The varzea forests in Amazonia: flooding and the highly dynamic geomorphology interact with natural forest succession

Although flooding and the highly dynamic geomorphology influence ecophysiology of trees in Amazonian whitewater forests (varzea), information about the extent of these environmental conditions on distribution and richness of tree species is scarce. To better understand dynamic of natural forest succession and the development of different varzea forest types, we inventoried structure and floristic composition of trees >=10 cm DBH in a total area of 5.24 ha in varzea forests near Tefe and near Manaus, Brazilian Amazon. The forests were of different successional stages and situated on different sites along the flood-level gradient. Topography and average inundation of all inventoried trees was measured with a theodolite. Sedimentation was recorded during the aquatic phase 2000 and the soil texture in each site determined. The low-varzea forests were composed of different successional stages subjected to annual floods between 7 and 3 m. Stand density per hectare averaged 490 individuals in the early successional stage, up to 1000 individuals in the early secondary stage and 434 individuals in the late successional stage, species richness amounted to 4, 45 and 91 tree species, respectively. The high-varzea forests were late successional stages subjected to annual inundations above 3 m. Stand density per hectare averaged 407 individuals and species richness amounted to 68 species in a frequently exploited forest near Manaus and to 172 species in a undisturbed forest near Tefe. Species richness and distribution along the gradients of flooding and sedimentation was well defined. Only 2.6% of 222 recorded tree species occurred over the whole flood-level gradient. Sedimentation was highest in the early successional stages near the slip-off slopes of the main-river channels, and decreased with increasing topographic level of the forested sites and proceeding succession, reaching lowest rates in the high-varzea forests. Simultaneously, soil texture changed from coarse-grained soils in early stages to fine-grained soils in subsequent stages. Thus, natural succession in varzea forests is linked to the biogenical induced silting up of the sites, because dunes and large stems building trees slow down water energy and favour the deposition of fine grained sediment. A special vegetation unit within the low varzea is the ‘chavascal’, a species-poor swamp forest, which establishes in the marginal zone of lakes or in ancient silted up river arms. Since it is located off the direct influence of the main-river channels, it seems to be characterised by longstanding stable environmental conditions, and therefore cannot rank with the successional sequence near the highly dynamic river systems.