Tree age,site and climate controls on tree ring cellulose δ18O: A case study on oak trees from south-western France

A main concern of dendroclimatic reconstruction is to distinguish in the tree ring proxy the influence of the climate variables of interest from other controlling factors. In order to investigate age, site and climate controls on tree ring width and cellulose δ¹⁸O, measurements have been performed in nearby groups of young (145 years old) and older (310–405 years old) oak trees in south-western France, covering the period 1860–2010.Within a given site, inter-tree deviations are small, pointing to a common climatic signal. Despite a similar inter-annual variability, the average level of cellulose δ¹⁸O in the young tree group is ∼0.8‰ higher than in the old trees. Such offsets might be caused by different soil properties and differences in the fraction of the source water used by trees from different depths. The δ¹⁸O of water in the top soil layer is directly related to the current growing season precipitation, while deeper water can have a lower and more constant δ¹⁸O. Local cave drip waters at 10 m depth indeed show a constant isotopic composition, which corresponds to pluri-annual mean precipitation.A 2‰ increasing trend is observed in cellulose δ¹⁸O of young trees in the first 30 years of growth, during a period when no trend is visible in older trees. This increase can be quantitatively explained by humidity gradients under the forest canopy, and a changing microclimate around the crown as trees grow higher.While relationships between tree ring width and climate appear complex, the isotopic composition of cellulose is strongly correlated with summer maximum temperature, relative humidity and evapotranspiration (r ≈ 0.70). Weaker correlations (r ≈ 0.40) are identified with precipitation δ¹⁸O from a 15-year long local record and from the REMOiso model output. These results imply that leaf water enrichment has a stronger control on the inter-annual variability of cellulose δ¹⁸O than the δ¹⁸O of precipitation.This study demonstrates the suitability of oak tree ring cellulose δ¹⁸O for reconstructing past summer climate variability in south-western France, provided that the sampling and pooling strategy accounts for the fact that trees from different sites and of different age can introduce non-climatic signals.     

Climatic potential of δ18O of Abies spectabilis from the Nepal Himalaya

A 50-year tree-ring δ¹⁸O chronology of Abies spectabilis growing close to the tree line (3850 m asl) in the Nepal Himalaya is established to explore its dendroclimatic potential. Response function analysis with ambient climatic records revealed that tree-ring δ¹⁸O is primarily governed by rainfall during the monsoon season (June–September), and the regression model accounts for 35% of the variance in rainfall. Extreme dry years identified in instrumental weather data are detected in the δ¹⁸O chronology. Further, tree-ring δ¹⁸O is much more sensitive to rainfall fluctuations than other tree-ring parameters such as width and density typically used in dendroclimatology. Correlation analyses with Niño 3.4 SST reveal time-dependent behavior of ENSO–monsoon relationships.     

The oxygen isotope composition of planktonic foraminifera from the Guaymas Basin,Gulf of California: Seasonal,annual, and interspecies variability

Sediment trap samples collected over a seven-year period (February 1991–October 1997) from Guaymas Basin in the Gulf of California were used to study the oxygen isotope composition of five species of planktonic foraminifera, Globigerinoides ruber (white), Globigerina bulloides, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii. The δ¹⁸O data were analyzed for temporal and interspecies variability and were compared to local hydrography to evaluate the use of each species in reconstructing past oceanographic applications. The two surface dwelling species, G. ruber and G. bulloides displayed the lowest δ¹⁸O values (～ 0.0 to ? 5.0‰), while δ¹⁸O values for the thermocline dwelling N. dutertrei, P. obliquiloculata, and G. menardii were higher (～ 0.0 to ? 2.0‰). The δ¹⁸O of G. ruber most accurately records measured sea surface temperatures (SSTs) throughout the year. G. bulloides δ¹⁸O tracks SSTs during the winter–spring upwelling period but for the remainder of the year records slightly colder, subsurface temperatures. The difference between the δ¹⁸O of the surface dwelling species, G. ruber and G. bulloides, and that of the thermocline dwelling species, N. dutertrei, P. obliquiloculata, and G. menardii, was used to estimate the surface to thermocline temperature gradient. During the winter these δ¹⁸O differences are small (～ 0.50‰) reflecting a well-mixed water column. These interspecies δ¹⁸O differences increase during the summer (～ 1.90‰) in response to the strong thermal stratification that exists at this time of year.     

