Geographic rainfall partitioning
In order to identify all potential water sources in the soil and rocks used by the two tree species, we first generated a sketch map of the Local Meteoric Water Line (LMWL) using the δ18O and δD values of rainwater of each of 52 precipitation events (Fig. 1). Since the Atlantic basin (Golf of Mexico) is closer to the study area than the Pacific basin (351.56 vs 426.44 km, Fig. S1), distinct continental effects resulted in well-defined isotopic signatures of summer rainwater originating from the Pacific and the Atlantic. Between January 2012 (early winter) and December 2014 (late autumn), we tracked each continental ingress of moisture from tropical storms deriving from the Pacific Ocean basin (18 events), the Gulf of Mexico (16 events), and from cold fronts from the North (12 events). Five rain collectors (All-weather rain gauge, Forestry Supplies Inc.) partially filled with mineral oil to avoid evaporative water loss were installed equidistantly in the different forest (pure and mixed pine and oak) stands along the 3.5 km transect. Water from each rain event was collected and stored in glass vials with screw caps to prevent isotopic fractionation; all vials were stored in iced coolers and transported to the laboratory in the Division of Environmental Sciences at the Instituto Potosino de Investigación Científica y Tecnológica, A.C., in San Luis Potosí, Mexico, where vials were refrigerated (6 °C) until later isotopic analysis. To track each meteorological rainfall event, we used forecasts from The National Meteorological Service of Mexico (http://smn.cna.gob.mx/), the weather network from INIFAP (http://clima.inifap.gob.mx/), The Weather Channel (http://weather.com/) and The Tropical Weather Center (http://wx.hamweather.com/tropical/).
Examining spatial and temporal water use and niche utilization