Importance of the subsurface limestone and pockets of soil as sources of water for tree species in Yucatan, Mexico

The importance of the subsurface limestone and pockets of soil as sources of water for tree species in Northern Yucatan, Mexico was investigated. Three limestone layers were identified: the laja (0.3-2.5 m), the sascab (2.5-5 m) and the coquina (5-9 m). Empty cavities and soil pockets represented 10% of the total substrate volume. Physical properties of laja restricted vertical root growth. However, the ten most abundant tree species of the forest had roots growing deeper into the sascab and coquina layers, mainly through rock fissures and soil pockets. Soil pockets held more water than topsoil during the dry season. Rock layers do not hold as much water as the topsoil, but some of them may hold enough to help trees to survive the dry season. Most of the water in the vadose zone of Northern Yucatan is below the topsoil and is retained in the bedrock and soil pockets. Timing of leaf shedding in Piscidia piscipula and Gymnopodium floribundum was not related to their use of different water sources. P. piscipula (late deciduous) took water primarily from shallow sources regardless of the season; rain also appeared to be an important source of water for this species. G. floribundum (early deciduous) took water from topsoil and bedrock but the latter could be a more important source in the dry season. Performance of the two species was not related to the sources of water they used although P. piscipula had always lower water potentials than G. floribundum. There was a positive correlation between oxygen and carbon isotopes in both species suggesting a relatively stable photosynthetic rate and relatively greater variation in the stomatal conductance.