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1. Water is a key resource in tropical savannas. Changes in vegetation structure due to land-use change and increased fire frequency may affect the availability of water and the flux of water through these ecosystems.2. We compared the seasonal soil moisture dynamics of two adjacent savanna ecosystems with contrasting tree densities in central Brazil. Our goal was to investigate the influence of tree density on deep water uptake, soil water dynamics and evapotranspiration.3. Soil water was measured using a depth of 7.5 m beneath the tree-dominated cerrado denso ecosystem and to 4 m beneath the grass-dominated campo sujo ecosystem. Plant-available water (PAW) throughout the cerrado denso soil profile ranged from 293 mm at the end of the dry season to 689 mm during the wet season. In the grass-dominated site, PAW in the profile ranged from 155 to 362 mm.4. During the dry season, approximate to 82% of the water used in cerrado denso and 67% in campo sujo was extracted from the profile below 1 m. The tree-dominated cerrado denso used 137 mm more water than the grass-dominated campo sujo. Significant deep soil water uptake was also observed during the wet season of 1998, when rainfall was below average.5. Evapotranspiration (ET) rates (estimated as change in soil moisture over time) were higher in cerrado denso than in campo sujo during both seasons. Estimated ET ranged from 1.4 mm day(-1) during the dry season to 5.8 mm day(-1) for the wet season in cerrado denso, and from 0.9 mm day(-1) in the dry season to 4.5 mm day(-1) in early wet season in campo sujo. The differences in PAW and ET rates between the two ecosystems are associated not only with differences in root distribution, but also with differences in tree densities and the phenology of full-leaf canopies.6. Our results suggest that deep-rooted plants may contribute significantly to the water balance of cerrado ecosystems, and that the hydrological cycle of this biome could change as woody vegetation is replaced by exotic grasses and agricultural crops.

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