Mg/Ca and δ18O in the brackish shallow-water benthic foraminifer Ammonia ‘beccarii’

Specimens of the benthic foraminifer Ammonia beccarii were cultured in the laboratory in order to determine the relation between temperature and Mg/Ca and oxygen isotope values in their tests. Asexual reproduction in this species provides a large number of juveniles that were allowed to grow into maturity at temperatures ranging from 10 to 27 °C and at salinities ranging from 18 to 33 PSU. The Mg/Ca in a calcite increase exponentially and δ¹⁸O decreases linearly with the temperature. Salinity has no significant impact on either Mg/Ca or δ¹⁸O. We show how the combination of these two parameters can be used to reconstruct seawater δ¹⁸O and temperature in shallow marine habitats.     

Investigating the potential for Southern Hemisphere climate reconstruction using stable isotopes in Tasmanian tree rings

Few annually dated stable isotope records exist across Oceania. Stable carbon and oxygen isotope ratios have the potential to enhance climate reconstructions currently reliant on tree ring width chronologies. The purpose of this study is to explore the sources of variability in a stable oxygen isotope chronology derived from A. selaginoides from Mount Read, Tasmania. This high elevation site receives abundant rainfall throughout the year and is ∼130 km from the Global Network of Isotopes in Precipitation (GNIP) site at Cape Grim. We crossdated 10 new tree core samples against an existing ring width chronology (954–2011 CE) and analyzed the δ¹⁸O from the individual rings for the period 1960–2018. Using high resolution (0.25 degrees) climate data and ECMWF ERA5 reanalysis data, we disentangled the effects of local climate and source region on the isotopic signatures recorded in the annual rings. In addition, we used HYSPLIT backward trajectory analysis to characterize the source region of precipitation to Mount Read and whether the source region has influence over the δ¹⁸O_TR series. Median δ¹⁸O_TR (n = 10) is correlated with local temperature and vapor pressure deficit in the early growing season. In addition, spatial correlations reveal that median δ¹⁸O_TR is positively correlated with temperature and negatively correlated with precipitation in the source region. However, measurements of δ¹⁸O_TR exhibit high inter-tree variation, particularly between 1960 and 1990. Our results indicate that this δ¹⁸O_TR proxy may provide additional information about past moisture conditions during the growing season, potentially contributing to more robust reconstructions of the Southern Hemisphere climate dynamics; however, additional sampling may be necessary to resolve inter-tree variation in δ¹⁸O_TR.     

Cribra orbitalia as a potential indicator of childhood stress: Evidence from paleopathology,stable C,N, and O isotopes,and trace element concentrations in children from a 17th⿿18th century cemetery in JĿkabpils,Latvia

Cribra orbitalia (CO), or porotic hyperostosis (PH) of the orbital roof, is one of the most common pathological conditions found in archaeological subadult skeletal remains. Reaching frequencies higher than 50% in many prehistoric samples, CO has been generally attributed to a variety of factors including malnutrition (e.g., megaloblastic anemia) and parasitism. In this study, we tested the relationship between CO, trace element concentrations, and stable isotope values (δ¹³C, δ¹⁵N, δ¹⁸O) in subadult skeletons from a 17^th to 18^th century cemetery in the historic town of JĿkabpils, Latvia. A total of 28 subadults were examined, seven of which (25%) showed evidence of CO. Bioarchaeological evidence indicated high mortality for children in this cemetery: half of the burials were subadults under the age of 14, while a third were under the age of four. Life expectancy at birth was estimated to have been only 21.6 years. Trace element concentrations measured by Inductively Coupled Plasma ⿿ Mass Spectrometry (ICP-MS) showed no relationship between presence or absence of CO and levels of manganese, zinc, strontium, barium, copper, cadmium, or lead in the bones (p > 0.05). However, a significant correlation (p < 0.05) was found between the presence of CO and decreased levels of iron. The correlations between CO and decreased levels of copper and lead approached significance (p = 0.056 for both elements). Individuals with CO furthermore displayed significantly lower δ¹⁵N isotope values, suggesting greater consumption of lower trophic level food resources than those unaffected by CO; δ¹³C and δ¹⁸O values, in contrast, showed no significant differences. These results suggest that the prevalence of CO may be related to dietary deficiencies. In this case, low iron levels may also signify a diet low in other key vitamins (e.g., B₉ and B₁₂), which are known to cause megaloblastic anemia.     

