Water economy of Neotropical savanna trees: six paradigms revisited
Guillermo Goldstein (1, 2, 3), Frederick C. Meinzer (4), Sandra J. Bucci (5), Fabian G. Scholz (5), Augusto C. Franco (6) and William A. Hoffmann (7)
1. Department of Biology, University of Miami, P.O. Box 249118, Coral Gables, FL 33124, USA / 2. Laboratorio de Ecología Funcional, Departamento de Ecologia, Genetica y Evolucion, Facultad de Ciencias Exactas y Naturales,
Universidad de Buenos Aires, Ciudad Universitaria, Nuñez, Buenos Aires, Argentina / 3. Corresponding author () / 4. USDA Forest Service, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97331, USA / 5. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Laboratorio de Ecologia Funcional, Departamento de
Biología, Universidad Nacional de la Patagonia San Juan Bosco, 9000 Comodoro Rivadavia, Argentina / 6. Departamento de Botanica, Universidade de Brasília, Caixa Postal 04357 Brasília, DF 70919-970, Brazil / 7. Department of Botany, Campus Box 7612, North Carolina State University, Raleigh, NC 28695-7612, USA / Received February 2, 2007; accepted June 22, 2007; published online January 2, 2008
Summary
Biologists have long been puzzled by the striking morphological and anatomical characteristics of Neotropical savanna trees
which have large scleromorphic leaves, allocate more than half of their total biomass to belowground structures and produce
new leaves during the peak of the dry season. Based on results of ongoing interdisciplinary projects in the savannas of central
Brazil (cerrado), we reassessed the validity of six paradigms to account for the water economy of savanna vegetation. (1)
All savanna woody species are similar in their ability to take up water from deep soil layers where its availability is relatively
constant throughout the year. (2) There is no substantial competition between grasses and trees for water resources during
the dry season because grasses exclusively explore upper soil layers, whereas trees access water in deeper soil layers. (3)
Tree species have access to abundant groundwater, their stomatal control is weak and they tend to transpire freely. (4) Savanna
trees experience increased water deficits during the dry season despite their access to deep soil water. (5) Stomatal conductance
of savanna species is low at night to prevent nocturnal transpiration, particularly during the dry season. (6) Savanna tree
species can be classified into functional groups according to leaf phenology.
We evaluated each paradigm and found differences in the patterns of water uptake between deciduous and evergreen tree species,
as well as among evergreen tree species, that have implications for regulation of tree water balance. The absence of resource
interactions between herbaceous and woody plants is refuted by our observation that herbaceous plants use water from deep
soil layers that is released by deep-rooted trees into the upper soil layer. We obtained evidence of strong stomatal control
of transpiration and show that most species exhibit homeostasis in maximum water deficit, with midday water potentials being
almost identical in the wet and dry seasons. Although stomatal control is strong during the day, nocturnal transpiration is
high during the dry season. Our comparative studies showed that the grouping of species into functional categories is somewhat
arbitrary and that ranking species along continuous functional axes better represents the ecological complexity of adaptations
of cerrado woody species to their seasonal environment.
Keywords:
cerrado, nighttime transpiration, tropical savannas, water deficit, water uptake.
