Water economy of Neotropical savanna trees: six paradigms revisited

GOLDSTEIN G, MEINZER FC, BUCCI SJ, SCHOLZ FG, FRANCO AC AND HOFFMANN WA

TREE PHYSIOLOGY

Heron Publishing

Lugar: Victoria Canada; Año: 2008 vol. 28 p. 395 - 404

0829-318X

Biologists have long been puzzled by the striking morphological and anatomical characteristics of neotropical savanna trees (e.g. large scleromorphic leaves and more than 50% of the total biomass allocated to belowground structures) and their apparent anomalous leaf phenology (e.g. new leaves are produced during the peak of the dry season). We use recent results of ongoing interdisciplinary projects in the savannas of central Brazil (Cerrado) to assess the validity of the following paradigms: 1. Savanna woody plants have similar patterns of water uptake enabling them to take up water from deep soil layers where its availability is relatively constant throughout the year; 2. There are no substantial competitive interactions between grasses and trees in terms of water resources during the dry season because grasses exclusively explore upper soil layers, whereas trees are able to access deeper soil layers; 3. Because 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 is very low at night to prevent nocturnal transpiration, particularly during the dry season when water is less available; 6. Savanna tree species can be naturally classified into distinct functional groups according to their leaf phenology. We evaluate each of the above paradigms and demonstrate differences in the patterns of water uptake between deciduous and evergreen tree species, as well as within evergreen tree species, having profound implications for regulation of tree water balance. The absence of resource interactions between herbaceous and woody plants is refuted by our observations that herbaceous plants utilize water released from deep-rooted trees into the upper soil layer by hydraulic lift. We also provide evidence of strong stomatal control of transpirational losses and show that most species exhibit homeostasis in maximum water deficit experienced, with midday water potentials being nearly identical in the wet and dry seasons. Although stomatal control is strong during the day, nocturnal transpiration is relatively high during the dry season.  Finally, our comparative studies show that grouping of species into functional groups is somewhat arbitrary and that continuous functional axes rather than distinct categories better represent the ecological complexity of adaptations of Cerrado woody species to their seasonal environment.