Evaluating tree-ring climate relationships from various climate data sources as predictors: A case study from the South-Eastern Tibetan Plateau

Abstract HKT-ISTP 2013 B

Intra-Annual Radial Growth of Pinus kesiya var. langbianensis Is Mainly Controlled by Moisture Availability in the Ailao Mountains, Southwestern China

Intra-annual monitoring of tree growth dynamics is increasingly applied to disentangle growth-change relationships with local climate conditions. However, such studies are still very limited in subtropical regions which show a wide variety of climate regimes. We monitored stem radius variations (SRV) of Pinus kesiya var. langbianensis (Szemao pine) over five years (2012–2015 and 2017) in the subtropical monsoon mountain climate of the Ailao Mountains, Yunnan Province, southwest China. On average, the stem radial growth of Szemao pine started in early March and ended in early October, and the highest growth rates occurred during May to June. Stem radius increments were synchronous with precipitation events, while tree water deficit corresponded to the drought periods. Correlation analysis and linear mixed-effects models revealed that precipitation and relative humidity are the most important limiting factors of stem radial increments, whereas air temperature and vapor pressure deficit significantly affected tree water balance and may play an important role in determining the growing season length and seasonality (i.e., duration, start, and cessation). This study reveals that moisture availability plays a major role for tree growth of P. kesiya var langbianensis in the Ailao Mountains, southwest China

Two Nothofagus Species in Southernmost South America Are Recording Divergent Climate Signals

Recent climatic trends, such as warming temperatures, decrease in rainfall, and extreme weather events (e.g., heatwaves), are negatively affecting the performance of forests. In northern Patagonia, such conditions have caused tree growth reduction, crown dieback, and massive die-back events. However, studies looking at these consequences in the southernmost temperate forest (Nothofagus betuloides and Nothofagus pumilio) are much scarcer, especially in southernmost South America (SSA). These forests are also under the influence of the positive phase of Antarctic Oscillation (AAO, also known as Southern Annular Mode, SAM) that has been associated with increasing trends in temperature, drought, and extreme events in the last decades. This study evaluated the growth patterns and the climatic response of eight new tree-ring chronologies from Nothofagus species located at the upper treeline along different environmental gradients in three study areas: Punta Arenas, Yendegaia National Park, and Navarino Island in SSA. The main modes of the ring-width index (RWI) variation were studied using principal component analysis (PCA). We found that PC1 has the higher loadings for sites with precipitation values over 600 mm/yr, PC2 with N. betuloides sites, and PC3 with higher loadings for sites with precipitation values below 600 mm/yr. Our best growth-climate relationships are between N. betuloides and AAO and the most northeastern site of N. pumilio with relative humidity (which coincides with heatwaves and extreme drought). The climatic signals imprinted in the southernmost forests are sensitive to climatic variability, the climate forcing AAO, and the effects of climate change in the last decades.Fil: Soto-Rogel, Pamela. Universidad de Magallanes; Chile. Universitat Erlangen-Nuremberg; Alemania. Cape Horn International Center; ChileFil: Aravena, Juan Carlos. Universidad de Magallanes; Chile. Cape Horn International Center; ChileFil: Villalba, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Bringas, Christian. Universidad de Magallanes; ChileFil: Meier, Wolfgang Jens-Henrik. Universitat Erlangen-Nuremberg; AlemaniaFil: Gonzalez Reyes, Álvaro. Centro Fondap de Investigación En Dinámica de Ecosistemas Marinos de Altas Latitudes; Chile. Universidad Mayor; ChileFil: Grießinger, Jussi. Universitat Erlangen-Nuremberg; Alemani

Late Holocene Glacial Fluctuations of Schiaparelli Glacier at Monte Sarmiento Massif, Tierra del Fuego (54°24′ S)

