67 cm). In the case of the Liriodendron chinense tree species, the annual height growth was the highest (58.64 cm) while that of the Osmanthus fragrans tree species was the lowest (34.00 cm). By analyzing the height growth estimates of trees each month, it was concluded that there were significant differences among various tree species. In the case of the Liriodendron chinense tree species, the growth season occurred primarily from April to July. During this season, 56.92 cm of growth was recorded, which accounted for 97.08% of the annual growth. In the case of the Ficus concinna tree species, the tree height was in a state of growth during each month of the year. The changes in the height growth estimates of the tree were higher from May to August (44.24 cm of growth, accounting for 77.09% of the annual growth). After applying the time difference correction to the regional tree growth estimates, the extraction results of the changes in the height growth estimates of the tree (based on a monthly scale) were correlated with the height of the UAV image-derived tree. The correlation coefficients of R2 = 0.99 and RMSE = 0.26 m were obtained. The results demonstrate that changes in the height growth estimates on a monthly scale can be accurately determined by employing UAV stereo images. Furthermore, the results can provide basic data for the correction of the time differences in the growth of regional trees and further provide technical and methodological guidance for regional time difference correction of other forest structure parameters.

CPSDv0: a forest stand structure database for plantation forests in China

ABSTRACT Forest stand structure is not only a crucial factor for regulating forest functioning but also an important indicator for sustainable forest management and ecosystem services. Although there exists a few national/global structure databases for natural forests, a country-wide synthetic structure database for plantation forests over China, the world’s largest player in plantation forests, has not been achieved. In this study, we built a country-wide synthetic stand structure database by surveying more than 600 peer-reviewed literature. The database covers tree species, mean stand age, mean tree height, stand density, canopy coverage, diameter at breast height, as well as the associated ancillary in-situ topographical and soil properties. A total of 594 published studies concerning diverse forest stand structure parameters were compiled for 46 tree species. This first synthesis for stand structure of plantation forests over China supports studies on the evolution/health of plantation forests in response to rapid climate change and intensified disturbances, and benefits country-wide sustainable forest management, future afforestation or reforestation planning. Potential users include those studying forest community dynamics, regional tree growth, ecosystem stability, and health, as well as those working with conservation and sustainable management. This dataset is freely accessible at http://www.doi.org/10.11922/sciencedb.j00076.00091.

The Spatially Inhomogeneous Influence of Snow on the Radial Growth of Schrenk Spruce (Picea schrenkiana Fisch. et Mey.) in the Ili-Balkhash Basin, Central Asia

Snow has an important impact on forest ecosystems in mountainous areas. In this study, we developed 14 tree-ring-width chronologies of Schrenk spruce (Picea schrenkiana Fisch. et Mey.) for the Ili-Balkhash Basin (IBB), Central Asia. We analyzed the response of radial growth to temperature, precipitation and snow parameters. The results show that previous winter and current summer precipitation have an important influence on the radial growth of P. schrenkiana. Further, we find spatially inhomogeneous effects of snow on subsequent growing-season tree growth in IBB. The radial growth response of P. schrenkiana to snow shows a weak–strong–weak trend from west to east across the Ili-Balkhash Basin. This spatial difference is mainly related to precipitation, as snow has little effect on tree growth in regions that receive more precipitation. Thus, winter snow has an important influence on the radial growth of trees in regions that receive limited amounts of precipitation.

Environmental Dynamics of the Ribbon-Like Pine Forests in the Parklands of North Kazakhstan

