Tree age, site and climate controls on tree ring cellulose δ18O: A case study on oak trees from south-western France

Abstract

A main concern of dendroclimatic reconstruction is to distinguish in the tree ring proxy the influence of the climate variables of interest from other controlling factors. In order to investigate age, site and climate controls on tree ring width and cellulose δ18O, measurements have been performed in nearby groups of young (145 years old) and older (310–405 years old) oak trees in south-western France, covering the period 1860–2010.Within a given site, inter-tree deviations are small, pointing to a common climatic signal. Despite a similar inter-annual variability, the average level of cellulose δ18O in the young tree group is ∼0.8‰ higher than in the old trees. Such offsets might be caused by different soil properties and differences in the fraction of the source water used by trees from different depths. The δ18O of water in the top soil layer is directly related to the current growing season precipitation, while deeper water can have a lower and more constant δ18O. Local cave drip waters at 10m depth indeed show a constant isotopic composition, which corresponds to pluri-annual mean precipitation.A 2‰ increasing trend is observed in cellulose δ18O of young trees in the first 30 years of growth, during a period when no trend is visible in older trees. This increase can be quantitatively explained by humidity gradients under the forest canopy, and a changing microclimate around the crown as trees grow higher.While relationships between tree ring width and climate appear complex, the isotopic composition of cellulose is strongly correlated with summer maximum temperature, relative humidity and evapotranspiration (r≈0.70). Weaker correlations (r≈0.40) are identified with precipitation δ18O from a 15-year long local record and from the REMOiso model output. These results imply that leaf water enrichment has a stronger control on the inter-annual variability of cellulose δ18O than the δ18O of precipitation.This study demonstrates the suitability of oak tree ring cellulose δ18O for reconstructing past summer climate variability in south-western France, provided that the sampling and pooling strategy accounts for the fact that trees from different sites and of different age can introduce non-climatic signals.