The Magallanes–Tierra del Fuego region, Southern Patagonia (53–56° S) features a plethora of fjords and remote and isolated islands, and hosts several thousand glaciers. The number of investigated glaciers with respect to the multiple Neoglacial advances is based on a few individual studies and is still fragmentary, which complicates the interpretation of the glacial dynamics in the southernmost part of America. Schiaparelli Glacier (54°24′ S, 70°50′ W), located at the western side of the Cordillera Darwin, was selected for tree-ring-based and radiocarbon dating of the glacial deposits. One focus of the study was to address to the potential dating uncertainties that arise by the use of Nothofagus spp. as a pioneer species. A robust analysis of the age–height relationship, missing the pith of the tree (pith offset), and site-specific ecesis time revealed a total uncertainty value of ±5–9 years. Three adjacent terminal moraines were identified, which increasingly tapered towards the glacier, with oldest deposition dates of 1749 ± 5 CE, 1789 ± 5 CE, and 1867 ± 5 CE. Radiocarbon dates of trunks incorporated within the terminal moraine system indicate at least three phases of cumulative glacial activity within the last 2300 years that coincide with the Neoglacial phases of the Southern Patagonian Icefield and adjacent mountain glaciers. The sub-recent trunks revealed the first evidence of a Neoglacial advance between ~600 BCE and 100 CE, which so far has not been substantiated in the Magallanes–Tierra del Fuego region

New perspective on spring vegetation phenology and global climate change based on Tibetan Plateau tree-ring data

Phenological responses of vegetation to climate, in particular to the ongoing warming trend, have received much attention. However, divergent results from the analyses of remote sensing data have been obtained for the Tibetan Plateau (TP), the world's largest high-elevation region. This study provides a perspective on vegetation phenology shifts during 1960-2014, gained using an innovative approach based on a well-validated, process-based, tree-ring growth model that is independent of temporal changes in technical properties and image quality of remote sensing products. Twenty composite site chronologies were analyzed, comprising about 3,000 trees from forested areas across the TP. We found that the start of the growing season (SOS) has advanced, on average, by 0.28 d/y over the period 1960-2014. The end of the growing season (EOS) has been delayed, by an estimated 0.33 d/y during 1982-2014. No significant changes in SOS or EOS were observed during 1960-1981. April-June and August-September minimum temperatures are the main climatic drivers for SOS and EOS, respectively. An increase of 1 °C in April-June minimum temperature shifted the dates of xylem phenology by 6 to 7 d, lengthening the period of tree-ring formation. This study extends the chronology of TP phenology farther back in time and reconciles the disparate views on SOS derived from remote sensing data. Scaling up this analysis may improve understanding of climate change effects and related phenological and plant productivity on a global scale

Long-term decrease in Asian monsoon rainfall and abrupt climate change events over the past 6,700 years.

Asian summer monsoon (ASM) variability and its long-term ecological and societal impacts extending back to Neolithic times are poorly understood due to a lack of high-resolution climate proxy data. Here, we present a precisely dated and well-calibrated tree-ring stable isotope chronology from the Tibetan Plateau with 1- to 5-y resolution that reflects high- to low-frequency ASM variability from 4680 BCE to 2011 CE. Superimposed on a persistent drying trend since the mid-Holocene, a rapid decrease in moisture availability between ∼2000 and ∼1500 BCE caused a dry hydroclimatic regime from ∼1675 to ∼1185 BCE, with mean precipitation estimated at 42 ± 4% and 5 ± 2% lower than during the mid-Holocene and the instrumental period, respectively. This second-millennium-BCE megadrought marks the mid-to late Holocene transition, during which regional forests declined and enhanced aeolian activity affected northern Chinese ecosystems. We argue that this abrupt aridification starting ∼2000 BCE contributed to the shift of Neolithic cultures in northern China and likely triggered human migration and societal transformation

The influence of decision-making in tree ring-based climate reconstructions.

Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794-2016 CE at 0.79 (p < 0.001), reveals summer cooling in the years following large volcanic eruptions, and exhibits strong warming since the 1980s. Differing in their mean, variance, amplitude, sensitivity, and persistence, the ensemble members demonstrate the influence of subjectivity in the reconstruction process. We therefore recommend the routine use of ensemble reconstruction approaches to provide a more consensual picture of past climate variability

Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE

This study was funded by the WSL-internal COSMIC project (5233.00148.001.01), the ETHZ (Laboratory of Ion Beam Physics), the Swiss National Science Foundation (SNF Grant 200021L_157187/1), and as the Czech Republic Grant Agency project no. 17-22102s.Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770–780 and 990–1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.Publisher PDFPeer reviewe

Untersuchungen zur Klimavariabilität auf dem Tibetischen Plateau - ein Beitrag auf der Basis stabiler Kohlenstoff- und Sauerstoffisotope in Jahrringen von Bäumen waldgrenznaher Standorte

To contribute to the available information on climate history in high Asia, stable oxygen and carbon isotopes from about 18.000 individual tree rings from three sites and two species in southeast Tibet have been analysed using a mass spectrometer. The first species is a very long-living juniper tree ($\textit{Juniperus tibetica}$) which was complemented by spruce trees ($\textit{Picea balfouriana}$) at one site. The longest isotope time series goes back to 490 AD, thus encompassing a time span of about 1520 years. The study region in southeast Tibet (25-45°N/70-110°E) is situated between the deeply incised gorges of the great Asian rivers, which are strongly influenced by monsoonal rainfalls, and the semi-arid, continental alpine meadows and steppes of the Tibetan plateau. The largest distance between the sites is about 800 km. However, the climatic conditions at all sites during the growth season are quite similar, caused by the influence of the Indian summer monsoon and the Asian summer monsoon, respectively. Thus, both monsoons cause a characteristic seasonal change in the regional climate, which strongly influences tree growth. The three sites, located near the upper timber line, are aligned along a moisture gradient from the cool and moist climate of the river gorge region to the dry and warm climate of the steppe forests in southern Tibet. This thesis focuses on the detection of local climate signals stored in the variations of stable carbon and oxygen isotopes in tree-ring cellulose. Therefore, four annually resolved, centennial chronologies for $\delta^{13}$C and $\delta^{18}$O from subalpine forest sites in southeast Tibet were developed. After conducting analyses of correlation and gleichlaeufigkeit (GLK), distinct relationships between $\delta^{13}$C and $\delta^{18}$O isotopes of tree-rings and locally relevant climate elements were established. In most cases, these results are statistically significant and plausible from an ecological point of view. The variations of carbon and oxygen isotopes in juniper and spruce trees of ecologically different sites are highly influenced by several superimposed climate factors. A comparison of $\delta^{13}$C and $\delta^{18}$O time series from juniper and spruce trees also revealed remarkable results: while $\delta^{18}$O variations in both tree species are clearly influenced by a common, exogenous factor, the time series of $\delta^{13}$C for both species show only weak correlations. On the other hand, the highest correlations between $\delta^{18}$O time series of the different sites do not occur between the three juniper chronologies, but between the individual juniper and the spruce chronology, respectively. Thus, the $\delta^{18O}$ juniper chronologies seem to represent local site conditions or precipitation, while the $\delta^{18}$O spruce chronology seems to represent a regional climate signal. This combination of dendrochronological methods, including ring width and stable isotopes of $\delta^{13}$C and $\delta^{18}$O, which was applied to all four sites, is highly relevant for further studies on climate variability and ecology of southeast Tibet. Since the different tree ring parameters are independent of each other, individual analyses of the climatic forcing for each parameter and for all sites are possible. Additionally, the combination of these parameters allows the reconstruction of different seasonal climate patterns at each site. A more detailed knowledge of the relationship between the different spatial and temporal climate patterns will improve palaeoclimatic analyses. Furthermore, it will also contribute to a better evaluation of the effects of climate change on forests in the southeastern part of the Tibetan Plateau