VS-oscilloscope: A new tool to parameterize tree radial growth based on climate conditions

It is generally assumed in dendroecological studies that annual tree-ring growth is adequately determined by a linear function of local or regional precipitation and temperature with a set of coefficients that are temporally invariant. However, various researchers have maintained that tree-ring records are the result of multivariate, often nonlinear biological and physical processes. To describe critical processes linking climate variables with tree-ring formation, the process-based tree-ring Vaganov–Shashkin model (VS-model) was successfully used. However, the VS-model is a complex tool requiring a considerable number of model parameters that should be re-estimated for each forest stand. Here we present a new visual approach of process-based tree-ring model parameterization (the so-called VS-oscilloscope) which allows the simulation of tree-ring growth and can be easily used by researchers and students. The VS-oscilloscope was tested on tree-ring data for two species (Larix gmeliniiand Picea obovata) growing in the permafrost zone of Central Siberia. The parameterization of the VS-model provided highly significant positive correlations (p < 0.0001) between simulated growth curves and original tree-ring chronologies for the period 1950–2009. The model outputs have shown differences in seasonal tree-ring growth between species that were well supported by the field observations. To better understand seasonal tree-ring growth and to verify the VS-model findings, a multi-year natural field study is needed, including seasonal observation of the thermo-hydrological regime of the soil, duration and rate of tracheid development, as well as measurements of their anatomical features.     

High-elevation inter-site differences in Mount Smolikas tree-ring width data

We present the longest high-elevation tree-ring width dataset in the Mediterranean reaching back to the 6th century CE. The network includes 101 living and 92 relict Pinus heldreichii Christ trees from four differently exposed sites in the 2100–2200 m a.s.l. elevation range of Mt. Smolikas in the Pindus Mountains in Greece. Though the sites were all sampled within a distance of <1 km, inter-site correlations are surprisingly low (r_1550–2014 = 0.65–0.87), indicating site exposure might affect tree-ring formation. We here explore the consequence of exposure differences on the climate signals in an eastern Mediterranean treeline ecotone. Temporally stable growth/climate relationships reveal similar seasonal patterns among the four sites, but differences in signal strength. P. heldreichii growth at Mt. Smolikas is significantly controlled by temperature in April (r_1951–2014 = 0.33–0.50) and precipitation in June-July (r_1951–2014 = 0.23–0.42), which emphasizes the overall importance of an early growth onset and subsequent moisture conditions. The association between stem growth and April climate is strongest in the South-facing stand, supporting the significance of higher insolation rates at this thermally privileged site. Strongest summer precipitation signals are found in the NE-facing stand, where trees seem to benefit least from an early growth onset and where reduced meltwater supply may enhance the dependency on early summer precipitation. The significance of spring temperature on tree growth in all four sites constrains the emergence of a distinct summer precipitation signal in the Mt. Smolikas high elevation ecotone. Exploration of the site-specific influences on a new millennium-long tree-ring width dataset is an important step towards an improved understanding of long-term climate variability in the Eastern Mediterranean. Site-related differences in climate sensitivity in the high-elevation tree-ring network at Mt. Smolikas indicate that both temperature and precipitation during different seasons could potentially be reconstructed if distinct site exposures (S versus NE) are considered.     

Age-dependent tree-ring growth responses of Schrenk spruce (Picea schrenkiana) to climate—A case study in the Tianshan Mountain,China

For both its climatic and ecological importance, Schrenk spruce (Picea schrenkiana) is a crucial tree species living at mid-altitude on the western area of the Tianshan Mountains. It plays a key role on understanding climatic change in the Tianshan Mountains in the past 500 years. However, whether the relationship between tree growth and limiting climate factors is stable over time is still not well-known. In this study, standard and residual chronologies of four 100-year age classes (AC1 < 110a, 110a < AC2 < 210a, 210a < AC3 < 310a and AC4 > 310a) were established for detecting divergence in climate–growth relationships as well as comparing low-frequency and high-frequency variations. The results show that climate can account for a high amount of variance in tree-ring width and higher climate sensitivity was detected in younger trees. Younger trees (<210a) exhibit significantly negative growth responses to mean monthly air temperature of previous June and positive relationship with total monthly precipitation of current April and May, while mean monthly air temperature of current March may inhibit growth of older trees (>210a). Tree-ring chronology statistics and response function reveal that the age-growth patterns are non-monotonic. Our results together with previous studies demonstrate that the age effects on tree-ring growth–climate response is attributed to a combination of genetic characteristics and site microclimate, which suggests that it is necessary to consider both age-dependent and species-specific climate responses when using tree-ring measurements as a proxy for valid climate reconstructions.     

