Linking intra-seasonal variations in climate and tree-ring δ13C: A functional modelling approach

Stable carbon isotopic composition (delta C-13) in tree rings is a widely recognized tool for climate reconstruction, and several works suggest that seasonal information can be extracted from intra-ring delta C-13 variations. In this study, we explored the link between climate and intra-seasonal oak ring delta C-13 using a process-based modelling approach. The ISOCASTANEA model was developed to compute the seasonal dynamics of tree-ring delta C-13 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 H2O and CO2 fluxes measured by the eddy flux technique, and simulated delta C-13 values were compared to seasonal patterns of total organic matter delta C-13 measured in tree rings for 2006 and 2007 at the same site. With the inclusion of carbon translocation and with regard to C-13 enrichment of starch compared to soluble sugars, the model can reasonably simulate the intra-seasonal and inter-annual variability of tree-ring delta C-13 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 delta C-13 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 delta C-13 in the early part of the ring to carbohydrate metabolism suggests that intra-ring delta C-13 could be used to explore the relationship between tree decline or mortality and carbohydrate deficiency. (C) 2010 Elsevier By. All rights reserved.