Adult evergreen Ramón (Brosimum alicastrum Sw. Moraceae) trees can thrive on the shallow soils atop limestone of northern Yucatan despite limited soil water storage capacity, and do not require irrigation during the annual dry season. We hypothesized that early development of deep roots to reach moist subsoil layers or groundwater is critical to the performance of ramón trees growing on thin soils (∼5 cm deep) over limestone bedrock. We conducted an isotopic study in a non-irrigated ramón plantation aimed at determining the sources of water used by trees of different age (5 or 9 years old) at the peak of the dry season. Pit excavation showed that Brosimum alicastrum roots were mostly concentrated in the upper soil and bedrock layers. About 5-year-old trees showed a much sharper decrease in shoot water content (21%) than 9-year-old ones (10%) during the dry season. Foliar δ13C values were significantly higher in 5-year-old trees (−27.5 ± 0.3‰) than in 9-year-old ones (−28.5 ± 0.3‰), indicating greater water use efficiency and water stress levels in the younger age group. The δ18O signature of stem water did not differ significantly between tree age groups at either sampling date. Stem water δ18O values of Brosimum alicastrum trees at the peak of drought (−2.3 ± 0.3/−2.9 ± 0.3‰) were significantly different from groundwater (−4.3 ± 0.1‰) for both age groups. According to δ18O data, ramón trees utilized bedrock water from depths between 0.5 and 2.5 m during the late dry season. Better dry-season water status in 9-year-old trees compared to 5-year-old ones appeared to be more the result of greater volume of soil and bedrock explored for water than the outcome of deeper rooting depth. The ability to take up water stored in the upper few meters of the weathered limestone bedrock during the pronounced dry season is likely the key feature allowing Brosimum alicastrum to thrive under non-irrigated conditions in the shallow, rocky soils of the Yucatan. Locally adapted native tree species capable of efficiently extracting water from bedrock may be the only perennial crops suitable for rainfed cultivation in the shallow soils atop limestone of northern Yucatan.

This research was supported by a UC MEXUS-CONACYT grant awarded jointly by the University of California Institute for Mexico and the United States, and the Mexican Comisión Nacional de Ciencia y Tecnología. We also thank the financial support of the American Institute for Global Change Research (IAI Project: Biogeochemical cycles under land use change in the semiarid Americas). JI Querejeta acknowledges a postdoctoral Fulbright Fellowship from the Spanish Ministerio de Educacio´n y Ciencia, as well as financial support by the Biocomplexity Program (DEB 9981548) of the US National Science Foundation.

Peer reviewed