Climate-growth relationships for Rocky Mountain juniper (Juniperus scopulorum Sarg.) on the volcanic badlands of western New Mexico,USA

We sampled Rocky Mountain junipers (RMJ) to produce a multi-century tree-ring chronology from a relict lava flow, the Paxton Springs Malpais (PAX), in the Zuni Mountains of western New Mexico. Our objective was to assess crossdating potential for RMJ growing on the volcanic badlands of the region, investigate potential relationships between climate and RMJ growth, and investigate temporal variability in relationships identified between climate and RMJ growing at our site. We hypothesized that, similar to other drought stressed-conifers growing on the lava flows, RMJ responds to climate factors that influence and indicate moisture availability. We found a high average mean sensitivity value (0.53), which indicated the PAX chronology exhibited enough annual variability to capture fluctuations in environmental conditions. The average interseries correlation (0.74) indicated confident crossdating and a significant association of annual growth among trees within the stand. The positive correlation between the PAX chronology and total precipitation for the local water year was significant (r = 0.53; P < 0.001). Significant positive correlations also were identified between monthly PDSI, monthly total precipitation, and RMJ radial growth. Analyses of temporal stability indicated that the positive relationship between RMJ growth at the PAX site and monthly PDSI was the most stable relationship during the period of analysis (1895–2007). More importantly, we identified a unique inverse relationship between radial growth and monthly mean temperature during periods of the preceding year and current growing year, the first such finding of a strong temperature response for a low-mid elevation tree species in the American Southwest. Our results confirm that RMJ samples collected on the Paxton Springs Malpais are sensitive to climate factors that affect moisture availability, further suggesting that RMJ may be suitable for use in dendroclimatic research at additional locations across the broad distribution of the species.     

Evaluating the temperature sensitivity of radial growth patterns from whitebark pine in the western Canadian Cordillera

We present a network of thirteen annual ring-width chronologies from high elevation whitebark pine (Pinus albicaulis Engelm.) sites in the western Canadian Cordillera in order to assess the dendroclimatic potential of this long-lived tree species. The temperature signal within the chronologies is complex and strongly influenced by diverging trends in the summer temperature and ring-width records from across the region. A first differences transformation of the tree-ring and temperature records illustrates a loss of frequency coherence in growth response to summer temperatures following reduced radial growth in the 1950s. Prior to reduced growth, we note a positive association with summer temperatures for both first differenced (r_d = 0.60) and traditional (r = 0.50) records. Following reduced growth, the association at first differences is maintained (r_d = 0.49) whereas there is a change in the lower frequency component of tree growth response to summer temperatures (r = ?0.34). We suggest the cause of this reduced temperature sensitivity is related to the interaction between diurnal temperature and cloud cover patterns, the hydrological regime of snowpack, and site conditions which have been amenable to the initiation of moisture stress during the latter half of the 20th century. Reduced radial growth is coincident with the arrival of white pine blister rust (Cronatium ribicola J.C. Fisch. ex Raben) into the study region which suggests this infestation may be related to the observed reduction in radial growth. Whitebark pine has considerable potential for the field of dendroclimatology. Unfortunately, the decline of the species due to the combined effects of climate change, white pine blister rust, mountain pine beetle (Dendroctonus ponderosae Hopk.), and forest fire exclusion practices indicate this potential may remain unfulfilled.     

