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Water Relations and Stem Water Usage of Trees from the Central Amazonian Whitewater Floodplain (Várzea)


Müller,  Ewald
Working Group Tropical Ecology, Max Planck Institute for Limnology, Max Planck Institute for Evolutionary Biology, Max Planck Society;

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Müller, E. (2002). Water Relations and Stem Water Usage of Trees from the Central Amazonian Whitewater Floodplain (Várzea). In R. Lieberei, H. Bianchi, V. Boehm, & C. Reisdorff (Eds.), Neotropical Ecosystems: Proceedings of the German-Brazilian Workshop, Hamburg 2000 (pp. 623-627). Geesthacht: GKSS-Forschungszentrum.

A wide range of tree species from the Amazon whitewater floodplains are deciduous and show leaf shedding behavior mainly during the aquatic period. We, therefore, are hypothesizing severely physiological stress caused by anoxic conditions in the rooting environment. Those conditions are supposed to inhibit nutrient and water uptake by roots which at least leads to the reduction of transpiring surfaces via leaf shedding. The grade of root water uptake restrictions was estimated by means of stem water usage calculations. The daily stem water usage was studied on different tree species throughout the terrestrial and aquatic period of the year in a secondary stage whitewater floodplain forest near Manaus, Brazil. Stem water usage was calculated by simultaneous measurements of xylem sap flow at the stem base and the crown base of each model tree. Measurements were carried out by means of the "Granier-Method" using constant energy input thermal probes inserted in the sapwood. The daily stem water usage was calculated by comparing the time delay in water fluxes between stem base and crown base. Increasing water uptake restrictions during waterlogging are leading to a higher amount of stem water usage in most of the deciduous species but not in the evergreen species Nectandra amazonum. During the aquatic period the daily amount of water withdrawn from the stem water store during the day and subsequently replaced during the night ranged from 0 kg d-1 in Nectandra amazonum (evergreen) to 4.7 kg d-1 (22% of total daily sap flow) in Pseudobombax munguba (deciduous). The highest amount of stem water usage during the terrestrial period was 13% of total transpiration observed also in Pseudobombax munguba. Although the stem water store was used to a larger extent during the aquatic period, the total sap flow rates of the terrestrial period does not significantly exceed sap flow of the aquatic period in all species. Night time was always sufficient for refilling the sap wood of the stems. We, therefore, could not verify a causal correlation between limited water supply to transpiring leaves and the observed leaf shedding of deciduous species. Leaf shedding never followed decreasing xylem water fluxes but in the opposite sap fluxes decreased simultaneously with prolonged leaf shedding. No significant pre-shedding decrease of leaf water potential could be observed in all species which indicates functioning water supply to the transpiring crown during waterlogging. Root water uptake restrictions were effectively buffered by stem water usage during midday peaks of high water demand