The ribbon-like pine forests of North Kazakhstan represent the principal territorial intrazonal and azonal biotopes. Integrated bio-geographic studies of the pine forests’ status were performed in the Beskaragai and Chaldai Nature Reserves in the Pri-Irtysh River basin within, at present, the climate–change most susceptible transitional parkland-steppe zone of Central Asia, adjoining the West Siberian Lowland. The investigations followed the regional topographic gradient with a series of mapped sites characterizing the spatial relief patterns of the pristine forest distribution and the associated phytocenoses. The results revealed marked natural arboreal cover restoration differences between the geographically close upland and lowland forest ecosystems. The regional tree growth dynamics show the varying intensity of the pine seedlings’ succession, the tree stands’ biomass productivity and the environmental stability, weakened by the extreme continentality and progressing aridification along with adverse anthropogenic ecological impacts. The specific geomorphic, soil and hydrological conditions are the principal determining factors. The more vital plain and lowland pine forests host the floristically richer fescue-dominated communities compared to the more fragile and precipitation-poorer upland pine settings. The latter forest ecosystems display a higher vulnerability to the current climate change, generating tree drying, forest fires, and to modern human activities such as logging, herding and recreation. The research conclusions provide new insights on the natural ribbon-like pine forests’ sustainability and adaptation to the ongoing continental warming triggering fundamental environmental transformations in Central Asia’s parklands.

Drought Drives Growth and Mortality Rates in Three Pine Species under Mediterranean Conditions

Drought constrains tree growth in regions with seasonal water deficit where growth decline can lead to tree death. This has been observed in regions such as the western Mediterranean Basin, which is a climate-warming hotspot. However, we lack information on intra- and inter-specific comparisons of growth rates and responses to water shortage in these hotspots, considering tree species with different drought tolerance. We sampled several sites located in north-eastern Spain showing dieback and high mortality rates of three pine species (Pinus sylvestris, Pinus pinaster, Pinus halepensis). We dated death years and reconstructed the basal area increment of coexisting living and recently dead trees using tree ring data. Then, we calculated bootstrapped Pearson correlations between a drought index and growth. Finally, we used linear mixed-effects models to determine differences in growth trends and the response to drought of living and dead trees. Mortality in P. sylvestris and P. pinaster peaked in response to the 2012 and 2017 droughts, respectively, and in sites located near the species’ xeric distribution limits. In P. halepensis, tree deaths occurred most years. Dead trees showed lower growth rates than living trees in five out of six sites. There was a strong growth drop after the 1980s when climate shifted towards warmer and drier conditions. Tree growth responded positively to wet climate conditions, particularly in the case of living trees. Accordingly, growth divergence between living and dead trees during dry periods reflected cumulative drought impacts on trees. If aridification continues, tree drought mortality would increase, particularly in xeric distribution limits of tree species.

Intra-Annual Wood Formation of Cryptomeria fortunei and Cunninghamia lanceolata in Humid Subtropical China

Monitoring cambial activity is important for a better understanding of the mechanisms governing xylem growth responses to climate change, providing a scientific basis for tree-ring-based climate reconstructions and projections about tree growth under future climate scenarios. It plays an even more important role in investigating evergreen tree growth in regions with less distinct seasonal cycles. Subtropical evergreen forests have been studied in recent years for their sensitivity to climate change, but it remains unclear how xylem growth is driven by subtropical climates. To further understand the climate-growth response strategies of subtropical conifers, we micro-cored Cryptomeria fortunei and Cunninghamia lanceolata weekly in 2016 and 2017 at the humid subtropical Gushan Mountain in southeastern China. Our weekly growth monitoring showed that the vegetation periods of these two species were both approximately 2–3 months longer than trees in temperate and boreal forests. The growth of C. fortunei in 2016 and 2017 and C. lanceolata in 2017 showed a bimodal pattern of xylogenesis, which was induced by summer drought. The results also indicated that the earlier end of the xylem formation was related to the yearly drought stress. These findings provide more specific information about tree growth and evidence of how climate influences wood production at the cellular level in subtropical regions.

