Can climate variations be inferred from tree-ring parameters and stable isotopes from Larix decidua? Juvenile effects, budmoth outbreaks, and divergence issue

Larch wood has been used for centuries as a building material. Hence, the study of the tree-ring width, the latewood maximal density, and the oxygen and carbon isotope composition of the cellulose of this tree provides potential and valuable insights when reconstructing past climate variability. We explore the qualities and limitations of these proxies, focusing on a forest standing the Nevache valley (French Alps). The analysis of 15 trees demonstrates a small intra-tree variability in comparison with the inter-tree variability of delta C-13 and delta O-18, and shows that 6 trees, at least, must be pooled to make a population-representative sample. Our results show no juvenile effect for delta C-13. Unlike tree-ring width and density, delta C-13 and delta O-18 are not altered by budmouth outbreaks. These two parameters therefore appear well suited for climate reconstructions, and depict a strong correlation with July-August temperature and relative humidity. The delta O-18 of larch cellulose is also strongly linked with the previous winter (December-March) oxygen isotopic composition of the precipitation. This is consistent with a winter water recharge of soil and ground. The past variations of July-August maximum temperature and relative humidity were established using two different combinations of the isotopic ratios. Uncertainties on the reconstructions are estimated respectively at +/- 1.4 degrees C and 3.6%. Inter-annual variations of temperature and relative humidity are well reproduced. However, the reconstructed July-August temperature series diverges from the instrumental one, being lower, after ca. 1990. The effects of the variation through time of the depth of source water and of the ecophysiological response of trees to rising CO2 on the isotope composition are discussed as possible causes of divergence. This 'divergence problem' strongly questions the possibility of producing appropriate isotope-based temperature calibration. The relationships between isotopes and the July-August relative humidity are more stationary than those with temperature. This may reflect the first order control of the relative humidity on delta C-13 via the stomatal conductance and its influence on the evaporative enrichment of the oxygen of the leaf-water. Our study suggests that past variations of relative humidity in the French Alps can be accurately estimated using the stable isotope composition of larch cellulose. (C) 2011 Elsevier B.V. All rights reserved.