Utilization of bedrock water by Brosimum alicastrum trees growing on shallow soil atop limestone in a dry tropical climate

Adult evergreen Ramon ( 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 ramon trees growing on thin soils (similar to 5 cm deep) over limestone bedrock. We conducted an isotopic study in a non-irrigated ramon 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 delta C-13 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 delta O-18 signature of stem water did not differ significantly between tree age groups at either sampling date. Stem water delta O-18 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 delta O-18 data, ramon trees utilized bedrock water from depths between 0.5 and 2.5 m during the late dry season. Better dryseason 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.