A Tree-Ring-Based Assessment of Pinus armandii Adaptability to Climate Using Two Statistical Methods in Mt. Yao, Central China during 1961–2016

Assessing the characteristics and limiting factors of tree growth is of practical significance for environmental studies and climatic reconstruction, especially in climate transition zones. In this study, four sites of Pinus armandii Franeh are investigated to understand regional climate-tree growth response in Mt. Yao, central China. Based on the high similarity of four residual chronologies and high correlations between chronologies and climatic factors, we analyzed the correlations of regional residual chronology with monthly climatic factors and the self-calibrating Palmer Drought Severity Index (scPDSI) from 1961–2016. The results indicate that the hydrothermal combination of prior August and current May and the scPDSI in May are main limiting factors of regional tree growth in Mt. Yao. The results of stepwise regression models also show that temperature and scPDSI in May are the main limiting factors of tree growth, but the limiting effect of scPDSI is more than temperature in this month. Through the analysis of the number of tree growth years corresponding to high temperature and high scPDSI, it was further confirmed that scPDSI in May is the main limiting factor on the growth of P. armandii in Mt. Yao. However, the influence of scPDSI in May has weakened, while temperature in May has increasingly significant influence on tree growth. The above findings will help improve our understanding of forest dynamics in central China under global climate change.

High-altitude tree growth responses to climate change across the Hindu Kush Himalaya

AbstractAimsRapid warming at high altitudes may lead to a higher sensitivity in tree growth to temperature. The key factors constraining tree radial growth and to what extent regional tree growth has suffered from climatic changes are unclear.MethodsTree-ring width data were collected from 73 sites across the Hindu Kush Himalaya (HKH), including three dominant genera (Abies, Juniperus and Picea) at high altitudes over 3000 m. Dynamic time warping was introduced to develop subregional chronologies by considering the synchrony of annual tree growth among different sites. We quantified the contribution of the climate variables, and analyzed the spatiotemporal variation of the growth–climate relationship.Important FindingsThe site chronologies were grouped into three clusters, corresponding to the three distinct bioclimatic zones, i.e. the western HKH, central-eastern HKH and southeastern Tibetan Plateau (TP). Tree growth was positively correlated to winter and spring precipitation in the drier western HKH, and to winter temperature and spring precipitation in the humid southeastern TP. Tree growth was markedly constrained by the minimum temperature, especially in winter, with its importance increasing from the west toward the east. As shown by moving correlation analysis, the signal of winter temperature in tree growth was weakened in the western and central-eastern HKH, while it was enhanced in the southeastern TP following rapid warming since the 1980s. Our results highlight that continuous warming may cause forest recession due to warming-induced moisture deficit in the western HKH, but forest expansion in the southeastern TP.

Kauri Tree‐Ring Stable Isotopes Reveal a Centennial Climate Downturn Following the Antarctic Cold Reversal in New Zealand

AbstractThe dynamics of the Late Glacial have been demonstrated by numerous records from the Northern Hemisphere and far fewer from the Southern Hemisphere (SH). SH paleoclimate records reveal a general warming trend, interrupted by a deglaciation pause Antarctic Cold Reversal (ACR; ∼14,700–13,000 cal BP). Here, we present decadal tree‐ring stable isotope chronologies (δ18O, δ13C) from New Zealand (NZ) subfossil kauri trees (n = 6) covering the post‐ACR millennium from 13,020 to 11,850 cal BP. We find a distinct, simultaneous downturn (∼12,625–12,375 cal BP) in all tree‐ring proxies paralleling regional tree growth declines, suggesting a widespread climate deterioration. This downturn was characterized by sustained high precipitation, low temperatures, and high relative humidity in NZ with incoming weather fronts from the South Ocean. Despite these promising results, questions remain about what drove the Kauri Downturn and how the hydroclimatic conditions were altered during this time period.

Seasonal radial growth dynamics of Scots pine (Pinus silvestris L.) in Voronezh region (Russia)

One of the insufficiently studied areas in dendroclimatology is the seasonal dynamics of the radial growth of trees in regions with different climatic conditions. The urgency of this problem has increased due to the observed climate changes. This paper presents the results of a three-year experimental study (2016-2018) on the seasonal growth dynamics of Scots pine in the Voronezh region, as well as the weather conditions during these growing seasons. It has been established that the Scots pine ecotype under the conditions of the forest-steppe zone demonstrates a multi-peaked cyclical dynamics of cell growth rate within the growing season. At the same time, the culmination of growth differs significantly in different calendar years. Annual averages for the following growth traits were registered for the Voronezh region: the duration of the formation of the annual ring (4 months), the onset (25.04-03.05) and the end (29.08) of annual-ring formation, the end of earlywood formation and the onset of the formation of the transition zone from earlywood to latewood (01.07) and the onset of latewood formation (01.08).

