Deperissement des chenaies et changements climatiques en Sologne Bourbonnaise, Auvergne, Allier.

BREYSSE Bernard

RESUME
L’Auvergne comporte une importante surface forestiere (700 000 ha) majoritairement composee d’especes feuillues, les chenes sessile et pedoncule etant preponderants. Cette surface forestiere est detenue a 85% par des proprietaires forestiers prives. Certaines chenaies du departement de l’Allier font l’objet d’une notoriete internationale pour la qualite de leurs bois (foret domaniale de Troncais). Ces bois etant destines a la fabrication de tonneaux pour d’elevage de grands crus. Depuis plusieurs decennies des phenomenes de deperissements ont ete observes dans ces peuplements de chenes, en foret publique et en foret privee. Notre etude se concentre sur la partie Allier de la Sologne Bourbonnaise, region naturelle caracterisee par un relief de plateau et des sols a nappe d’eau perchee temporaire. Notre hypothese initiale est que les signes de deperissements sont lies a la double contrainte hydrique (ennoyage-deficit) que subissent les arbres sur ce type de sol. Les deficits hydriques en saison de vegetation risquent de devenir de plus en plus importants dans le contexte de changements climatiques. Sur deux sites geographiquement proches et deux peuplements de chenes d’aspect regulier en foret privee, nous avons realise un inventaire des especes, avec mesure des circonferences a 1,30 m du sol, sur la base de quatre placettes par hectare pour une surface d’environ 1200 m2 ha-1. En meme temps, nous avons note l’abondance-dominance de la molinie bleue et de la fougere aigle avec les coefficients de Braun-Blanquet. Nous avons evalue l’importance des deperissements dans ces peuplements avec le protocole DEPEFEU, en saison de vegetation, et le protocole ARCHI, en periode hivernale. A partir des resultats de cet inventaire, un echantillon d’arbres representatifs de la distribution des circonferences et des notes DEPEFEU a ete choisi. Sur les individus selectionnes nous avons preleve des biopsies au pied des arbres qui nous ont permis de mesurer les accroissements annuels radiaux depuis 2000. Les parametres edaphiques ont egalement ete apprecies (profondeur du plafond de la nappe d’eau perchee temporaire, reservoir utile). Nous avons aussi integre les variables climatiques a partir des releves de la station meteorologique d’Yzeure (03). Nos investigations concluent que les protocoles utilises sont pertinents pour evaluer le deperissement des chenes et que les individus les plus affectes aujourd’hui ont manifeste des baisses significatives d’accroissement radial depuis le milieu des annees 2000.

https://hal-ephe.archives-ouvertes.fr/hal-01645086/document

Dépérissement des chênaies et changements climatiques en Sologne Bourbonnaise, Auvergne, Allier.

The characterization of natural climate variability is important in order to understand the climate response to natural forcings and to identify anthropogenic influences. The aim of this thesis is to reconstruct climate changes in the southwest of France, a region which is characterised by recurrent drought periods, where high resolution proxy records of the last millennia were lacking.The reconstruction is based on multiple proxies from two continental archives: speleothems and tree rings. Their combination can make use of the strengths of each archive while compensating their weaknesses. There are two principal objectives: first, to gain a better understanding of the climatic and non-climatic influences on each proxy; and second, to reconstruct drought periods in the past.The oxygen isotopic composition (δ18O) of speleothem fluid inclusions and tree ring cellulose is controlled to a large extent by the δ18O of precipitation, which can serve as a tracer of the atmospheric circulation. In order to interpret these proxies in terms of climate, it is necessary to understand how the climate signal becomes recorded in the proxy, and which processes modify the original signal during the formation of the archive.Measurements of δ18O in precipitation, cave drip water, and fluid inclusions in modern speleothem samples from Villars Cave demonstrated that the isotopic composition of cave drip water corresponds to the pluri-annual average precipitation. The speleothem fluid inclusions, in turn, preserve the isotopic composition of the drip water. Based on this calibration, it is possible to reconstruct drip water isotope variability using fluid inclusions in a more than 2000 year old stalagmite, which has been dated by laminae counting, as well as U-Th and 14C measurements. Changes in the cave environment, e.g. the vegetation cover, are indicated by other proxies from the same stalagmite (stable isotopes in calcite and trace element concentrations), but these changes do not seem to impact the fluid inclusion δ18O significantly.The isotopic composition of tree ring cellulose from Quercus spp. in the study area is strongly influenced by climate conditions during the summer. However, non-climatic influences on the isotopic composition of cellulose are identified. They are linked to the age of the trees and to site hydrology, and must be accounted for in the sampling and analytical procedures. Crossdated cores from living trees and timber wood in historic buildings near Angouleme are used to build an annually resolved chronology of cellulose δ18O. Significant correlations with meteorological data enable a calibration and a reconstruction of drought periods since 1360 AD.Lastly, this thesis explores a novel approach of integrating oxygen isotope records from speleothem fluid inclusions and tree ring cellulose from closely located sites to reconstruct both high- and low-frequency variability of droughts in the past.

