Statistical analysis
To compare the stable isotopic composition (D/H and18O/16O isotope ratios) considering natural abundance ( ‰) in xylem sap, plant (Ψleafin MPa) and soil (Ψsoil in MPa) water potentials, we implemented a multifactorial repeated-measures analysis of variance using a mixed model, which included as classification factors, species with two levels (Q. potosina and P. cembroides , fixed effect), forest stand with two levels (pure and mixed, fixed effect) and time (sampling dates, random effect) with 56 levels (biweekly dates). For the fracture water injection assay, we implemented an analysis of variance to examine the isotopic composition of xylem sap, using a mixed model with species (Q. potosina and P. cembroides , fixed effect), forest stand (fixed effect), tree distance (0-2, 2-4, 4-6 m; fixed effect) and time (sampling dates, random effect) as classification factors. Also, to compare changes of diameter at breast height and wood moisture content a 2x2 mixed model factorial including as factors species with two levels (pine and oak) and ecohydrological periods with five levels (depletion 2013, recovery 2013, wet 2013, depletion 2014, and wet 2014) was applied to compare changes of diameter at breast height and wood moisture content. In addition, a nested two-way factorial was used to compare treatment effects on root frequency for each diameter class. The model included forest stand with three levels (pure and mixed pine and oak stands; fixed effect) and soil depth with four levels (0-10, 10-20, 20-30 and >30 cm; nested effect). In all cases, we conducted Tukey’s post hoc mean comparison test. Regarding the labeled water treatment in P. cembroides trunks, we analyzed the presence of isotopic label in twigs using a one-way ANOVA followed by a Tukey’s post hoc mean comparison test. We ran Type I regression analyses to examine the relationships between wood moisture content and tree diameter at breast height for P. cembroides andQ. potosina . Polynomial quadratic regression analyses were implemented to examine the relationship between soil water potential (Ψsoil , MPa) and resistivity (ERT, Ω m−1) between October 2013 and May 2014. Before statistical analyses, response variables were examined for normality using the Shapiro–Wilk’s test (Shapiro & Wilk, 1965). Spatial analysis of species-specific vertical root distribution at different soil depths and soil electrical resistivity were examined using the Kriging interpolation method (Empirical Bayesian Kriging Simulations). All statistical analyses were run in SAS University Edition (Free Statistical Software) using PROC MIXED for the repeated-measures analysis and PROC GLM for generalized linear model for the univariate analysis. All geostatistical analyses were run with ArcGIS v. 10.1 for Windows (ArcGIS Desktop, ESRI 2011). The relative contributions of different sources to xylem water were estimated by Bayesian mixing models using the Stable Isotope Analysis in R (siar) package (Parnell, Inger, Bearhop, & Jackson, 2010).