Long-term physiological and growth responses of Himalayan fir to environmental change are mediated by mean climate.

High-elevation forests are experiencing high rates of warming, in combination with CO2 rise and (sometimes) drying trends. In these montane systems, the effects of environmental changes on tree growth are also modified by elevation itself, thus complicating our ability to predict effects of future climate change. Tree-ring analysis along an elevation gradient allows quantifying effects of gradual and annual environmental changes. Here, we study long-term physiological (ratio of internal to ambient CO2 , i.e., Ci /Ca and intrinsic water-use efficiency, iWUE) and growth responses (tree-ring width) of Himalayan fir (Abies spectabilis) trees in response to warming, drying, and CO2 rise. Our study was conducted along elevational gradients in a dry and a wet region in the central Himalaya. We combined dendrochronology and stable carbon isotopes (δ13 C) to quantify long-term trends in Ci /Ca ratio and iWUE (δ13 C-derived), growth (mixed-effects models), and evaluate climate sensitivity (correlations). We found that iWUE increased over time at all elevations, with stronger increase in the dry region. Climate-growth relations showed growth-limiting effects of spring moisture (dry region) and summer temperature (wet region), and negative effects of temperature (dry region). We found negative growth trends at lower elevations (dry and wet regions), suggesting that continental-scale warming and regional drying reduced tree growth. This interpretation is supported by δ13 C-derived long-term physiological responses, which are consistent with responses to reduced moisture and increased vapor pressure deficit. At high elevations (wet region), we found positive growth trends, suggesting that warming has favored tree growth in regions where temperature most strongly limits growth. At lower elevations (dry and wet regions), the positive effects of CO2 rise did not mitigate the negative effects of warming and drying on tree growth. Our results raise concerns on the productivity of Himalayan fir forests at low and middle (<3,300m) elevations as climate change progresses.

Uneven winter snow influence on tree growth across temperate China.

Winter snow is an important driver of tree growth in regions where growing-season precipitation is limited. However, observational evidence of this influence at larger spatial scales and across diverse bioclimatic regions is lacking. Here, we investigated the interannual effects of winter (here defined as previous October to current February) snow depth on tree growth across temperate China over the period of 1961-2015, using a regional network of tree ring records, in situ daily snow depth observations, and gridded climate data. We report uneven effects of winter snow depth on subsequent growing-season tree growth across temperate China. There shows little effect on tree growth in drier regions that we attribute mainly to limited snow accumulation during winter. By contrast, winter snow exerts important positive influence on tree growth in stands with high winter snow accumulation (e.g., in parts of cold arid regions). The magnitude of this effect depends on the proportion of winter snow to pre-growing-season (previous October to current April) precipitation. We further observed that tree growth in drier regions tends to be increasingly limited by warmer growing-season temperature and early growing-season water availability. No compensatory effect of winter snow on the intensifying drought limitation of tree growth was observed across temperate China. Our findings point toward an increase in drought vulnerability of temperate forests in a warming climate.

Growth response of alpine treeline forests to a warmer and drier climate on the southeastern Tibetan Plateau

Forest growth at high altitudes and latitudes is sensitive to climate warming. However, warming-induced drought stress has decreased forest growth and survival rates, and constitutes a key uncertainty in projections of forest ecosystem dynamics. A fast warming rate has occurred over the Tibetan Plateau (TP), and the response pattern of alpine forest growth on the TP to a warmer and possibly drier climate is still unknown. By compiling tree-ring width records from ten alpine treeline ecotones (ATEs), we developed an index of regional tree growth in ATEs (RTGA) on the southeastern TP, which is a major forested region of the TP. Our results showed a stable and clear coherence between RTGA and the regional summer (June-August) minimum temperature during the studied period (1950–2012, R2 = 0.59, P < 0.001), despite a prominent drying trend since the 1990s. We conclude that warming-induced drought stress has not limited ATE forest growth on the moist southeastern TP.

