Interpreting high-resolution stable isotope signals in tree ring cellulose in terms of seasonal climate variations using a simple model of carbohydrate reserves

The carbon and oxygen stable isotope composition of cellulose in annual tree rings is a useful signal to understand water use and carbon allocation in trees. Recent high-resolution stable isotope measurements in tree ring cellulose have revealed distinct seasonal patterns and inter-annual variability. Yet the interpretation of this isotopic signal is complex because it does not appear to record the signature of current assimilates directly (or its driving climate variables) but also that of carbohydrate reserves. Here, we present a simple model of carbohydrate reserves in trees in order to "explore this gap" and study the link between the cellulose signal and seasonal climate variations. To test this model, we cored a Pinus pinaster tree that had been equipped with sap flow sensors and micro- dendrometers in 1997-1998. The phenology of the tree (shoot and needle elongation periods), its architecture and biomass were also recorded. At that time, the tree was representative of the stand where it was growing, a CarboEurope site instrumented with micrometeorological and eddy flux measurements. The two annual rings corresponding to years 1997 and 1998 were divided into 100μm-thick slices and the cellulose was extracted for isotopic analysis. Both carbon and oxygen stable isotope signals recorded in the annual rings exhibit distinct seasonal patterns but with differences between the two years. During this talk I will show how our simple model of carbohydrate reserves, forced by sap flow and eddy covariance measurements (downscaled to the tree level), enables us to interpret these seasonal and inter-annual patterns in terms of spring precipitation, air water vapour deficit or incoming solar radiation. A sensitivity analysis of the model will also be presented to try quantifying how much information on water use and carbon allocation in trees we can get from these isotopic profiles and how they are linked to climate variability.