The variations in δD of tree rings and the implications for climatic reconstruction

The delta D values of cellulose nitrate of tree rings (delta D-cn) have been used to reconstruct the climate records, because trees obtain hydrogen from meteorologically derived water that contains information on atmospheric conditions (e.g., temperature). However, many hydrological and biological factors may affect how hydrogen isotopic characteristics of meteoric water are recorded in tree rings as hydrogen from precipitation is transferred into cellulose. In addition, these factors may cause random isotopic variations within a single site or between different species. In this study, the delta D-cn was analyzed for Douglas-fir (Pseudotsuga menziesii) and subalpine fir (Abies lasiocarpa) trees from five sites within the Olympic Mountains located in the northwest of Washington State, USA. These sites are distributed on both west and east sides of the mountains, which differ in climatic regimes (e.g., dry and wet) and non-climatic conditions (e.g., hydrology, topography and soil permeability). We found that within-site variations of delta D-cn differ from one site to another, depending upon the degree of hydrological homogeneity. A relatively flat and wet site has smaller isotopic variability than a dry, rocky, and hilly site. For limited observations, the variation between Douglas-fir and subalpine fir is no greater than the within-species variation of Douglas-fir. A one-to-one relationship between the delta D-cn and the delta D of source water (delta D-sw) exists among sites. Most of our delta D-cn time series significantly correlate with annual mean temperature, but temperature only explains up to 26% of the total variance in delta D-cn. The sensitivity of the delta D-cn response to temperature ranges from 4.7 to 13.4 parts per thousand/degrees C, which is typically higher and more variable compared to the isotopic response of precipitation to temperature (4.0-5.2 parts per thousand/degrees C). We argue that the more sensitive response of delta D-cn to temperature is caused by inter-annual covariation of humidity with temperature. Copyright (C) 2000 Elsevier Science Ltd.