Time delays between environmental forcing and isotopic signatures of tree-rings
The use of multiple data from tree-rings, including isotopic ratios and xylogenesis monitoring, can enhance our interpretations on tree functioning and on tree-environment relations. Here, we explored whether understanding of carbon deposition to tree-rings could be improved using: (1) monitoring of wood cell formation during the growing season, (2) intra-annual data of d13C in tree-ring cellulose and (3) ecophysiological modelling. We collected wood micro-cores to monitor wood cell formation of six Silver fir trees at two forest sites in the Vosges mountains (France) and detect periods of cell division, cell enlargement and cell wall thickening of 10 tree-ring subsectors per year over three years (2007-2009). The same trees were sampled to measure d13C of tree-ring cellulose (d13Ccell) over similar 10 tree-ring subsectors of the same three years. We then used an ecosystem model, integrating a model for sugar availability and carbon isotope fractionation, to identify periods of carbon deposition in each tree-ring subsector based on the agreement between simulated and observed d13Ccell values. Our results show how intra-annual patterns of 13C/12C ratios of tree-ring cellulose were driven by vapor pressure deficit variations over the growing season and that cellulose deposited in the tree-ring subsectors integrated carbon that started to be assimilated in the cells during the cell division phase and before the cell enlargement and wall thickening. This means that there is a time delay between the environmental forcing and the isotopic signature of tree-rings. Our study provides useful insights on carbon allocation to wood in Silver fir that can be used to improve interpretations of isotopic time series of this species.