Age-related drought sensitivity of Atlas cedar (Cedrus atlantica) in the Moroccan Middle Atlas forests

Age-related tree responses to climate change are still poorly understood at the individual tree level. In this paper, we seek to disentangle the relative contribution of tree age to growth decline and growth–climate relationships in Atlas cedar (Cedrus atlantica Manetti) trees at the Middle Atlas Mountains, northern Morocco. Dendrochronological methods were applied to quantify growth–climate relationships using tree-ring width indices (TRWi) calculated for cedars of two contrasting age groups (old trees, age ≥150 years; young trees, age <150 years). TRWi–climate relationships were assessed at the site and tree levels by using response functions and linear mixed-effects models, respectively. Growth of the studied Atlas cedars was negatively affected by recurrent droughts and by the steep temperature rise since the 1970s. Response functions and mixed-effects models indicated that the decline in tree growth was mainly explained by diminishing precipitation. The negative association between cedar growth and temperature was stronger in old than in young trees. Vulnerability to temperature-induced drought stress in old cedar trees may lead to an impending growth decline. We argue that the age dependence of growth sensitivity to drought must be quantified and considered at the individual tree level when predicting the future dynamics and persistence of cedar forests in the Moroccan Middle Atlas.     

Spatio-temporal growth variability of three Pinus species of Northeast Himalaya with relation to climate

This research aimed to evaluate spatio-temporal growth variability of three Pinus species viz. Pinus kesiya (Khasi pine), Pinus merkusii (Merkus pine) and Pinus wallichiana (Blue pine) along with the existence of species differentiation among the taxa in northeast India. Several statistical analyses were used, namely Pearson correlation and multivariate approaches involving UPGMA Cluster Analysis; ordination methods by Principal Component Analysis (PCA) and Non-metric multidimensional scaling (NMDS) on tree-ring width chronologies from 13 sites. The tree growth-climate relationships were assessed with both correlation and bootstrap response function using regional climate datasets of each sampling site prepared by averaging the nearest grid points of 0.5 × 0.5° of CRU TS-2.1 climate dataset. Pronounced species differentiation in the growth pattern among the three Pinus taxa was inferred. The observed spatio-temporal variability revealed inter-species tree growth variations were not uniform suggesting no common factor influenced the radial tree growth in this region, which may be related to anthropogenic impact or non-climatic factors. The tree growth-climate relationship showed that climatic factors limiting the radial growth of Pine are mostly similar for intra-species but diverse in inter-species. This study is extremely relevant in terms of species and site selection for the long-term climate reconstruction and forest management in the Northeast Himalaya.     

Toward consistent measurements of carbon accumulation: A multi-site assessment of biomass and basal area increment across Europe

The use of tree-ring data in carbon cycle research has so far been limited because traditional study designs are not geared toward quantifying forest carbon accumulation. Existing studies that assessed biomass increment from tree rings were often confined to individual sites and used inconsistent sampling schemes. We applied a consistent biomass-oriented sampling design at five managed forest sites located in different climate zones to assess the annual carbon accumulation in above-ground woody tissues (i.e. stems and branches) and its climate response. Radial growth and biometric measurements were combined to reconstruct the annual biomass increment in individual trees and upscaled to the site level. In addition to this, we estimated that 32–60 trees are required at these five sites to robustly quantify carbon accumulation rates. Tree dimensions and growth rates varied considerably among sites as a function of differing stand density, climatic limitations, and management interventions. Accordingly, mean site-level carbon accumulation rates between 65 g C m⁻² y⁻¹ and 225 g C m⁻² y⁻¹ were reconstructed for the 1970–2009 period. A comparison of biomass increment with the widely used basal area increment (BAI) revealed very similar growth trends but emphasized the merits of biomass assessments due to species-specific BAI/biomass relationship. Our study illustrates the benefits and challenges of combining tree-ring data with biometric measurements and promotes the consistent application of a standardized sampling protocol across large spatial scales. It is thus viewed as a conceptual basis for future use of tree-ring data to approach research questions related to forest productivity and the terrestrial carbon balance.     

Responses to climate change in radial growth of Picea schrenkiana along elevations of the eastern Tianshan Mountains,northwest China

