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