Gerhard Helle

• Dr. rer. nat.
• Helmholtz Centre for Geosciences

The world’s longest tree-ring chronology comprises thousands of oak and pine series from Germany and continuously covers the Holocene back to 12,325 cal BP. A lack of relict wood from the Younger Dryas cold reversal ca. 12,900–11,700 cal BP, however, challenges the extension of this abso lutely dated ring width record further back in time. Here, we...

Tropical forests and woodlands are key components of the global carbon and water cycles. Yet, how climate change affects these biogeochemical cycles is poorly understood because of scarce long-term observations of tropical tree growth. The recent rise in tropical tree-ring studies may help to fill this gap, but a large-scale quantitative analysis o...

This study investigates the relationship between oxygen isotope ratios (δ18O) in oak tree ring cellulose and past drought variability in Letea Forest, Romania. A δ18O site chronology spanning 1803–2020 was compiled from seven individual time series. δ18O values exhibited a significant negative correlation with moisture-related variables (cloud cove...

This study investigates the relationship between oxygen isotope ratios (δ¹⁸O) in oak tree ring cellulose and past drought variability in Letea Forest, Romania. A δ¹⁸O site chronology spanning 1803-2020 was compiled from seven individual time series. δ¹⁸O values exhibited a significant negative correlation with moisture-related variables (cloud cove...

Stable oxygen isotopes records (δ18O) in tree-rings are commonly used to assess the response of trees to environmental variability being a valuable tool for studying past climate at different temporal and spatial scales. This is particularly relevant in semi-arid regions like the southern Tropical Andes, where ongoing environmental changes coincide...

The response of evapotranspiration to anthropogenic warming is of critical importance for the water and carbon cycle. Contradictory conclusions about evapotranspiration changes are caused primarily by their brevity in time and sparsity in space, as well as the strong influence of internal variability. Here, we present the first gridded reconstructi...

The vapor pressure deficit reflects the difference between how much moisture the atmosphere could and actually does hold, a factor that fundamentally affects evapotranspiration, ecosystem functioning, and vegetation carbon uptake. Its spatial variability and long-term trends under natural versus human-influenced climate are poorly known despite bei...

The response of evapotranspiration to anthropogenic warming is of critical importance for the water and carbon cycle. Contradictory conclusions about evapotranspiration changes are caused primarily by their brevity in time and sparsity in space, as well as the strong influence of internal variability. Here, we present the first gridded reconstructi...

In recent decades, Europe has experienced more frequent flood and drought events. However, little is known about the long-term, spatiotemporal hydroclimatic changes across Europe. Here we present a climate field reconstruction spanning the entire European continent based on tree-ring stable isotopes. A pronounced seasonal consistency in climate res...

High-quality interviews that follow best-practice guidelines are the best means available to frontline child protective service (CPS) workers and specially trained police officers to investigate and detect abuse and maltreatment. In Norway, the CPS and police are trained in the same interview method. In the current quantitative study, we investigat...

Poor drought tolerance of European beech trees raised concerns in Europe. We hypothesized that beech could show an opposite physiological response to the same level of climatic drought with change in edaphic drought. We performed a combined analysis of δ 13 C and δ 18 O in tree rings to reveal retrospective temporal physiological responses of trees...

Robust reconstruction of past climate and environmental change based on proxy data obtained from natural archives requires an in-depth understanding of the processes and mechanisms that form and determine these proxies. Here we present comprehensive long-term monitoring projects for seasonally laminated (varved) lake sediments and tree rings in the...

The tree-ring stable C, O and H isotope compositions have proven valuable for examining past changes in the environment and predicting forest responses to environmental change. However, we have not yet recovered the full potential of this archive, partly due to a lack understanding of fractionation processes resulting from methodological constraint...

This is the first Europe-wide comprehensive assessment of the climatological and physiological information recorded by hydrogen isotope ratios in tree-ring cellulose (δ²Hc) based on a unique collection of annually resolved 100-year tree-ring records of two genera (Pinus and Quercus) from 17 sites (36°N to 68°N). We observed that the high-frequency...

The response of evapotranspiration to anthropogenic warming is of critical importance to the water and carbon cycle. Con-flicting observations about changes of evapotranspiration stem mostly from the brevity of observations in time and space as well as a high degree of internal variability. Here we present the first gridded reconstruction of the Eu...