Climate Variability in Southwest France During the Last 2000 Years : Proxy Calibration and Reconstruction of Drought Periods Based on Stable Isotope Records from Speleothems and Tree Rings

When evaluating tree architecture, condition and vitality, often, the prevalence and morphology of epicormic shoots are ignored or misinterpreted. However, epicormic shoots can play a fundamental role in tree survival and need to be taken into account when they occur. The presence and abundance of three morphologically different types of epicormic shoots can be used to distinguish between trees which are healthy, stressed, resilient (i.e. likely to recover after being stressed), undergoing natural crown retrenchment, or in a state of irreversible decline. Orthotropic and plagiotropic epicormic shoots, which respectively replicate the architecture of young trees and branches, act as the tree's “life insurance” against future catastrophe. On the other hand, ageotropic epicormic shoots, which have the characteristics of old age, indicate irreversible decline. Analysis of annual growth-ring width has shown that the architectural characters assessed for pedunculate oak (Quercus robur L.) are good indicators of...

The diagnostic method ARCHI applied on declining pedunculate oaks

In palaeoclimate reconstructions, the combination of proxy records measured in different climate archives is challenging because of the uncertainties associated with each proxy, but it can also help reduce some of these uncertainties. Here, we present a novel approach to combine speleothem and tree ring proxies for a drought reconstruction of the last 640 years: a fluid inclusion δ18O record from a stalagmite from Villars Cave (southwest France) and a tree ring cellulose δ18O record of Quercus spp. from the nearby Angouleme area. The δ18O of the fluid inclusions is taken as an estimate of the δ18O of the trees’ source water. Then, the cellulose and source water δ18O are used to calculate the leaf water isotopic enrichment, as well as relative humidity, which is the dominant controlling factor of this enrichment. The reconstructed long-term trends in relative humidity differ from a previously published reconstruction of moisture variability based on the tree ring record alone. Further measurements will be necessary to support either reconstruction. Nevertheless, this investigation demonstrates the great potential for combining isotope proxies from speleothems and tree rings to reconstruct both the low- and high-frequency variability of drought.

/pdf/a-multi-proxy-approach-to-drought-reconstruction-4u66xxpmhs.pdf

A multi-proxy approach to drought reconstruction

Des premiers résultats riches de renseignements Durant la saison de végétation 2009, 184 peuplements de chênes (sessile, pédonculé ou en mélange avec du pubescent notamment) ont été observés dans les forêts privées des régions Pays de la Loire et Poitou-Charentes (figures 1 et 2, p. 51). Le niveau de dépérissement des chênes de chaque placette a été évalué en employant le protocole DEPEFEU (protocole de mesure du dépérissement des feuillus) du Département santé des forêts. L’espèce chêne identifiée, de multiples mesures dendrométriques (circonférence, diamètre de houppier, hauteur dominante, surface terrière,...) ont été effectuées. Un relevé phytoécologique (sol, flore, topographie, pH) a également été réalisé sur chaque placette. Chaque placette a ensuite été localisée avec le GPS. Leurs données climatiques (normale climatique trentenaire 1961 – 1990) ont ensuite été collectées. Cette étude a également bénéficié de l’apport de l’étude de Catherine Mercadier (DRAAF Poitou-Charentes 1997) « Étude des dépérissements de chênes dans les Deux-Sèvres et dans la Vienne ». 56 peuplements dépérissants de chênes y ont été décrits. Les chênaies atlantiques face aux changements climatiques globaux : comprendre et agir Jean Lemaire* ,Yves Lacouture**,Maël Soleau et Christian Weben***, Marc Mounier et Arnaud Guyon ****(1)

Y. Lacouture

Papers

Les chênaies atlantiques face aux changements climatiques globaux: comprendre et agir