There is no temperature dependence of net biochemical fractionation of hydrogen and oxygen isotopes in tree-ring cellulose

The isotopic composition of tree-ring cellulose was obtained over a two-yea r period from small diameter, riparian zone trees along an elevational tran sect in Big Cottonwood Canyon, Utah, USA to test for a possible temperature dependence of net biological fractionation during cellulose synthesis. The isotope ratios of stream water varied by only 3.6 parts per thousand and 0 .2 parts per thousand in deltaD and delta O-18, respectively, over an eleva tion change of 810m. The similarity in stream water and macroenvironment ov er the short (13 km) transect produced nearly constant stem and leaf water deltaD and delta O-18 values. In addition, what few seasonal variations obs erved in the isotopic composition of source water and atmospheric water vap or or in leaf water evaporative enrichment were experienced equally by all sites along the elevational transect. The temperature at each site along th e transect spanned a range of greater than or equal to 5 degreesC as calcul ated using the adiabatic lapse rate. Since the deltaD and delta O-18 values of stem and leaf water varied little for these trees over this elevation/t emperature transect, any differences in tree-ring cellulose deltaD and delt a O-18 values should have been associated with temperature effects on net b iological fractionation. However, the slopes of the regressions of elevatio n versus the m) and delta O-18 values of tree-ring cellulose were not signi ficantly different from zero indicating little or no temperature dependence of net biological fractionation. Therefore, cross-site climatic reconstruc tion studies using the isotope ratios of cellulose need not be concerned th at temperatures during the growing season have influenced results.