Here, we present the first gridded reconstruction of the European summer vapour pressure deficit (VPD) for the past four centuries. The gridded reconstruction is based on 26 European tree-ring oxygen isotope records and is performed using a Random Forest approach. Furthermore, we provide summer VPD data derived from 20CRV3 for the period 1836 to 20...

In this chapter, we give some basic information on the chemical and isotopic properties of wood constituents and describe their relative contribution to the isotopic signature of wood. Based on these considerations we review studies that have compared stable isotope signals of wood with those of corresponding cellulose. We exemplify how relationshi...

Large earthquakes can increase the amount of water feeding stream flows, raise groundwater levels, and thus grant plant roots more access to water in water‐limited environments. We examine growth and photosynthetic responses of Pine plantations to the Maule Mw 8.8 earthquake in headwater catchments of Chile's Coastal Range. We combine high‐resoluti...

Tropical forests uptake more atmospheric CO 2 and transpire more water than any other forest in the world and are critical components of the global carbon and hydrological cycles. Both cycles depend to a great extent on the carbon and water balance of individual trees. Such adjustments are usually evaluated through well-established and newly-emergi...

The African baobab, Adansonia digitata L., has great paleoclimatological potential because of its wide distributional range and millennial length life span. However, dendroclimatological approaches are hampered by dating uncertainties due to its unique, parenchyma-dominated stem anatomy. Here, securely dated time series of annual wood increment gro...

We investigate the climate signature of δ18O tree-ring records from sites distributed all over Europe covering the last 400 years. An empirical orthogonal function (EOF) analysis reveals two distinct modes of variability on the basis of the existing δ18O tree-ring records. The first mode is associated with anomaly patterns projecting onto the El Ni...

Purification protocols to extract pollen from lake sediments contain chemicals that alter the carbon and oxygen pollen‐isotope values according to pollen characteristics and family affiliation. Modern (raw) pollen of broad‐leaved (Alnus glutinosa, Betula pendula, Carpinus betulus, Corylus avellana, Fagus sylvatica and Quercus robur) and coniferous...

The dynamics of the Late Glacial (LG) have been demonstrated by numerous records from the Northern Hemisphere (NH) and far fewer from the Southern Hemisphere (SH). SH paleoclimate records reveal a general warming trend, interrupted by a deglaciation pause (ACR: Antarctic Cold Reversal, ~14,700-13,000 cal BP). Here we present decadal tree-ring stabl...

Previous studies have suggested that the Late Glacial period (LG; ∼14 600–11 700 cal BP) was characterised by abrupt and extreme climate variability over the European sector of the North Atlantic. The limited number of precisely dated, high-resolution proxy records, however, restricts our understanding of climate dynamics through the LG. Here, we p...

Stable carbon and oxygen isotope ratios of raw pollen sampled from nine abundant tree species growing in natural habitats of central and northern Europe were investigated to understand the intra- and inter-specific variability of pollen-isotope values. All species yielded specific δ¹³Cpollen and δ¹⁸Opollen values and patterns, which can be ascribed...

As the worldwide standard for radiocarbon ( ¹⁴ C) dating over the past ca. 50,000 years, the International Calibration Curve (IntCal) is continuously improving towards higher resolution and replication. Tree-ring-based ¹⁴ C measurements provide absolute dating throughout most of the Holocene, although high-precision data are limited for the Younger...

We investigate the annual variability of δ 18 O tree ring records from sites distributed all over Europe covering the 15 last 400 years. An Empirical Orthogonal Function (EOF) analysis reveals two distinct modes of variability on the basis of the existing δ 18 O tree ring records. The first mode of δ 18 O variability is associated with anomaly patt...

Advances in accelerator mass spectrometry have resulted in an unprecedented amount of new high-precision radiocarbon ( ¹⁴ C) -dates, some of which will redefine the international ¹⁴ C calibration curves (IntCal and SHCal). Often these datasets are unaccompanied by detailed quality insurances in place at the laboratory, questioning whether the ¹⁴ C...

The African baobab, Adansonia digitata, has great paleoclimatological potential because of its wide distributional range and millennial lifespan. However, dendroclimatological approaches are hampered by dating uncertainties due to its parenchyma-dominated wood anatomy. Here, securely-dated time series of annual wood increment growth and intra-ring...