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.     

Ontogenetic effects on stable carbon and oxygen isotopes in tests of live (Rose Bengal stained) benthic foraminifera from the Pakistan continental margin

We determined the stable oxygen and carbon isotopic composition of live (Rose Bengal stained) tests belonging to different size classes of two benthic foraminiferal species from the Pakistan continental margin. Samples were taken at two sites, with water depths of about 135 and 275 m, corresponding to the upper boundary and upper part of the core region of the oxygen minimum zone (OMZ). For Uvigerina ex gr. Uvigerina semiornata and Bolivina aff. Bolivina dilatata, δ¹³C and δ¹⁸O values increased significantly with increasing test size. In the case of Uvigerina ex gr. U. semiornata, δ¹³C increased linearly by about 0.105‰ for each 100-μm increment in test size, whereas δ¹⁸O increased by 0.02 to 0.06‰ per 100 μm increment. For Bolivina aff. B. dilatata the relationship between test size and stable isotopic composition is better described by logarithmic equations. A strong positive linear correlation is observed between δ¹⁸O and δ¹³C values of both taxa, with a constant ratio of δ¹⁸O and δ¹³C values close to 2:1. This suggests that the strong ontogenetic effect is mainly caused by kinetic isotope fractionation during CO₂ uptake. Our data underline the necessity to base longer δ¹⁸O and δ¹³C isotope records derived from benthic foraminifera on size windows of 100 μm or less. This is already common practice in down-core isotopic studies of planktonic foraminifera.     

Cenozoic long-term terrestrial climatic evolution in Germany tracked by δ18O of rodent tooth phosphate

Even though the Cenozoic has been recognized as a period of important climate change, long-term climatic changes that took place in the continental domain are still questioned. For an area, southern Germany, for which other long-term palaeoclimatic records exist, analysis of oxygen isotope composition of small mammal teeth has been carried out for localities ranging in age from the late Eocene (c.36 Ma) to the middle Miocene (c.10 Ma). Comparison of this long-term continental δ¹⁸O record with the marine record reveals comparable trends. The major Cenozoic climatic events are thus recognized in the continental oxygen isotope record.Through comparison with other quantitative palaeoclimatic reconstructions available for the studied area, temperatures in southern Germany are broadly estimated to have fluctuated between 12 °C and 25 °C from late Eocene to early late Miocene. According to the different available chronological frameworks, either a ～ 2 °C (considering the classical biostratigraphy) or a ～ 6 °C (considering a revised biostratigraphy) decrease in mean air temperature is estimated for the Eocene/Oligocene boundary.     

The influence of climate and sea-level change on the Holocene evolution of a Mediterranean coastal lagoon: Evidence from ostracod palaeoecology and geochemistry

Coastal lagoons provide an excellent basis for the study of processes controlling the evolution of a coastal zone. We examine the relative importance of these processes during the middle to late Holocene through a study of an 8.5 meter-long sediment record from the Albufera de Valencia (Spain). We combine sedimentological analyses with investigations into the palaeoecology, taphonomy and geochemistry (Mg/Ca, Sr/Ca, δ¹⁸O and δ¹³C) of ostracod valves in order to assess the effects of sea-level changes, storm events and effective moisture on the evolution of a Western Mediterranean coastal wetland. The late Pleistocene sediments represent a subaerial environment, which was followed by a hiatus in deposition. The first Holocene unit (～8700–7500 calendar yr. BP) is composed of typical lagoon-barrier and backshore sediments, deposited when seawater intruded into the lake and the climate was arid. The upper part of the sequence (between 7500 and 3400 yr.) is characterized by two sedimentary units, which correspond to Holocene progradation phases and humid climate associated with an increased freshwater influx to the lake accompanied by several high-energy events (palaeostorms). Overall, the record shows that an arid climate prevailed in the western Mediterranean area between 8400 and 7600 yr. The main marine transgression and accompanying progradational phases occurred between 7000 and 3400 yr., which is confirmed by other studies of coastal evolution along the Mediterranean coast. The multiproxy reconstructions demonstrate that controls on sedimentation and palaeoecology in this Mediterranean coastal lagoon were complex.     

