Linking intra-seasonal variations in climate and tree-ring δ 13C: A functional modelling approach

Abstract

Stable carbon isotopic composition (δ 13C) in tree rings is a widely recognized tool for climate reconstruction, and several works suggest that seasonal information can be extracted from intra-ring δ 13C variations. In this study, we explored the link between climate and intra-seasonal oak ring δ 13C using a process-based modelling approach. The ISOCASTANEA model was developed to compute the seasonal dynamics of tree-ring δ 13C for deciduous species from half-hourly climatic data by accounting for photosynthetic discrimination and carbon translocation and allocation at the tree scale and in tree rings. The model was applied from March 2005 to December 2007 in a 150-year-old deciduous oak forest. Canopy photosynthesis and stomatal conductance were calibrated using H 2O and CO 2 fluxes measured by the eddy flux technique, and simulated δ 13C values were compared to seasonal patterns of total organic matter δ 13C measured in tree rings for 2006 and 2007 at the same site. With the inclusion of carbon translocation and with regard to 13C enrichment of starch compared to soluble sugars, the model can reasonably simulate the intra-seasonal and inter-annual variability of tree-ring δ 13C using the same parameter values for 2006 and 2007. The amplitude of the seasonal carbon isotope pattern in tree rings was influenced by both photosynthetic and post-photosynthetic processes (starch enrichment and reserve use). The δ 13C variations in the early part of the ring, i.e., mainly in the earlywood, were related mostly to carbohydrate metabolism, although diluted information about environmental conditions during the previous year could also be found. The last part of the ring, consisting mainly of latewood, was found to be a good recorder of current-year environmental conditions, in particular relative humidity, at a fine temporal resolution when the growth rate was high. The sensitivity of the δ 13C in the early part of the ring to carbohydrate metabolism suggests that intra-ring δ 13C could be used to explore the relationship between tree decline or mortality and carbohydrate deficiency.