The main focus of the TERENO Northeastern German Lowland Observatory (TERENO-Northeast) is the regional impact of Global Change. Since 2011, the observatory has recorded changes in the geo-, hydro-, bio-and atmosphere at six main study sites. The year 2018, particularly in northeast Germany, was record-breaking in regard to dryness and heat. The me...

Aim The aim was to decipher Europe‐wide spatio‐temporal patterns of forest growth dynamics and their associations with carbon isotope fractionation processes inferred from tree rings as modulated by climate warming. Location Europe and North Africa (30‒70° N, 10° W‒35° E). Time period 1901‒2003. Major taxa studied Temperate and Euro‐Siberian tre...

Tree-rings are recorders of environmental signals and are therefore often used to reconstruct past environmental conditions. In this paper, we present four annually resolved, multi-centennial tree-ring isotope series from the southeastern Tibetan plateau. The investigation site, where juniper and spruce trees jointly occur, is one of the highest kn...

The carbon isotope composition (δ 13 C) in tree rings were used to derive the intrinsic water-use efficiency (iWUE) of Araucaria ara-ucana trees of northern Patagonia along a strong precipitation gradient. It is well known that climatic and ontogenetic factors affect growth performance of this species but little is known about their influence in th...

The Northeast German Lowland Observatory (TERENO-NE) was established to investigate the regional impact of climate and land use change. TERENO-NE focuses on the Northeast German lowlands, for which a high vulnerability has been determined due to increasing temperatures and decreasing amounts of precipitation projected for the coming decades. To fac...

The Northeast German Lowland Observatory (TERENO-NE) was established to investigate the regional impact of global change. TERENO-NE focuses on the Northeast German lowlands, for which a high vulnerability has been determined due to increasing temperatures and decreasing amounts of precipitation projected for the coming decades. In order to facilita...

Nearly 13,000 years ago, the warming trend into the Holocene was sharply interrupted by a reversal to near glacial conditions. Climatic causes and ecological consequences of the Younger Dryas (YD) have been extensively studied, however proxy archives from the Mediterranean basin capturing this period are scarce and do not provide annual resolution....

Nearly 13,000 years ago, the warming trend into the Holocene was sharply interrupted by a reversal to near glacial conditions. Climatic causes and ecological consequences of the Younger Dryas (YD) have been extensively studied, however proxy archives from the Mediterranean basin capturing this period are scarce and do not provide annual resolution....

This paper describes devices to extract α-cellulose from small whole wood samples developed at the Lamont-Doherty Earth Observatory Tree-Ring Lab and explains the procedures for chemical extractions and for the dual analysis of carbon (δ¹³C) and oxygen (δ¹⁸O) stable isotopes. Here, we provide the necessary steps and guidelines for constructing a ce...

Stable isotopes are of great interest to conduct ecophysiological and paleoenvironmental researches. The analyses of the tree-ring 13C/12C ratios (δ13C) allow to study the environmental changes effects on tree development and water use over time. Thus, the δ13C values allow to estimate the intrinsic water-use efficiency (iWUE) of trees. Moreover, t...

The 10-year juvenile records of three hybrid poplar and two aspen cultivars (Populus spp.) from a short rotation coppice (SRC) were assessed by measuring tree-ring width (annual radial increment, ir) and stable isotope ratios of carbon and oxygen (δ¹³C and δ¹⁸O) of α-cellulose. All cultivars showed common ‘juvenile trends’ that were modeled with no...

The rate and magnitude of temperature variability at the transition from the Last Glacial Maximum into the early Holocene represents a natural analog to current and predicted climate change. A limited number of high-resolution proxy archives, however, challenges our understanding of environmental conditions during this period. Here, we present comb...

The Himalayas have a large impact on the surrounding lowlands and provide vital services including drinking water, hydropower generation and agricultural sustainability. Climate change is currently affecting the provision of these services, with substantial economic and social consequences. To date, very little is known about the sensitivity of the...

Anthropogenic activity is now recognised as having profoundly and permanently altered the Earth system, suggesting we have entered a human-dominated geological epoch, the ‘Anthropocene’. To formally define the onset of the Anthropocene, a synchronous global signature within geological-forming materials is required. Here we report a series of precis...

Climate change has altered precipitation patterns and impacted the spatio-temporal distribution and availability of water in high mountain environments. For example, intensification of the Indian Summer Monsoon (ISM) increases the potential for moisture laden air to breach the Himalayan orographic barrier and penetrate into the arid, elevated south...