Fluxes and isotopic composition of planktonic foraminifera off Hainan Island,northern South China Sea: Implications for paleoceanographic studies

Planktonic foraminifera collected from a sediment trap deployed off Hainan in the northwestern South China Sea (SCS-NW) between July 2012 and April 2013 were studied to evaluate their seasonal variability and ecology as well as to infer the factors controlling their shell fluxes. The total planktonic foraminifera flux, as well as the fluxes of the dominant species (Globigerinoides ruber, Globigerinoides sacculifer and Neogloboquadrina dutertrei), showed three distinct maxima during SW-monsoon in August 2012, the SW-NE intermonsoon in October 2012 and the NE-monsoon in December 2012–February 2013. These periods were characterized by upwelling, aerosol fallout, and intense wind mixing, respectively, from which the foraminiferal assemblages benefitted, as indicated by the close correlation between wind speed, sea surface temperature (SST), chlorophyll a concentration (Chl-a), δ¹⁸O of G. ruber and the shell fluxes. The correlation also suggests that temperature and food availability might have been the primary drivers of the observed changes in foraminiferal abundance. The offset between the SST deduced from flux-weighted of G. ruber δ¹⁸O and annual mean SST is only ∼0.3 °C, much lower than ∼5.2 °C between the summer and winter temperature, indicating a balanced seasonality bias in the shell flux. The linear regression between the satellite-derived sea surface temperature and G. ruber δ¹⁸O reveals the strong potential of this species, at least in the studied region, as an ecological indicator for past oceanic environments.     

Tillage effects on carbon footprint and ecosystem services of climate regulation in a winter wheat–summer maize cropping system of the North China Plain

Inappropriate farm practices can increase greenhouse gases (GHGs) emissions and reduce soil organic carbon (SOC) sequestration, thereby increasing carbon footprints (CFs), jeopardizing ecosystem services, and affecting climate change. Therefore, the objectives of this study were to assess the effects of different tillage systems on CFs, GHGs emissions, and ecosystem service (ES) values of climate regulation and to identify climate-resilient tillage practices for a winter wheat (Triticum aestivum L.)-summer maize (Zea mays L.) cropping system in the North China Plain (NCP). The experiment was established in 2008 involving no-till with residue retention (NT), rotary tillage with residue incorporation (RT), sub-soiling with residue incorporation (ST), and plow tillage with residue incorporation (PT). The results showed that GHGs emissions from agricultural inputs were 6432.3–6527.3 kg CO₂-eq ha⁻¹ yr⁻¹ during the entire growing season, respectively. The GHGs emission from chemical fertilizers and irrigation accounted for >80% of that from agricultural inputs during the entire growing season. The GHGs emission from agricultural inputs were >2.3 times larger in winter wheat than that in the summer maize season. The CFs at yield-scale during the entire growing season were 0.431, 0.425, 0.427, and 0.427 without and 0.286, 0.364, 0.360, and 0.334 kg CO₂-eq kg⁻¹ yr⁻¹ with SOC sequestration under NT, RT, ST, and PT, respectively. Regardless of SOC sequestration, the CFs of winter wheat was larger than that of summer maize. Agricultural inputs and SOC change contributed mainly to the component of CFs of winter wheat and summer maize. The ES value of climate regulation under NT was ¥159.2, 515.6, and 478.1 ha⁻¹ yr⁻¹ higher than that under RT, ST, and PT during the entire growing season. Therefore, NT could be a preferred “Climate-resilient” technology for lowering CFs and enhancing ecosystem services of climate regulation for the winter wheat–summer maize system in the NCP.     

Oxygen isotope fractionation between crocodilian phosphate and water

Oxygen isotope compositions of phosphate (δ¹⁸O_p) were measured in tooth enamel from captive and wild individuals of 8 crocodilian species. A rough linear correlation is observed between the δ¹⁸O_p of all the studied species and the oxygen isotope composition of ambient water (δ¹⁸O_w). Differences in mean air temperature, diet and physiology could contribute significantly to the large scatter of δ¹⁸O_p values. The combination of these parameters results in a fractionation equation for which the slope (0.82) is lower than that expected (≥ 1) from predictive model equations that assume temperature and diet as fixed parameters. Taking into account large uncertainties, the observed oxygen isotope fractionation between phosphate and ambient water does not statistically differ from that formerly established for aquatic turtles. Case studies show that δ¹⁸O_p values of fossil crocodile tooth enamel can be used to discriminate between marine and freshwater living environments within a precision of about ± 2‰ only.     

