Ecological gradients within riparian forests of the lower Caura River, Venezuela

The distribution of woody species within the 260 km-long riparian corridor of the Lower Caura River, a nutrient-poor tributary of the Orinoco River, draining the Guiana Shield in Venezuela is related to environmental variables. TWINSPAN clustering and a Canonical Correspondence ordination were used on abundance data for the 110 most common tree species in 51 sample plots. Four groups are identified: (i) upstream of La Mura Rapids, narrow floodplains in constrained valleys with steep slopes show marked differences between sites dominated by channel lateral accretion (levees, Group B) and overbank deposition (swamps, Group A), (ii) downstream of the rapids, levees (Group C) are differentiated from a more heterogeneous floodplain (Group D) influenced by a backwater effect caused by the ponding of the Caura River by the Orinoco. A Multiple discriminant analysis of these groups resulted in a function including depth of inundation, ratio of alkaline/alkaline earth major cations, and soil phosphorous content, which accounted for 83% of the variance between the four groups. Inundation level and phosphorous content were also the most significant variables in the ordination, within which the first two axes explained 48% of the species- environment relationships. Tree density, species richness and diversity (H') are shown to change significantly along the lower Caura with highest values associated with levees in sectors upstream of the La Mura Rapids; effects of terrestrialization and intermediate disturbance are proposed to explain these patterns. Floristic elements typical of both Amazonian Igapo and Varzea forests are shown to occur along the whole riparian corridor of the lower Caura, but the majority occur downstream of La Mura Rapids. The intermediate nutritional status of the Caura River and a hydroecological confluence effect associated with higher flooding depths and stronger biogeochemical gradients along the lower reach are suggested to explain the co-occurrence of Igapo and Varzea species.