Patterns and processes in a seasonally flooded tropical plain: the Apure Llanos, Venezuela

Aim The factors and processes determining the ecology and distribution of ecosystems in the Apure lowlands, Venezuela, are critically discussed. Location The Middle Apure region, an alluvial Quaternary plain built by the Arauca and Apure rivers, occupies a subsidence area caused by faulting of the geological basement during the last and most active phases of the Andean uplift. Methods To assess the determinants of the hydrological and ecological variability in this region, we used remote imagery and field surveys. Results The hydrological and ecological variability responds to a set of interconnected processes of different nature: geological, climatic and paleoclimatic and hydrographic, which together have conditioned the geomorphological history of the regional landscape. This evolution, in turn, has guided the soil forming processes and the functional behaviour of the different types of savanna, forest and wetland ecosystems. The continuously subsiding trend of the Llanos that accompanied the Andean Orogeny, together with the Quaternary climatic oscillations, settled the framework in which successive depositional and erosional events took place. Five Quaternary depositional events, gave rise to the actual landscape pattern. The youngest, Qoa is the actual flood plain, Qob is the Holocene flood plain, Qi is from Upper Pleistocene, Q2 from Middle Pleistocene, while Q3 the oldest sedimentary material in this area, is of Lower Pleistocene age. Within each sedimentary unit, we analysed the pattern of land forms, soils and natural ecosystems. The Q0 and Q, land units show a characteristic unstable drainage system, with frequent changes in the course of rivers and their affluents and diffluents. Soil genesis mainly correlates with the contrasting characteristics of the wet and dry tropical climate, and with the water regime of each land form on each sedimentary unit. So, a sequence from entisols, through inceptisols and alfisols, to ultisols, correlates with the sequence of land units from Q0 to Q3. Different vegetation types predominate on each land form/soil unit. Gallery forest (evergreen and semi-evergreen) exclusively occurs on QOa entisols, semi-deciduous forest characterizes Qob and Q, inceptisols, semiseasonal savanna is almost entirely restricted to Q0 and Q, units, hyperseasonal savanna widely dominates on Q2 and Q3 alfisols and ultisols, while seasonal savanna just occurs on the well-drained Q, and Q2 alfisols, and on dunes. Main conclusions The genesis and dynamics of the various land forms and soils throughout the Quaternary, provide the key to their particular hydrology, and determines the kind of plant formation able to occupy each habitat, thus explaining the complex pattern of quite different ecosystems found in the region.