Hydrogen and oxygen isotope ratios of tree-ring cellulose for riparian trees grown long-term under hydroponically controlled environments

Saplings of three riparian tree species (alder, birch and cottonwood) were grown for over 5 months in a hydroponics system that maintained the isotopi c composition of source water in six treatments, ranging from -120 to +180 parts per thousand delta D and -15 to +10 parts per thousand delta(18)O. Th e trees were grown in two greenhouses maintained at 25 degrees C and at eit her 40 or 75% relative humidity, creating differences in transpiration rate s and leaf water isotopic evaporative enrichment. The cellulose produced in the annual growth ring was linearly related to source water with differenc es in both slope and offset associated with greenhouse humidity. The slope of the isotopic composition of source water versus tree-ring cellulose was less than 1 for both delta D and delta(18)O indicating incomplete isotopic exchange of carbohydrate substrate with xylem water during cellulose synthe sis. Tests using the outer portion of the tree-ring and new roots were simi lar and showed that the tree-ring values were representative of the cellulo se laid down under the imposed environmental conditions. The fraction of H and O in carbohydrate substrate that isotopically exchange with medium wate r was calculated to be 0.36 and 0.42 respectively, and biochemical mechanis ms for these observed fractions are discussed. A mechanistic model of the b iochemical fractionation events for both delta D and delta(18)O leading to cellulose synthesis was robust over the wide range of cellulose stable isot ope ratios. The experimental results indicate that both water source and hu midity information are indeed recorded in tree-ring cellulose. These result s help to resolve some of the disparate observations regarding the interpre tation of stable isotope ratios in tree-rings found in the literature.