Soil water content and above-ground biomass accumulation, above 10 cm high, were measured monthly in a flooded savanna ecosystem under grazing pressure and under cattle exclusion, during two growth cycles. Near-to-the-ground and below-ground biomass were measured three times during this period. Besides, composition, species richness and diversity were obtained through a floristic inventory. Despite a relatively high floristic richness and diversity, Panicum laxum is the dominant species throughout the study area, while three other perennial grasses, Paspalum chaffanjonii, Leersia hexandra and Axonopus purpusii, also reach high values of cover and biomass. Each of them reacts specifically to flooding, drought and grazing conditions. This ecosystem shows a strongly seasonal behaviour, with primary production, mortality and decomposition sharply timed by soil relative water content. Both drought and water excess seem to limit plant production, even more during wet years when the savanna might remain flooded for up to 4 mo. Some structural and functional differences between the grazed and the protected systems are demonstrated, but under the actual, relatively low stocking rate, the grazed savanna produces as much forage as the ungrazed one.

In tropical moist forests, length of the dry period may have a profound influence on leaf dynamics, plant growth and survival. To evaluate the role of light and water availability on seedling performance, a 1-y experiment was carried out in a tropical moist forest in the Bolivian Amazon in which seedlings of three tree species (Brosimum lactescens, Cedrela odorata and Schizolobium amazonicum) were planted in gaps and the understorey. Variation in length of the dry period was simulated by subjecting part of the seedlings to a water treatment at the end of the dry period. Gaps and understorey had a similar soil moisture content, which varied between 39% in the wet season and 16% in the dry season. Height and leaf growth rates were higher in gap compared to understorey plants, and in the wet compared to the dry season. A high growth during the wet season provided gap plants with a decisive size advantage over understorey plants during the dry season. Their larger root system allowed gap plants to explore a larger surface area and deeper soil layers for water. Consequently, gap plants of Cedrela experienced a shorter deciduous period (22 d) compared to understorey plants (61 d). Watering at the end of the dry season cued the flushing of new leaves by Cedrela, although it did not lead to a higher plant growth.