Infidelity in the outback: climate signal recorded in Δ18O of leaf but not branch cellulose of eucalypts across an Australian aridity gradient.

The isotopic composition of leaf water in terrestrial plants is highly dependent upon a plant's environment. This isotopic signature can become integrated into organic molecules, allowing the isotopic composition of biomarkers such as cellulose to be used as sensitive paleo and climatic proxies. However, the mechanisms by which cellulose isotopic composition reflect environmental conditions are complex, and may vary between leaf and woody tissues. To date few empirical tests have been made on the relative roles of leaf-water enrichment and source water on the isotopic composition of leaf and wood cellulose within the same plant. Here, we study both leaf and branch wood cellulose, as well as xylem/source water of eucalypts across a 900 km aridity gradient in NE Australia. Across 11 sites, spanning average annual precipitation of 235-1400 mm and average relative humidity of 33-70%, we found a strong and consistent trend in leaf cellulose. However, once the effect of altered source water was considered we found wood cellulose to show no trend across this environmental gradient. We consider potential mechanisms that could explain the 'damping' of a climatic signal within wood cellulose and consider the implication and limitations on the use of tree-ring cellulose as a climate proxy.