ISOTOPIC ENRICHMENT OF WATER IN THE WOODY TISSUES OF PLANTS - IMPLICATIONS FOR PLANT WATER SOURCE, WATER-UPTAKE, AND OTHER STUDIES WHICH USE THE STABLE ISOTOPIC COMPOSITION OF CELLULOSE

The stable hydrogen isotopic ratio (deltaD) of stem water is shown to deviate from source water values in young, nonsuberized woody stems, reflecting an isotopic enrichment associated with cuticular water loss and/or transpiration. The extent of this enrichment was greater in trees which produce a new set of leaves annually (e.g., deciduous-leaved species) than in trees which retain their leaves for two or more years (e.g., coniferous or evergreen-leaved species). Stem-water enrichment was also greater in rapidly growing stem tissues than in mature, fully suberized or lignified stems which had ceased growing and showed little or no isotopic enrichment. These results are discussed in terms of their implications for studies which use xylem-sap deltaD values to evaluate plant water sources and uptake. We also discuss our findings in the context of long-term investigations that use cellulose or cellulose nitrate deltaD values for climatic reconstruction or to infer past water-use or distributional patterns in woody vegetation.