Application of tree-ring isotopic analyses to reconstruct historical water use of riparian trees

Historical patterns of water source use by trees inferred from long-term records of tree-ring stable isotopic content could assist in evaluating the impact of human alterations to natural stream flow regimes (e.g., water impoundments, stream flow diversions, and groundwater extraction). Our objective was to assess the utility of the hydrogen stable isotopic composition (δD) of tree rings as an index of historical water source use by riparian trees. We investigated the influence of site conditions that varied in climate and hydrology on the relationship between δD of Populus xylem water (δD_xyl) and tree-ring cellulose (δD_cell). δD_xyl and δD_cell were strongly correlated across sites (r² = 0.89). However, the slope of this relationship was less than 1, indicating that factors other than δD_xyl influenced δD_cell Inverse modeling with an isotopic fractionation model for tree-ring cellulose suggested that the lack of one-to-one correspondence between δD_xyl and δD _cell was due to the influence of the hydrogen isotopic content of the atmospheric water vapor (δD_atm). Empirically measured values of δD_cell were typically within the seasonal range of δD_cell predicted from the fractionation model. Sensitivity analyses showed that changes in δD_xyl generally had a greater influence at high-elevation montane sites, whereas δD_xyl and δD_atm had about equal influence on δD_cell at low-elevation desert sites. The intrasite relationship between δD _cell and δD_xyl among individual trees was poor, perhaps because of the within-site spatial variation in hydrologic conditions and associated tree physiological responses. Our study suggests that historical variation in δD_cell of Populus provides information on historical variation in both time-integrated water source use and atmospheric conditions; and that the influence of atmospheric conditions is not consistent over sites with large differences in temperature and humidity. Reconstruction of xylem water sources of Populus in riparian ecosystems from δD _cell will be more direct at higher elevation mountain sites than at low-elevation desert sites.