The sources of water used by woody vegetation growing on karst soils in seasonally dry tropical regions are little known. In northern Yucatan (Mexico), trees withstand 4-6 months of annual drought in spite of the small water storage capacity of the shallow karst soil. We hypothesized that adult evergreen trees in Yucatan tap the aquifer for a reliable supply of water during the prolonged dry season. The naturally occurring concentration gradients in oxygen and hydrogen stable isotopes in soil, bedrock, groundwater and plant stem water were used to determine the sources of water used by native evergreen and drought-deciduous tree species. While the trees studied grew over a permanent water table (9-20 m depth), pit excavation showed that roots were largely restricted to the upper 2 m of the soil/bedrock profile. At the peak of the dry season, the delta(18)O signatures of potential water sources for the vegetation ranged from 4.1 +/- 1.1 per thousand in topsoil to -4.3 +/- 0.1 per thousand in groundwater. The delta(18)O values of tree stem water ranged from -2.8 +/- 0.3 per thousand in Talisia olivaeformis to 0.8 +/- 1 per thousand in Ficus cotinifolia, demonstrating vertical partitioning of soil/bedrock water among tree species. Stem water delta(18)O values were significantly different from that of groundwater for all the tree species investigated. Stem water samples plotted to the right of the meteoric water line, indicating utilization of water sources subject to evaporative isotopic enrichment. Foliar delta(13)C in adult trees varied widely among species, ranging from -25.3 +/- 0.3 per thousand in Enterolobium cyclocarpum to -28.7 +/- 0.4 per thousand in T. olivaeformis. Contrary to initial expectations, data indicate that native trees growing on shallow karst soils in northern Yucatan use little or no groundwater and depend mostly on water stored within the upper 2-3 m of the soil/bedrock profile. Water storage in subsurface soil-filled cavities and in the porous limestone bedrock is apparently sufficient to sustain adult evergreen trees throughout the pronounced dry season.

Theophrastus (372-287 BC), Cato (234-149 BC), and Xenophanes (44 BC), pointed out the importance of Traditional weed control practices in Mexico use legumes as cover legumes and grasses as mulches. Other common organic crops or manures. Legumes used in these practices play a dual role in agroecosystems by protecting the soil from erosion and by enriching fertilizers included dung of birds and bats (guano), it with organic matter and N through Rhizobium symbiosis. Farmers fish fertilizer, dry blood, and dry meat. The Maya people in the tropical regions of Mexico use Mucuna spp., Canavalia spp. fertilized their crops with dead leaves from the tropical and other legumes to control weeds in their fields. We conducted in forest, secondary and savanna plant communities, and vitro bioassays and greenhouse experiments to evaluate the toxic animal manures. All these organic compounds help imeffect of four legumes velvetbean [Mucuna deeringiana (Bort) Merr.], prove the soil by increasing water retention capacity, jackbean [Canavalia ensiformis (L.) DC.], jumbiebean [Leucaena

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Adult evergreen Ramó n (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 ramó n trees growing on thin soils (~5 cm deep) over limestone bedrock. We conducted an isotopic study in a non-irrigated ramó n 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 not differ significantly between tree age groups at either sampling date. Stem water d 18 O 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 d 18 O data, ramó n 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

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