A NEW METHOD OF TREE XYLEM WATER EXTRACTION FOR ISOTOPIC ANALYSIS FOR TRACKING TREE WATER SOURCES IN THE SACRAMENTO MOUNTAINS, NEW MEXICO

The Sacramento Mountains watershed study, which is designed to assess tree thinning as an effective method of increasing local and regional groundwater recharge, uses a soil water balance approach to quantify the partitioning of local precipitation within a small mountain watershed both before and after tree thinning. Due to thin soils and the complexity of the local system, trees may be extracting water from different sources (e.g. groundwater, fractured bedrock, and soils). Therefore, in order to accurately calculate the soil water balance and to assess how tree thinning will affect the water balance, it is important to identify the sources from which trees extract water. We plan to use the stable isotopes of oxygen and hydrogen to identify these different water sources. We are in the process of developing a new method of determining the isotopic composition of tree water.

Previous studies have shown that the isotopic composition of xylem water accurately reflects that of source water while phloem water has been fractionated. Distillation of water from twigs is often used to extract tree water for isotopic analysis but it may cause fractionation due to the phase changes and it involves the possible extraction of fractionated phloem waters. Employing a new mixing method, the composition of the twig water (TW) can be determined by putting twigs of unknown isotopic water composition into waters of known compositions or initial waters (IW), allowing diffusive processes to proceed to equilibrium, measuring the composition of the resulting mixture or final water (FW) then, solving a mixing equation.

To evaluate this method, we collected several twig samples from Douglas Firs in the Sacramento Mountains. Twig water was prepared for isotopic analysis both by cryogenic distillation and the mixing method. Soil in close proximity to these trees was also sampled and soil moisture was extracted by cryogenic distillation.

Preliminary results show that the isotopic composition of distilled twig water and soil waters plot below the evaporation line, approximately 4‰ heavier with respect to d¹⁸O than results obtained from the mixing method. The mixing method results plot close to the LMWL and indicate snowmelt as a source. This difference is quite significant when interpreting results and is being addressed with additional experiments.