Source water, phenology and growth of two tropical dry forest tree species growing on shallow karst soils

Seasonally dry tropical forests are dominated by deciduous and evergreen tree species with a wide range of leaf phenology. We hypothesized that Piscidia piscipula is able to extend leaf senescence until later in the dry season due to deeper and more reliable water sources than Gymnopodium floribundum, which loses leaves earlier in the dry season. Physiological performance was assessed as timing of leaf production and loss, growth, leaf water potential, depth of water uptake determined by stable isotopes, and leaf stable isotopic composition of carbon (δ¹³C) and oxygen (δ¹⁸O). P. piscipula took water primarily from shallow sources, whereas G. floribundum took water from shallow and deep sources. The greatest variation in water sources occurred during the onset of the dry season, when G. floribundum was shedding old leaves and growing new leaves, but P. piscipula maintained its leaves from the previous wet season. P. piscipula showed greater relative growth rate, greater leaf expansion rates, and more negative predawn and midday water potentials than G. floribundum. P. piscipula also exhibited greater leaf organic δ¹³C and lower δ¹⁸O values, indicating that the decrease in photosynthetic carbon isotope discrimination was associated with greater stomatal conductance and greater photosynthesis. Our results indicate that the contrasting early and late dry season leaf loss phenology of these two species is not simply determined by rooting depth, but rather a more complicated suite of characteristics based on opportunistic use of dynamic water sources, maximizing carbon gain, and maintenance of water potential during the dry season

Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate

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 δ18O signatures of potential water sources for the vegetation ranged from 4.1 ± 1.1‰ in topsoil to −4.3 ± 0.1‰ in groundwater. The δ18O values of tree stem water ranged from −2.8 ± 0.3‰ in Talisia olivaeformis to 0.8 ± 1‰ in Ficus cotinifolia, demonstrating vertical partitioning of soil/bedrock water among tree species. Stem water δ18O 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 δ13C in adult trees varied widely among species, ranging from −25.3 ± 0.3‰ in Enterolobium cyclocarpum to −28.7 ± 0.4‰ 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.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 American Institute for Global Change Research (IAI Project: Biogeochemical Cycles under Land Use Change in the Semiarid Americas) for support. J. I .Querejeta acknowledges a postdoctoral Fulbright Fellowship from the Spanish Ministerio de Educación y Ciencia.Peer reviewe

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

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 reviewe

Supplemental Material - Taxonomic and Functional Diversity of Bees in Traditional Agroecosystems and Tropical Forest Patches on the Yucatan Peninsula

Supplemental Material for Taxonomic and Functional Diversity of Bees in Traditional Agroecosystems and Tropical Forest Patches on the Yucatan Peninsula by Laura P. Serralta-Batun, Juan J. Jiménez-Osornio, Virginia Meléndez-Ramírez and Miguel A. Munguía-Rosas in Tropical Conservation Science</p

Supplemental Material - Taxonomic and Functional Diversity of Bees in Traditional Agroecosystems and Tropical Forest Patches on the Yucatan Peninsula

Supplemental Material for Taxonomic and Functional Diversity of Bees in Traditional Agroecosystems and Tropical Forest Patches on the Yucatan Peninsula by Laura P. Serralta-Batun, Juan J. Jiménez-Osornio, Virginia Meléndez-Ramírez and Miguel A. Munguía-Rosas in Tropical Conservation Science</p