Cellulose oxygen isotopic composition of teak (Tectona grandis) collected from Java Island: a tool for dendrochronological and dendroclimatological analysis

The oxygen isotopic composition (δ¹⁸O) of tree-ring cellulose has been recognized as a powerful tool for dendroclimatological and dendrochronological investigations in Asia. Only a few studies of cellulose δ¹⁸O so far published, however, have been conducted in Indonesia, and the spatial coherence of cellulose δ¹⁸O has yet to be clarified. In this study we measured cellulose δ¹⁸O of teak (Tectona grandis) collected from four sites on Java Island to evaluate the similarity between the different teak δ¹⁸O values and the potential for using teak δ¹⁸O both as a climate proxy and as a tool for cross-dating. Cellulose δ¹⁸O time series of Javanese teaks were found to be significantly correlated in all of the comparisons between sites, even between sites separated by a distance as great as 400 km. While significant correlations did not appear in the ring width data between our samples (expressed population signal (EPS) = 0.64; Rbar = 0.23; sample depth = 10), they were found between the cellulose δ¹⁸O values (EPS = 0.89; Rbar = 0.58; sample depth = 10). These results suggest that teak δ¹⁸O variations have higher spatial coherence and might be a useful tool for cross-dating. A significant negative correlation was observed between cellulose δ¹⁸O and the relative humidity/precipitation during the rainy season, indicating that the cellulose δ¹⁸O of Javanese teak is an effective proxy for relative humidity/precipitation during the rainy season. Cellulose δ¹⁸O was found to be positively correlated with precipitation during the dry season preceding the growing season, whereas it showed no correlations with the temperature and Palmer Drought Severity Index (PDSI), the key constraints of δ¹⁸O on the Indochina Peninsula.     

Magnesium and strontium compositions of recent symbiont-bearing benthic foraminifera

Minor and trace elements in foraminiferal carbonates are potential paleo-proxies of climate, nutrient and seawater composition. There are very few reports of trace element composition of symbiont-bearing, larger foraminifera that are known to be important constituents of shallow-marine, modern and ancient carbonates. In this paper we examine the range of variation in Mg and Sr content of Recent species of these foraminifera from a lagoon of Lakshadweep Atoll (Indian Ocean) and Akajima Islands, Japan. Two hyaline species, Amphistegina lessonii and Neorotalia calcar,and two porcellaneous species, Amphisorus hemprichii and Marginopora vertebralis were collected live from Lakshadweep islands. Mg / Ca in these foraminifera is of an order of magnitude higher than the values reported for planktonic and symbiont-free benthic foraminifera. The Sr / Ca values are, however, comparable with the reported values in other foraminiferal taxa and they are found to vary within a narrow range. Electron-probe micro-analysis of three symbiont-bearing benthic species indicates spatial heterogeneity of high orders in Mg / Ca composition in all the species. The annual variation in temperature and pH of the lagoon water cannot explain the observed amplitude of the compositional variation. The photosynthesis and respiration of the symbionts and host foraminifera are possibly the major cause of compositional heterogeneity in individual tests, as has also been recently postulated for symbiont-bearing planktonic foraminiferal species. It highlights the need to isolate biological factors and necessitates species-specific paleotemperature scale in paleoclimatic analysis. We also analyzed δ¹⁸O, δ¹³C, Ca, Mg and Sr in carefully dissected chambers of a reef-dwelling, porcellaneous benthic foraminifer, Marginopora kudakajimaensis, collected live in four seasons. A moderate positive correlation is observed between Mg / Ca and temperature. However, large inter- and intra-test variation in Mg limits the precision of Mg / Ca as palaeotemperature proxy. The Sr / Ca of the test calcite is unrelated to temperature of the sea water. The δ¹³C of M. kudakajimaensis does not correlate with δ¹⁸O, Mg / Ca or Sr / Ca.     

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.     