Intensified variability of the El Niño–Southern Oscillation enhances its modulations on tree growths in southeastern China over the past 218 years

Lack of long‐term tree‐ring records in the core regions of the Asian summer monsoon in southeastern China limits our ability of evaluating the current climate change in a historical context. In this study, we developed the first 218‐year tree‐ring chronology (1798–2015) of Pinus massoniana in Zhangping area, Fujian Province, humid subtropical China. This chronology is positively correlated with winter–spring (January–March) temperature (r = 0.359, p &lt; .01) and summer (July–September) precipitation (r = 0.351, p &lt; .01). Although the correlations between our tree rings with sea surface temperatures (SSTs) are not very high, the correlation pattern is very close to the correlation pattern with the El Niño–Southern Oscillation variability (ENSO). These suggest that the ENSO could be the major large‐scale regulator on the growth of our tree rings. The strength of the correlations between our tree rings and the ENSO (r = 0.30, N = 66) matches closely with the ENSO variability during 1950–2015. The modulations of the ENSO on regional tree growth have been the most conspicuous since the 1950s, which corresponds to its enhanced inter‐annual variability. The extreme growth anomalies match quite well with the extreme years of the moisture‐sensitive chronologies. The dry epoch from 1935 to 1958 is the most severe long‐lasting drought in our tree rings, which is a widely distributed pattern in southeastern China and is likely modulated by the La Niña‐like modes in that period.

Climate-Driven Synchronized Growth of Alpine Trees in the Southeast Tibetan Plateau.

Knowledge about the spatiotemporal tree growth variability and its associations with climate provides key insights into forest dynamics under future scenarios of climate change. We synthesized 17 tree-ring width chronologies from four tree species at the high-elevation sites in the southeast Tibetan Plateau (SETP) to study the regional tree growth variability and climate-growth relationships. Despite of diverse habitats and different physiological characteristics of these species, these tree-ring chronologies shared a significant common variance in SETP. An unprecedented increase in the shared variance is found along the latter half of the 20th century, coinciding with the enhancement of the frequency of extreme rings among chronologies. It is found that minimum winter temperature tends to be the dominant climate for trees in this region. The site-specific responses in cold (1965–1980) and warm (1990–2005) intervals by means of Fuzzy Cmeans (FCM) clustering reveal that the remarkable enhancement of growth synchrony among trees mainly occur in warm conditions. This is different from previous findings indicating that increased consistence among temperature sensitive tree rings in cold periods. This may be related to the reduced temperature sensitivity of regional tree growth as winter minimum temperature is lower than a certain threshold, which is in agreement with the “principle of ecological amplitude”. In addition, it is worth noting that precipitation in June have started to restrain the tree growth since the beginning of the 1980s, which is possibly an important contributor for synchronized growth among trees in SETP.

Climatic forcing of xylem formation in Qilian juniper on the northeastern Tibetan Plateau

Cambial activity of ~100-year-old Qilian juniper trees initiated before the middle of May at an elevation of 3200 m a.s.l.; June and July were the main stem radial increment period. Around the middle of August, all xylem cell differentiation periods are completed. Precipitation or relative humidity is the main limiting factor for tree radial growth on the northeastern Tibetan Plateau. Previous studies have found that annual tree-ring width series of Qilian juniper (Sabina przewalskii Kom.) in the northeastern Tibetan Plateau (TP) are mostly moisture controlled, irrespective of site elevation. Knowing precisely the cambial growth dynamics during a vegetation period can lead to a better understanding of the climatic factors driving regional tree growth patterns. We observed wood formation with micro-cores taken at weekly intervals from late April to early October 2013, and monitored daily stem radial changes with high-resolution electronic point dendrometers in 30-min intervals. Dormant cambium contained 5–6 cell layers in the cambium zone. Cambial activity initiated before the middle of May in all the monitored trees. About three quarters of the total cell production or radial growth formed during June and July. Lignification of secondary walls of new xylem cells continued from end of May to middle of August. After the middle of August, the cambium entered into the inactive period. Dendrometer measurements confirmed that June and July were the main stem radial increment period. Nonparametric Kendall’s Tau correlations indicate that daily relative humidity and precipitation significantly influence stem growth in May. No obvious relationships were detected in June. In July, significant negative influences of maximum temperature and positive effects of relative humidity prevailed. Overall, growing season precipitation or associated relative humidity is the limiting factor for tree stem radial increments on the intra-annual scale. Our results present a thorough understanding of Qilian juniper xylogenesis and its climate forcing within the whole growing season on the northeastern TP.