Tree growth is largely driven by climate conditions in arid and alpine areas. A strong change in climate from warm-dry to warm-wet has already been observed in northwest China. However, little is known about the impacts of regional climate variability on the radial growth of trees along elevations of the eastern Tianshan Mountains. Consequently, we developed three tree-ring width chronologies of Schrenk spruce (Picea schrenkiana Fisch. et Mey.) ranging in elevation from 2159 to 2552 m above sea level (a.s.l.), which play an important role in the forestry ecosystem, agriculture, and local economy of Central Asia. In our study, the correlation analyses of growth-drought using the monthly standardized precipitation-evapotranspiration index (SPEI) at different temporal scales demonstrated that drought in growing season was the main factor limiting tree growth, regardless of elevation. The relationships between radial growth of Schrenk spruce and main climate factors were relatively stable by moving correlation function, and the trend of STD chronologies and basal area increment (BAI) also showed a synchronous decline across the three elevations in recent decades. And meanwhile, slight differences in responses to climate change in radial growth along elevations were examined. The drought stress increased as elevations decreased. Radial growth at the higher elevation depended on moisture availability due to high temperature, as indicated by the significant negative correlation with mean temperature in the late growing season of the previous year (August-September, p < 0.001). However, radial growth at the lower elevation were restricted by drought stress due to less precipitation and higher temperatures, as demonstrated by the significant negative correlation with mean temperature but positive with total precipitation in the early growing season of the current year (April-May, p < 0.05). In addition, the decline of radial growth (BAI) at the higher elevation (3.710 cm² yr⁻¹/decade, p < 0.001) was faster than that of the middle elevation (2.344 cm² yr⁻¹/decade, p < 0.001) and the lower elevation (3.005 cm² yr⁻¹/decade, p < 0.001) since 2000, indicating that the trees at higher elevation of a relatively humid environment were more susceptible to the effects of climate change due to their poor adaptability to water deficit. Therefore, the forest ecosystems would be suppressed as a result of increasing drought stress in the future, especially in the high-elevation forests of arid and semi-arid areas.     

Environmental heterogeneity and neighbourhood interference modulate the individual response of Juniperus thurifera tree-ring growth to climate

Individual variation of tree-ring growth response to climate and heterogeneity of the local environment are usually neglected in dendrochronological research. Even if there is evidence showing that individual responsiveness to climate may depend on intrinsic traits such as tree age, size or sex, its modulation by the local heterogeneity of extrinsic factors has been less studied. Using an extensive, strictly regular sampling scheme across a 3300 ha woodland, we assessed the individual variation of tree-ring growth responses to climate in 100 Juniperus thurifera L. trees. The climatic response was evaluated by bootstrapped correlations of both population- and individual-based tree-ring chronologies with monthly records of precipitation, cloudiness, minimum and maximum temperatures. We studied also the influence of extrinsic abiotic (elevation, slope, heat load, tree location) and biotic (competition from neighbouring trees) factors on the individual growth variation and its climatic response. At a population level, growth was controlled by February–March precipitation, April minimum temperature, and June water stress. A significant proportion of individuals did not respond to those variables, but were sensitive to others not relevant at the population level. Inter-annual growth variation was strongly modulated by competition, whereas trees under lower competition levels, in eastern and warmer areas, were the most responsive to climate. The individual climatic response was, at least partially, modulated by the local heterogeneity of extrinsic factors. By considering environmental heterogeneity and neighbourhood interference we can identify the spectrum of site-dependent climatic responses in a population, which in turn will enable more realistic predictions of tree responses to ongoing climate change.     

A 382-year reconstruction of August mean minimum temperature from tree-ring maximum latewood density on the southeastern Tibetan Plateau,China

Long-term summer temperature records are important for climate studies on the Tibetan Plateau (TP). Here, we used tree-ring maximum latewood density (MXD) to develop a well-replicated regional chronology back to the year 1630 for the southeastern TP. The MXD chronology is positively related to the observed August mean minimum temperatures (AMMT) in the period 1961–2011. Therefore, the AMMT was reconstructed from the MXD chronology. The reconstruction explained 42.6% of the total variance in the observed AMMT. During the past 382 years, warm periods were found during 1646–1694, 1770–1805, 1930–1971 and 1992–2011, and cold periods were found during 1630–1645, 1695–1749, 1806–1825, 1889–1929 and 1972–1991. Extreme cold summers (≤mean − 2 SD) occurred in the years 1701, 1777, 1810, 1817, 1835, 1843, 1857, 1871, 1911, 1914, 1915, 1939, 1983 and 1984, whereas the warm summers (≥mean + 2 SD) occurred in the years 1786, 1788, 2003, 2004 and 2005. A comparison with temperature records in surrounding regions showed general agreements, indicating the fidelity of our reconstruction and its ability to represent summer temperature variations over a broad geographic extent. Conspicuous in-phase relationships between our reconstruction and the Atlantic Multidecadal Oscillation (AMO) indicated a strongly positive association between large-scale climate circulations and summer temperature variability on the southeastern TP at multidecadal scales.     

