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.