On the influence of tree size on the climate–growth relationship of New Zealand kauri (Agathis australis): insights from annual, monthly and daily growth patterns

Many tree-ring-based climate reconstructions are based on the assumption that the climate reaction of trees is independent of their size. Here, we test this assumption for New Zealand kauri (Agathis australis), one of the longest tree ring-based proxies for the El Niño-Southern Oscillation (ENSO). The most recent kauri chronology contains a large amount of archaeological material, e.g. timber for which the original tree size is often unknown. We analyzed the climate–growth relationship of different-sized kauri in a pristine forest using different temporal scales, i.e. annually, monthly and daily data on tree growth and climate conditions. Trees of different life stages exhibited approximately the same seasonal growth peaks during austral spring (October and November). The dormancy period overlaps with the period where weekly air temperature maxima are below ca. 17–18 °C, and where the corresponding daily minima are below ca. 8 °C. However, both correlation functions between annual growth and seasonal climate as well as Kalman filter regressions between daily growth and climate conditions suggest an influence of tree size on the climate–growth relationship for kauri. Smaller trees (DBH < 40 cm) contain weaker climate signals than larger trees. Therefore, the precautionary stripping of near-pith material (first 20 cm) from kauri chronologies may result in more uniform responses to climate forcing and thus enhance the reliability of long-term climate reconstructions.     

The dendroclimatic value of oak stable isotopes

Tree-ring stable oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) are an important archive for climate reconstructions. However, it remains unclear whether the polyvinyl acetate emulsion, often used for the preservation and fixation of wood samples, influences δ¹⁸O and δ¹³C signals. Further uncertainties are associated with the possible effects of geographical origin and cambial age of historical samples. Here, we present annually-resolved and absolutely-dated δ¹⁸O and δ¹³C measurements of 21 living oaks (Quercus robur and Q. petraea) from the Czech Republic. We find that the δ¹⁸O and δ¹³C signals in the extracted alpha-cellulose are not affected by polyvinyl acetate treatment. Covering the entire 20^th century and reaching until 2018 CE, our dataset reveals spatial and temporal coherency within and between the individual δ¹⁸O and δ¹³C chronologies of different oak species, sample locations, and tree ages. Highly significant (p < 0.01) Pearson’s correlation coefficients of the site-specific δ¹³C and δ¹⁸O chronologies range from 0.48–0.77 and 0.36–0.56, respectively. The isotopic inter-series correlations of Q. robur and Q. petraea from the same site are 0.75 and 0.43 for the mean δ¹³C and δ¹⁸O values, respectively. Significant (p < 0.01) correlations of 0.49 and 0.84 are found for δ¹³C and δ¹⁸O, respectively, when all measurements from all sampling locations and tree ages are included. Our study shows that non-pooled oak δ¹⁸O and δ¹³C measurements from both species, different locations, and diverse tree ages can be combined into robust isotopic chronologies for climate reconstructions.     

Multiple afforestation programs accelerate the greenness in the ‘Three North’ region of China from 1982 to 2013

China has launched multiple afforestation programs since 1978, including the ‘Three North’ Shelterbelt Development Program (TNSDP), the Beijing–Tianjin Sand Source Control Program (BSSCP), the Nature Forest Conservation Program (NFCP), and the Grain to Green Program (GTGP). These programs focus on local environment restoration by planting trees in semi-arid and arid regions and by protecting natural forests. However, the effectiveness of these programs has been questioned by several previous studies. Here, we report an increasing trend of greenness in this region using the satellite-retrieved normalized difference vegetation index (NDVI) from GIMMS, GIMMS-3g and MODIS datasets in the past 32 years. The NDVI increase for the ‘Three North’ region was 0.28%–0.38% yr⁻¹ in 1982–2000 and 0.86%–1.12% yr⁻¹ in 2000–2013, which is much higher than the country's means of 0.060%–0.063% yr⁻¹ and 0.27%–0.30% yr⁻¹, respectively. Most of the increase occurred in low and sparsely vegetated areas; and enlarged the moderate vegetated area (growing season mean NDVI above 0.5) from 16.5% to 25.7% for the two time periods, respectively. We also analyzed changes in the length of the growing season and the climate conditions including temperature, precipitation and two drought indices. However, these environmental factors cannot completely explain the changes in vegetation activity. Our study suggests these multiple afforestation programs contributed to the accelerated greening trend in the ‘Three North’ region and highlight the importance of human intervention in regional vegetation growth under climate change condition.