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).