Influence of climatic conditions and industrial emissions on spruce tree-ring Pb isotopes analyzed at ppb concentrations in the Athabasca oil sands region

This study investigates Pb isotope ratios at low concentrations (parts per billion; ppb) in tree rings and soils in the Northern Athabasca Oil Sands Region (NAOSR), western Canada, to evaluate if: (1) climatic conditions influence on tree-ring Pb assimilation; and (2) such low Pb content allows inferring the regional Pb depositional history.Our results reflect the influence of winter snow cover and the importance of minimum temperature and precipitation in spring and summer on the bioavailability of Pb and its passive assimilation by trees in sub-arctic semi-humid climatic conditions. Winter conditions can influence the state of root systems that subsequently impacts the following growth period, while spring and summer conditions likely control microbial processes and water source, and may thus impact Pb assimilation by trees. Thus, the results of tree-ring Pb concentrations show interesting correlation with cumulated snow from November of the previous year to February (ρ = 0.53; P < 0.01; n = 36). Likewise, the ²⁰⁶Pb/²⁰⁷Pb ratios inversely correlate with minimum temperature from April to September (ρ = −0.67; P < 0.01; n = 40) and precipitation from May to August (ρ = −0.42; P < 0.01; n = 36). The isotopic results also suggest that the effects of climatic variations are superimposed by regional industrial Pb deposition: Western North American Aerosols (WNAA) and fugitive dust from the oil sands mining operations appear to be the most likely sources.Importantly, this study suggests that even at low Pb concentrations, tree-ring Pb isotopes are modulated by climatic conditions and potential input of regional and long-range transport of airborne Pb. These interpretations open the possibility of using Pb isotopes as an environmental tool for inferring the pollution history in remote regions, and improving our understanding of its natural cycle through the forest environment.     

A 1700-year Athrotaxis selaginoides tree-ring width chronology from southeastern Australia

Few Southern Hemisphere tree-ring chronologies exceed 1000 years in length. We present a ca. 1700 years of indexed values for the long-lived conifer Athrotaxis selaginoides at Cradle Mt in southeastern Australia and compare it with the only other published millennial-plus length tree-ring chronology for Australia: the nearby Mt Read Lagarostrobos franklinii. We use simple correlation function and pointer year analyses to compare the climate responses of the two species (temperature, precipitation and growing degree days). Both chronologies show accelerated growth at their modern ends, but this growth acceleration is not synchronous, beginning approximately a quarter of a century earlier at the Cradle Mt site. This discrepancy may highlight the relevance of chronology composition and/or physiological differences in the species. Although the seasonality of the climatic responses of the two species is similar, that of A. selaginoides is generally weaker than that of L. franklinii. Somewhat paradoxically, the only pointer years in common between the chronologies are 1898 and 1908 CE. The periods from 600 to 900 CE and ∼1200–1450 CE are conspicuous for their absence of positive pointer years while no negative pointer years occur for either site from ∼1200–1350 CE. It is possible that differing patterns of pointer years can be partially explained by a peak in establishment from ∼1150–1850 CE at the Mt Read L. franklinii site compared to continuous establishment at Cradle Mt. Although statistically significant and time-stable climate responses for the A. selaginoides chronology are too weak to base a single-chronology climate reconstruction on, the long chronology will likely make an important contribution to future multi-proxy temperature reconstructions for southeastern Australia.     

Carbon mineralization of Chinese fir (Cunninghamia lanceolata) soils under different temperature and humidity conditions

