Seasonal physiological interpretation of intra-ring δ13C variation in temperate species: experimental and modelling approach

International audienceRing δ13C is a physiological and climatic archive that offers resolution at the annual and seasonal time scales. In this contribution, we synthesize our results relative to the functional understanding of the intra-ring δ13C variations. Our experimental approach consists in studying processes that are likely to influence the seasonal δ13C variations. We focus on two deciduous European species: Quercus petraea and Fagus sylvatica.One main result in oak is the identification of a recurrent pattern of the seasonal variations of the δ13C. These variations were mainly related to the mobilization of carbon reserves at the beginning of growth (high δ13C values) and to the influence of environmental conditions on the photosynthetic discrimination during the rest of the growing season. The δ13C of the latewood was correlated to climatic conditions (especially vapor pressure deficit, soil water content). For Fagus, there was no apparent reserve effect at least in healthy trees. However, in declining beech trees, δ13C values tended to be higher (Granda et al. in prep) and an intra-ring δ13C decrease reveals a difference in starch storage dynamics compared to healthy trees (Michelot et al. in prep).The δ13C intra-annual variations of the ring bulk organic matter were little influenced by variations in biochemical composition (in particular, lignin and holocellulose contents, Eglin et al. 2009). Comparisons between water use efficiency (WUE, the ratio of carbon assimilation and stomatal conductance to water) calculated from a leaf carbon isotope discrimination model applied to δ13C ring data and simulated or measured WUE at the tree or ecosystem levels suggest that latewood δ13C is a seasonal recorder of WUE with a time lag corresponding to the mixing time of sugars in the phloem (Michelot et al. 2011). Recent results at the inter-annual scale over 8 years indicate that the maximal correlation between WUE deduced from δ13C and that obtained from C and H2O flux is obtained with a time lag of 1-20 days. Moreover, intra annual δ13C patterns exhibited important inter-annual and inter-individual variability which were mainly attributed to inter-annual climate variations and competitive status of the trees respectively.These data from experiments have been used to elaborate a mechanistic model simulating the carbon isotopic balance at tree-scale (Eglin et al. 2010). This model has been coupled with CASTANEA, a model simulating the carbon, water and energy balances at stand-scale, then calibrated and validated on measurements of intra-annual variations of δ13C within oak rings, sampled in the Barbeau forest (ICOS network, http://www.barbeau.u-psud.fr/). Results from ISOCASTANEA showed a strong attenuation of the carbon isotopic signal from the leaf to the ring. Key parameters influencing the simulated whole-tree-ring δ 13C was tested by a sensitivity analysis: budburst date, growth rate and the discrimination during respiration are particularly important. From a physiological point of view, use of inter-annual intra-ring δ13C associated with other variables like anatomy, microdensity could particularly relevant to examine the importance of the functional life history of the trees to explain their vulnerability to climate change