In forests, the increase of atmospheric CO2 concentrations (Ca) has been related to enhanced tree growth and intrinsic water-use efficiency (iWUE). However, in drought-prone areas such as the Mediterranean Basin it is not yet clear to what extent this ‘fertilizing’ effect may compensate for drought-induced growth reduction. We investigated tree gro...

Scots pine (Pinus sylvestris L.) is a widely used tree species in European dendroclimatology studies due to its common distribution across much of the continent. Almost all studies find radial growth strongly related to summer temperature, a result reflecting site selection at high elevation/latitude environments where trees grow at their ecophysio...

Tree rings are natural archives that annually record distinct types of past climate variability depending on the parameters measured. Here, we use ring-width and stable isotopes in cellulose of trees from the northwestern Iberian Peninsula (IP) to understand regional summer hydroclimate over the last 400 years and the associated atmospheric pattern...

Deciphering large-scale spatiotemporal patterns of tree performance is essential to forecast global responses of forest ecosystems to environmental changes and their function as carbon sinks. Long-term information can be gained through the analysis of stable isotopes in tree rings as surrogates of plant carbon and water economies. We characterized...

The comprehensive procedure of wood sample preparation, including tree-ring dissection, cellulose extraction, homogenization and packing for stable isotope analysis, is labour intensive and time consuming. Based on a brief compilation of existing methods, we present a methodological approach from pre-analyses considerations to wood sample preparati...

Drought is a key limiting factor for tree growth in the Mediterranean Basin. However, the variability in acclimation via xylem traits is largely unknown. We studied tree growth and vessel features of Quercus petraea (Matt.) Lieb. in five marginal stands across southern Europe. Tree-ring width (TRW), mean earlywood vessel area (MVA) and number of ea...

Tree ring patterns provide one of the best records of pre-instrumental environmental and climate variability. To date, tree ring chronologies were explored from woody plant species with C3 photosynthetic pathway, only. For the first time, we have studied wood growth periodicity and stable carbon isotope ratios of tree ring cellulose of a tree speci...

In recent decades, the Tibetan plateau (TP) experienced a distinctive temperature increase, with fundamental consequences for the hydrological system. As meteorological time-series extending back more than 60 years are scarce, there is a strong need for proxy data providing insight into the regional hydroclimatic history as well as the long-term va...

Developing long-term chronologies of tree-ring anatomical features to evaluate climatic relationships within species might serve as an annual proxy to explore and elucidate the climatic drivers affecting xylem differentiation. Pinus leucodermis response to climate was examined by analyzing vertical xylem resin ducts in wood growing at high elevatio...

The warming trend at the end of the last glacial was disrupted by rapid cooling clearly identified in Greenland (Greenland Stadial 1 or GS-1) and Europe (Younger Dryas Stadial or YD). This reversal to glacial-like conditions is one of the best known examples of abrupt change but the exact timing and global spatial extent remain uncertain. Whilst th...

div class="title">Decadally Resolved Lateglacial Radiocarbon Evidence from New Zealand Kauri–CORRIGENDUM - Volume 58 Issue 4 - Alan Hogg, John Southon, Chris Turney, Jonathan Palmer, Christopher Bronk Ramsey, Pavla Fenwick, Gretel Boswijk, Ulf Buntgen, Michael Friedrich, Gerhard Helle, Konrad Hughen, Richard Jones, Bernd Kromer, Amexandra Noronha,...

According to the Intergovernmental Panel on Climate Change (IPCC), all of Africa is very likely to warm up more than the global average during this century. Especially (semi-)arid regions are expected to experience particularly high warming and possibly catastrophic droughts. However, assessments of the impacts of climate change on these regions ar...

The Last Glacial–Interglacial Transition (LGIT; 15,000–11,000 cal BP) was characterized by complex spatiotemporal patterns of climate change, with numerous studies requiring accurate chronological control to decipher leads from lags in global paleoclimatic, paleoenvironmental, and archaeological records. However, close scrutiny of the few available...

The TRACE 2015 conference was held on 20-23 May 2015 in Sevilla, Spain. This was the first TRACE in the Iberian Peninsula. It was organized by the University Pablo de Olavide (UPO) and the Association for Tree-ring Research (ATR), in collaboration with Pyrenean Institute of Ecology-Spanish National Research Council (IPE-CSIC), University of Barcelo...