Burned and unburned mineral soils (0–10 cm) from a 40-year-old Chinese fir (Cunninghamia lanceolata) forest in Nanping, Fujian, China were incubated for 90 days at different temperatures (25 °C and 35 °C) and humidity [25%, 50%, and 75% of water holding capacity (WHC)] conditions. Carbon (C) mineralization of all soils was determined using CO₂ respiration method. The results showed that CO₂ evolution rates of the burned and control soils exhibited similar temporal patterns, and similar responses to temperature and moisture. CO₂ evolution rates for all soil samples decreased with incubation time. At different humidity conditions, average rate of C mineralization and cumulative mineralized C from burned and control soils were significantly higher at 35 °C than at 25 °C. This implied that C mineralization was less sensitive to soil moisture than to temperature. In both soils at 25 °C or 35 °C, the amount of soil evolved CO₂ over the 90 days incubation increased with increasing moisture content from 25% to 75% WHC. A temperature coefficient (Q₁₀) varied with soil moisture contents. The maximum values recorded for Q₁₀ were 1.7 in control soil and 1.6 in burned soil both at 25% WHC. However, there were no significant differences in Q₁₀ values between the control and burned soils over all moisture ranges (P > 0.05). The data of cumulative C–CO₂ released from control and burned soils were fitted to two different kinetic models. The two simultaneous reactions model described mineralization better than the first-order exponential model, which reflected the heterogeneity of substrate quality. Based on these results, it is possible to conclude that temperature and moisture are important in the controls of C mineralization, and the combined effects of these variables need to be considered to understand and predict the response of CO₂ release in subtropical ecosystems to climate change.     

Tree-ring growth of Gmelin larch under contrasting local conditions in the north of Central Siberia

While the forest-tundra zone in Siberia, Russia has been dendroclimatologically well-studied in recent decades, much less emphasis has been given to a wide belt of northern taiga larch forests located to the south. In this study, climate and local site conditions are explored to trace their influence on radial growth of Gmelin larch (Larix gmelinii (Rupr.) Rupr.) trees developed on permafrost soils in the northern taiga. Three dendrochronological sites characterized by great differences in thermo-hydrological regime of soils were established along a short (ca. 100 m long) transect: on a river bank (RB), at riparian zone of a stream (RZ) and on a terrace (TER). Comparative analysis of the rate and year-to-year dynamics of tree radial growth among sites revealed considerable difference in both raw and standardized tree-ring width (TRW) chronologies obtained for the RZ site, characterized by shallow soil active layer depth and saturated soils. Results of dendroclimatic analysis indicated that tree-ring growth at all the sites is mostly defined by climatic conditions of a previous year and precipitation has stronger effect on TRW chronologies in comparison to the air temperatures. Remarkably, a great difference in the climatic response of TRW chronologies has been obtained for trees growing within a very short distance from each other. The positive relation of tree-ring growth with precipitation, and negative to temperature was observed in the dry site RB. In contrary, precipitation negatively and temperature positively influenced tree radial growth of larch at the water saturated RZ. Thus, a complicate response of northern Siberian larch forest productivity to the possible climate changes is expected due to great mosaic of site conditions and variability of environmental factors controlling tree-ring growth at different sites. Our study demonstrates the new possibilities for the future dendroclimatic research in the region, as various climatic parameters can be reconstructed from tree-ring chronologies obtained for different sites.     

Estimating root zone soil moisture at distant sites using MODIS NDVI and EVI in a semi-arid region of southwestern USA

This study investigates the potential of using Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) to estimate root zone soil moisture at native in-situ measured sites, and at distant sites under the same climatic setting. We obtained in-situ data from Soil Climate Analysis Network (SCAN) sites near the Texas-New Mexico border area, and NDVI and EVI products from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on board the Terra satellite. Results show that soil moisture values of the same depth are highly correlated (r = 0.53 to 0.85) among sites as far as 150 km apart, and that NDVI and EVI are highly correlated at the same site (r = 0.87 to 0.91). Correlation based on raw time series of NDVI and soil moisture is in general higher than that based on deseasonalized time series at every depth. The correlation reaches maximum value when vegetation index (VI) lags soil moisture by 5 to 10 days. NDVI shows a slightly higher correlation with soil moisture than EVI does by using the deseasonalized time series of NDVI and soil moisture. It is found that deseasonalized time series of NDVI and soil moisture are correlated at native sites (r = 0.33 to 0.77), but not at sites where soil moisture is very low. Regression analysis was conducted using deseasonalized time series soil moisture and deseasonalized time series NDVI with a 5-day time lag. Regression models developed at one site and applied to a similar distant site can estimate soil moistures, accounting for 50–88% of the variation in observed soil moistures.     

