APPLICATION OF TREE‐RING ISOTOPIC ANALYSES TO RECONSTRUCT HISTORICAL WATER USE OF RIPARIAN TREES

E-ISSN： 1939-5582|18|2|421-437

ISSN： 1051-0761

Source： Ecological Applications, Vol.18, Iss.2, 2008-01, pp. : 421-437

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Abstract

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 (δDxyl) and tree‐ring cellulose (δDcell). δDxyl and δDcell were strongly correlated across sites (r2 = 0.89). However, the slope of this relationship was less than 1, indicating that factors other than δDxyl influenced δDcell. Inverse modeling with an isotopic fractionation model for tree‐ring cellulose suggested that the lack of one‐to‐one correspondence between δDxyl and δDcell was due to the influence of the hydrogen isotopic content of the atmospheric water vapor (δDatm). Empirically measured values of δDcell were typically within the seasonal range of δDcell predicted from the fractionation model. Sensitivity analyses showed that changes in δDxyl generally had a greater influence at high‐elevation montane sites, whereas δDxyl and δDatm had about equal influence on δDcell at low‐elevation desert sites. The intrasite relationship between δDcell and δDxyl 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 δDcell 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 δDcell will be more direct at higher elevation mountain sites than at low‐elevation desert sites.