Although solar radiation at the surface plays a determinant role in carbon discrimination in tree rings, stable carbon isotope chronologies (δ13C) have often been interpreted as a temperature proxy due to the co-variability of temperature and surface solar radiation. Furthermore, even when surface solar radiation is assumed to be the main driver of...

In geomorphologically complex mountainous environments, such as the High Himalayas, knowledge of spatial and altitudinal variations in orographic rainfall and surface water distribution can be used to link atmosphere and land surface on hydroclimatic processes. Because instrumental records are rare in the High Himalayas, hydro-climatic sensitive pr...

European lowlands experience many direct and indirect influences of global warming, particularly related to the hydrological cycle which lately faces increasing flood and drought events. Although important for humans and the ecosystems in which they live, little is known about the long-term spatiotemporal hydrological changes in various European re...

Tree-ring based temperature reconstructions form a substantial part of the international proxy data base used to examine and model global climate variations of the last Millennium. However, most tree-ring based reconstructions are derived from study sites in the high latitudes or high altitudes, paying very little attention to low elevation sites....

The procedure of wood sample preparation, including tree-ring dissection, cellulose extraction, homogenization and finally weighing and packing for stable isotope analysis is labour intensive and time consuming. We present an elaborated methodical guideline from pre-analyses considerations, wood sample preparation through semi-automated chemical ex...

The Earth's carbon and hydrologic cycles are intimately coupled by gas exchange through plant stomata. However, uncertainties in the magnitude and consequences of the physiological responses of plants to elevated CO 2 in natural environments hinders modelling of terrestrial water cycling and carbon storage. Here we use annually resolved long-term δ...

The Earth's carbon and hydrologic cycles are intimately coupled by gas exchange through plant stomata 1–3. However, uncertainties in the magnitude 4–6 and consequences 7,8 of the physiological responses 9,10 of plants to elevated CO 2 in natural environments hinders modelling of terrestrial water cycling and carbon storage 11. Here we use annually...

As a part of the “Global Change Observatory Central Asian” Project funded by the Helmholtz Centre Potsdam, a dendroecological monitoring of juniper and spruce has been setup in the southwestern part of Kyrgyzstan in the Sary-Chelek National Reserve (41°47'55.26"N, 71°56'36.82"E, 1500m asl.). The climate in this region is characterized by hot and dr...

The stable isotopes of carbon were analysed in total wood and cellulose from the tree-rings of the tropical wood species Cariniana micrantha (Ducke). The aim was to examine the isotopic relationship between total wood and its cellulose over the last two and a half centuries. Although the correlation for the whole time period is very high (r = 0.96)...

In recent decades, lake level fluctuations have been observed in the lake area of north eastern Germany. However our knowledge on these fluctuations mostly date back until the 1970ies when gauge records were initiated. To deepen our understanding of the observed lake level fluctuations, proxies which provide longer records of measurement would be o...

Forest decline played a pivotal role in motivating Europe's political focus on sustainability around 35 years ago. Silver fir (Abies alba) exhibited a particularly severe dieback in the mid-1970s, but disentangling biotic from abiotic drivers remained challenging because both spatial and temporal data were lacking. Here, we analyze 14 136 samples f...

A novel procedure has been developed to conduct cell structure measurements on increment core samples of conifers. The procedure combines readily available hardware and software equipment. The essential part of the procedure is the application of a confocal laser scanning microscope (CLSM) which captures images directly from increment cores surface...

Key message Carbon isotope ratios in growth rings of a tropical tree species show that treefall gaps stimulate diameter growth mainly through changes in the availability of light and not water. The formation of treefall gaps in closed canopy forests usually entails considerable increases in light and nutrient availability for remaining trees, as...

UV ‐laser‐based microscopic systems were utilized to dissect and sample organic tissue for stable isotope measurements from thin wood cross‐sections. We tested UV ‐laser‐based microscopic tissue dissection in practice for high‐resolution isotopic analyses (δ ¹³ C/δ ¹⁸ O) on thin cross‐sections from different tree species. The method allows serial i...

In the eastern Mediterranean in general and in Turkey in particular, temperature reconstructions based on tree rings have not been achieved so far. Furthermore, centennial-long chronologies of stable isotopes are generally also missing. Recent studies have identified the tree species Juniperus excelsa as one of the most promising tree species in Tu...