Tree-ring analysis over western Himalaya and its long-term association with vapor pressure and potential evapotranspiration

Multi species tree-ring chronologies of the western Himalaya revealed strong significant negative relationship with potential evapotranspiration (PET) and vapor pressure (VP), and positive with moisture index (MI) and Palmer Drought Severity Index (PDSI) during spring season (March to May). The preliminary study showed that the MI and PDSI particularly in spring season might have a large scale positive association in developing of annual ring-width patterns, whereas PET and VP during the season are found not to be conducive for the trees growth. PET and VP from the beginning of the year 1917 showed strong influence on tree growth. High and low PET/VP might be associated with low and high MI/PDSI of the region. Extremely narrow ring width indices were observed in the year of 1921, 1941, 1953, 1954 and 1985 at most of the tree sites which are under the severe moisture stress condition due to extremely high PET and VP of the region. Also, extremely low PET and VP were found during 1917, 1933 and 1982, reflecting ring-width index above the normal due to enough moisture supply. Thus, the released and suppressed tree growth over the region is probably linked with the high and low MI/PDSI of the region. Loss or accumulation of soil moisture of the region might be precondition before the starting of growing season of the trees. The recent observation also suggests a weakening of VP and PET's influence on tree growth during recent few decades as compared to early period in sliding 31-years windows over western Himalaya. Correlation analysis of PET with MI and VP as well as PDSI for the period 1902–2002 during spring season indicated statistically strong correlation (r = −0.53, 0.82, −0.50) respectively which is highly significant at 0.01% level.     

The influence of the de Vries (∼ 200-year) solar cycle on climate variations: Results from the Central Asian Mountains and their global link

Long-term climatic changes related to solar forcing were examined using millennium-scale palaeoclimatic reconstructions from the Central Asian mountain region, i.e. summer temperature records for the Tien Shan mountains and precipitation records for the Tibetan Plateau. The reconstructions were based on juniper tree-ring width records, i.e. Juniperus turkestanica for the Tien Shan and Sabina przewalskii for the Tibetan Plateau. The data were processed using spectral and wavelet analysis and filtered in the frequency range related to major solar activity periodicities. The results obtained for various tree-ring chronologies indicate palaeoclimatic oscillations in the range of the de Vries (～ 210-year) solar cycles through the last millennium.The quasi-200-year variations revealed in the palaeoclimatic reconstructions correlate well (R² = 0.58–0.94) with solar activity variations (Δ¹⁴C variations). The quasi-200-year climatic variations have also been detected in climate-linked processes in Asia, Europe, North and South America, Australia, and the Arctic and Antarctica. The results obtained point to a pronounced influence of solar activity on global climatic processes.Analysis has shown that climate response to the long-term global solar forcing has a regional character. An appreciable delay in the climate response to the solar signal can occur (up to 150 years). In addition, the sign of the climate response can differ from the solar signal sign. The climate response to long-term solar activity variations (from 10s to 1000s years) manifests itself in different climatic parameters, such as temperature, precipitation and atmospheric and oceanic circulation. The climate response to the de Vries cycle has been found to occur not only during the last millennia but also in earlier epochs, up to hundreds of millions years ago.     

Growth response of Sabina tibetica to climate factors along an elevation gradient in south Tibet

We examine the climate significance in tree-ring chronologies retrieved from Sabina tibetica Kom. (Tibetan juniper) at two sites ranging in elevation from 4124 to 4693 m above sea level (a.s.l.) in the Namling region, south Tibet. The study region is under the control of semi-arid plateau temperate climate. The samples were grouped into high- and low-elevation classes and standard ring-width chronologies for both classes were developed. Statistical analysis revealed a decreasing growth rate yet increasing chronology reliability with increasing elevation. Overall, correlation analyses showed that radial growth in S. tibetica at the study sites was controlled by similar climatic factors, regardless of elevation; these factors comprised early winter (November) and early summer (May–June) temperatures as well as annual precipitation (July–June). Slight differences in the correlation between tree growth along the elevation gradient and climate variables were examined. The correlations with early winter temperature varied from significantly positive at the low-elevation site to weakly positive at the high-elevation site, whereas the correlations between radial growth and early summer temperature increased from weakly negative at the low-elevation sites to strongly negative at the high-elevation sites. The abundant precipitation through the year may have masked variations in tree growth on different elevation aspects. Our results will aid future dendroclimatological studies of Namling tree rings in south Tibet and demonstrate the potential of S. tibetica Kom. for improving our understanding of environmental impacts on tree growth.