Tree water sources
Spatial niches were identified based on soil/geologic strata
characteristics and the temporal changes in the distribution of water
stable isotope signatures in these strata. To identify the sources of
water taken up by pine and oak trees, we determined the isotopic
signatures of each rainfall event, plant water extracted from woody
tissue (twigs), soil water and rock water at monthly intervals for a
total of 36 months. Xylem samples were obtained from collected twigs
that were immediately frozen with dry ice in air-tight tubes for
subsequent water extraction using cryogenic distillation (West et al.
2006) and posterior determination of stable isotope ratios for hydrogen
(D/H) and oxygen (18O/16O). The bark of four to six 3 cm long twigs was
peeled off and the stem stored in 30-mL glass vials sealed with
parafilm-lined caps. At each sampling date (72 samples), we collected
twigs fully covered with periderm from randomly selected adult trees
(four trees, two species and three stands) (n = 16 twig samples per
sampling date, with a study total of 1152 twigs). Soil water was
obtained through cryogenic distillation of soil core (n= 576) extracted
at 15 cm depth and 10-20 cm distance from the tree; at the same time
twigs of the same tree were sampled to identify the source of plant
water use. In addition, we sampled water from rock fractures
accumulating water and from a water spring within the watershed (14
samples total).