Stable isotope analysis
For all stable isotope analysis, water was extracted from soil, stem and twig samples with the cryogenic vacuum distillation technique using an extraction line of 5 ports (West, Patrickson, & Ehleringer, 2006). δD and δ18O values of the woody stems, twigs, soil, precipitation and fracture water were determined using a stable isotope ratio mass spectrometer (Picarro L1102-i water isotope analyzer, PICARRO, INC) at the Instituto Potosino de Investigación Científica y Tecnológica in San Luis Potosi, Mexico. Results are reported in delta values, representing deviations in per mil (‰) from the Vienna Standard Mean Ocean Water 2 and Standard Light Antarctic Precipitation 2 (VSMOW2/SLAP2). Overall, analytical precision of the spectrometer was ±0.1‰ for δ18O and ±0.6 ‰ for δD. Additionally, we used GISP (International Atomic Energy Agency) and SSM2 (spring water from SSMVC) as two controls for quality checks of the analysis.