PI : Nathalie Breda (UMR1137 – Ecologie et Ecophysiologie Forestières)
BEF, LERFOB, LIBio, LAE
Unité Forestière de Lorraine (UEFL, INRA Nancy)
Context — An increased frequency of extreme climatic events could be accompanied by higher risks of tree mortality. Forest managers and forest monitoring services have reported that abnormally high mortality rates of individual trees within stands follow sever drought events. However, the mechanisms driving tree death are today still poorly understood and have been the subject of controversial debate within the international scientific community.
Objective — The MEPiB-DEATH project aims to understand the physiological mechanisms and morphological changes impacted by severe stress conditions which induce tree death (defoliation and drought). In particular, this involves analysis of conditions of hydraulic failure and carbon depletion associated with tree dieback. Further, this project explores the diverse range of responses to these extreme stress conditions among 14 beech forest populations in Lorraine, France. An additional objective is to improve our understanding of the impacts of stress conditions on carbon allocation, which will be fully developed by the end of the project and made available to modeling teams within LabEx ARBRE.
Approach — An experimental approach in semi-controlled conditions was used to alter the water or carbon status of beech planted 10 years ago in the INRA nursery. Approximately 1,000 young beech trees were covered with a roof to control the water regime. Three treatments were installed: a controlled irrigation treatment, treatments of irrigation and defoliation in June 2014 and June 2015 aimed to mainly impact carbon status, a dry treatment starting in June 2014 to impact water status in particular. In each treatment, 14 populations are represented in two replicates of 12 trees.
Key results —
The 1,000 beech trees involved in the experiment were subjected to their second season of controlled constraints with a new defoliation treatment in June 2015 and a water deficit treatment continued from June 2014 in the dry treatment.
- Training through research: Pierre Antoine Chuste began his doctoral work in the fall of 2014 covering the study of growth and storage (C and N) functions (WP 1 and WP2). Rémi Wortemann studied the hydraulic aspect (WP3) in the context of his post beginning in February 2015. In 2015, 5 Master 1 and 2 students contributed to Mepib-Death work packages: on phenology and growth (WP1), on carbon assimilation and respiration (WP2), on water uptake and root growth, and on native embolism of the populations (WP3).
- 2015 was marked by the construction and implementation of innovative gas exchange measurement chambers for whole trees (photo), heavy campaigns of predawn leaf water potential measurements, achieving isotopic labeling (13C, 15N).
- Defoliation in 2014 caused a temporary slowdown in root growth, but showed little affect on primary and secondary growth. Transpiration and carbon assimilation were not affected, due to improved penetration of the radiation in the lower part of the canopy.
- The water deficit imposed in 2014 induced in 2015 a delayed bud break in the spring, severe primary and secondary growth reduction, decreased elementary leaf surface, limited transpiration and carbon assimilation due to stomatal regulation.
- The native embolism rates between 14 populations of beech, studied in the zone under natural conditions, showed a high plasticity, without significant differences among populations.
- The first tree moralities appeared during the dry treatment: In late summer 2014 in conjunction with an attack of woolly adelgid, and throughout 2015 (7 trees had no bud break in the spring and 11 trees died during the summer drought). Loss of hydraulic conductivity in the terminal twigs were measured. The trees’ carbon status has not yet been quantified.
- Characterization of trees water status was carried out in the late summer of 2015 in the Hesse 2 forest site. Predawn leaf water potential of trees in natural conditions or from the rainfall exclusion design were similar and markedly negative due to the 2015 summer drought. Beech reached the most negative potential compared to oak, hornbeam and birch, in connection with their respective rooting patterns.
Future perspectives — Non structural carbohydrate and nitrogen contents will be measured during the 2015-2016 winter, partly biochemically and partly by NIRS / MIRS spectroscopy: calibration is currently underway as part of a collaboration between EEF / BEF (LabEx WP2 – functional ecology work package). The vulnerability curves to cavitation of the 14 populations will be established using the cavitron equipment available at the UMR PIAF in Clermont. The evolution of native embolism after two years of stress will be assessed in 2016. The ecophysiological and morphological measures will be continued throughout the 2016 season, particularly during the spring to monitor the occurrence of death during the budburst and growth recovery. Wood anatomy will be characterized for individuals harvested in 2015. The analysis of secondary metabolism (WP4 – forest economy) will be conducted in 2016 with a Master 2 student.
Knowledge transfer — The Mepib-Death project provides support for the particpatory science project entitled “SURVIVORS” co-constructed between the pilot unit, LabEx ARBRE, the CPIE and the middle school College of Einville au Jars. Several events and communication activities associated with this project were held in 2015 and will continue through 2016 with a group of 80 middle school students. They will follow the project from their fifth to third classes. The “SURVIVORS” project was presented at the “Science & You” conference held in Nancy in June 2015, was presented again in September 2015 at the Regional Prize ceremony for the Lorraine region ‘Science and Society’, and was included in the “Climate Train” event at the Nancy train station in October 2015. Activities will continue throughout the 2015-2016 school year. Two scientific posters were presented during the Xylem international meeting held in Bordeaux in September 2015.
FREY-KLETT P., BRÉDA N., MASSONNET C., GALLEY C., MARCHAL I., DEGREMONT M. A. (2015) SURVIVORS : un projet de sciences participatives avec des collégiens. Sciences & You, Nancy, 1-6 juin 2015. (comm. orale + actes) http://www.science-and-you.com/fr/colloque/vendredi-5-juin.
CHUSTE P.A., MASSONNET C., MAILLARD P. (2015) What roles of carbon and nitrogen metabolisms in the death of beech trees ? Xylem International Meeting, Bordeaux 7-9 September 2015 (Abstract +Poster)
MASSONNET C., BRÉDA N. (2015) Vulnerability of Lorraine’s Beech Provenances to Drought: Disentangling Carbon and Hydraulic dysfunctions leading to Tree Mortality. Xylem International Meeting, Bordeaux 7-9 September 2015 (Abstract +Poster)
BREDA N. (2015) Interview “Forêt et Changement climatique”, par Samuel Goldschmidt, pour RTL Le Journal Inattendu de Marie Drucker, 31 octobre 2015
INRA. Des collégiens observent les arbres souffrir. Rapport Annuel 2014, L’institut, p 56.