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

Abstract

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 sea- son. 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 (~5 cm deep) over limestone bedrock. We conducted an isoto- pic 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 d 13 C 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 d 18 O signature of stem water did