Dendroclimatic reconstruction of May–June maximum temperatures in the central Zagros Mountains, western Iran

AbstractLong‐term instrumental records that provide estimates about the natural range of long‐term climate fluctuation are lacking in Iran. Ring‐width chronologies were developed from two oak sites in the central Zagros Mountains, dating back to 1705 A.D. Due to high correlations between the local site chronologies, a mean regional chronology (RC) was calculated. Correlations with regionalized climate data showed significant negative correlations with maximum temperature in December of the previous year and February, May, June and August of the current year. However, May–June maximum temperatures of the current growing season showed the highest negative correlation coefficient with regional tree growth patterns. Based on this relationship, maximum temperature of May–June was reconstructed over the last 170 years (1840–2010). The average length of cold periods was shorter than that of warm periods. Further sampling may allow the analysis of long‐term climatic trends in this region.

The effect of forest-thinning works on tree growth and forest environment

This research was conducted to provide effective forest management through forest tree production and forest land changes which took place after the forest-tending works in the plantation of Pinus koraiensis. It has been shown that the current annual diameter growth of trees in regions which implemented forest-tending projects in 2005 had increased by 142.9% compared to those which did not implement the forest-tending projects. In 2006, the current annual diameter growth of trees in implemented regions increased by 132.6% compared to those which did not implement forest-tending. Additionally, in the Pinus koraiensis plantation of Ⅲ·Ⅳ age classes where forest-tending projects works were implemented, the growth in diameter started to show a clear increase in the year following the implementation, and the maximum effects were displayed within 3–5 years. Thereafter, the growth gradually started to slow down. Therefore, implementing forest-tending works repetitively with a minimum period of 5 years would be considered appropriate. Soil moisture during the rainy season did not show significant differences as the soil moisture content remained at 40.4–43.0%. However, during the dry season, regions which did not implement forest-tending showed an average of 20.4%, and regions which implemented forest-tending showed an average of 26.6%, and the differences were greater when compared to the rainy season. Due to the control in the density of the number of forest trees due to forest-tending and forest thinning works, the number of understory woody plant species increased from 17 to 30 and the number of herbaceous plant species increased from 20to 44. Furthermore, as the density of standing trees was reduced to about 50% in places where forest-tending works were implemented, crown length ratios increased greatly from 46.1% to 62.1% and became a great help to the growth of forest trees.

Precipitation variations in the central Zagros Mountains (Iran) since A.D. 1840 based on oak tree rings

We established three tree-ring width chronologies from oak species in the central Zagros Mountains, Iran, where instrumental climate data only cover a short period (1951–2010). Due to high similarity of the individual ring-width series, we included all studied sites in a regional chronology covering 306years. Correlation functions with regional climate data revealed significant positive correlations (p<0.01) with total precipitation from previous October to current May. Thus, we reconstructed regional precipitation variability over the period 1840–2010 based on linear regression between regional tree growth and climate data. Dry conditions prevailed in the 1840s, 1850, 1870s, 1880s, 1900s, 1920s, 1940s, 1960s, 1980s and 1999–2003. In comparison to wet periods, the number and the intensity of dry periods have increased during the 1840–2010 study period. Many of the reconstructed wet years coincided with major El Niño events. Inverse association between the Southern Oscillation Index (SOI) and the reconstructed precipitation during the common period (1877–2010) revealed that the Southern Oscillation Index (SOI) plays an important role in regional precipitation variability.