Frédéric Guibal

Climate/growth relationships in a Pinus cembra high-elevation network in the Southern French Alps

Abstract•IntroductionIn the context of climate change, assessing climate–growth relationships is of high importance in order to understand how forest ecosystems evolve and to test climate models at regional scale.•ObjectivesThis study aims to identify the climate variables that explain most of the variation in Pinus cembra radial growth at different spatial–temporal scales, response functions and moving response functions were processed on chronologies from a dense tree ring network. This original network is centred on the French Alps influenced by the Atlantic, Continental, and Mediterranean synoptic systems.•ResultsA spatial clustering pattern matching the latitudinal climatic gradient was observed in tree ring indexed chronologies and climate responses. The stationary response functions computed for each cluster-averaged population evidenced a thermo-dependent northern cluster (Cl1), a rainfall-dependent southern cluster (Cl3), and an alternative response for the intermediate cluster in between (Cl2). Since the late nineteenth century, the moving response functions indicate an increasing influence of winter precipitation for the northern and the intermediate clusters.•ConclusionConsidering the increase in temperature and reduction both of summer precipitation predicted by general circulation models and of the snow cover and duration in the French Alps, stone pine is likely to be exposed to stressful conditions during the twenty-first century.RésuméDans le contexte actuel de changement climatique, l’amélioration des connaissances concernant les relations entre le climat et la croissance des arbres est primordiale pour comprendre l’évolution des écosystèmes forestiers et tester des modèles climatiques à l’échelle régionale. Afin de déterminer les variables climatiques expliquant significativement les variations de la croissance radiale du Pin cembro à différentes échelles spatiales et temporelles, un vaste réseau de données dendrochronologiques a été utilisé pour le calcul de fonctions de réponse fixes et mobiles. Ce vaste réseau original est centré sur les Alpes françaises soumises à des influences climatiques océaniques, continentales et méditerranéennes. Les analyses en composante principale réalisées sur les chronologies indicées et sur les résultats des fonctions de réponse révèlent l’influence d’un gradient climatique latitudinal permettant d’identifier différents groupes de populations. Les fonctions de réponses calculées pour chaque groupe de populations moyennées mettent en évidence une dépendance aux températures des populations septentrionales (Cl1), une dépendance aux précipitations des populations méridionales (Cl3) et une réponse alternative d’un groupe intermédiaire Cl2). Depuis la fin du 19ème siècle les fonctions de réponse mobiles révèlent une augmentation de l’influence positive des précipitations hivernales pour les populations des groupes septentrionale et intermédiaire. Compte tenu de l’augmentation des températures et de la réduction à la fois des précipitations estivales prédites par les Modèles Globaux de Circulation et de l’épaisseur et de la durée de la couche nivale dans les Alpes françaises, le Pin cembro pourrait être soumis à une augmentation des conditions de stress au cours du 21ème siècle.

Landscape evolution and agro-sylvo-pastoral activities on the Gorgan Plain (NE Iran) in the last 6000 years

The Gorgan Plain (NE Iran) is characterized by fertile soils formed on a loess plateau and is at present primarily exploited for intensive agriculture. However, the timing and intensity of the human impact on the landscape in the past are still unclear. A sediment core, taken from the centre of the eastern Gorgan Plain in the Kongor Lake covering the major part of the Holocene from 6.1 to 0.8 ka (all ages are calibrated before present), has been studied for pollen, non-pollen palynomorphs, botanical macroremains, insects, charcoal, geochemistry, biomarkers and magnetism in order to provide new insights into the evolution of the landscape and to estimate the intensity of human activities. The data obtained suggest a dry period between 5.9 and 3.9 ka and an increase in regional humidity afterwards with a maximum between 2.7 and 0.7 ka, during the period of the Persian empires (Achaemenid through Sasanian) and the Islamic era. The eastern part of the Gorgan Plain was characterized by open steppe landscapes during the last 6 ka, which most likely were used for pasture and at least since 2.7 ka for agriculture including arboriculture. The strongest anthropogenic impact on the environment around the Kongor site is documented during the Parthian and Sasanian Empires (200 BC–651 AD) and the Islamic era up to the eve of the Mongol invasion.

Wood 14C Dating with AixMICADAS: Methods and Application to Tree-Ring Sequences from the Younger Dryas Event in the Southern French Alps

The AixMICADAS facility is in part dedicated to research on radiocarbon ( 14 C) calibration by means of various archives. For this purpose, we are improving upon the capacity to accurately date subfossil wood. In the current study, nine chemical pretreatment protocols are tested on six wood samples of known ages. The optimization based on 14 C ages, 13 C/ 12 C ratios, carbon % and overall mass yield % leads us to favor the acid-base-acid-bleaching pretreatment (ABA-B). This efficient method is shown to provide a residue of holocellulose with optimal blanks equivalent to an age of 51,300 14 C BP with a standard deviation of 1500 yr based on 25 analyses. The seven wood samples from the Sixth International Radiocarbon Intercomparison (SIRI) are then analyzed as a further verification of the accuracy of our method. As a first scientific contribution, we studied two tree-ring sequences from subfossil pines (Barb12 and Barb17) collected in the southern French Alps. New 14 C analyses were performed at high resolution (every third year) and are shown to agree well with results obtained previously by high precision β-counting on CO 2 from large samples at lower resolution for Barb17 and accelerator mass spectrometry (AMS) data for Barb12. The new 14 C series are then matched to the Kauri and YDB chronologies: the new sequence of Barb12-17 tentatively corresponds to the interval between 12,836 and 12,594 cal BP within the Younger Dryas cold period. The 14 C comparison between the Barb12-17 sequence from France and the Kauri sequence from New Zealand allows calculating the 14 C Inter-Hemispheric Gradient (IHG), with an average value of ca. 57 yr. The IHG stayed relatively high throughout the studied period. Interestingly, the IHG exhibits a transient maximum value (ca. 100 yr) during the period of rapid Δ 14 C rise (12,750–12,720 cal BP), a behavior that could be due to a delayed response of the Southern Hemisphere.

Subfossil trees suggest enhanced Mediterranean hydroclimate variability at the onset of the Younger Dryas

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. Here, we report a hydroclimatic reconstruction from stable isotopes (δ18O, δ13C) in subfossil pines from southern France. Growing before and during the transition period into the YD (12 900–12 600 cal BP), the trees provide an annually resolved, continuous sequence of atmospheric change. Isotopic signature of tree sourcewater (δ18Osw) and estimates of relative air humidity were reconstructed as a proxy for variations in air mass origin and precipitation regime. We find a distinct increase in inter-annual variability of sourcewater isotopes (δ18Osw), with three major downturn phases of increasing magnitude beginning at 12 740 cal BP. The observed variation most likely results from an amplified intensity of North Atlantic (low δ18Osw) versus Mediterranean (high δ18Osw) precipitation. This marked pattern of climate variability is not seen in records from higher latitudes and is likely a consequence of atmospheric circulation oscillations at the margin of the southward moving polar front.