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Free keywords:
alpha diversity; Amazonia; floodplain forest; high varzea; low varzea; species distribution; species richness; species similarity; varzea
Abstract:
Aim: Attention has increasingly been focused on the floristic variation within forests of the Amazon Basin. Variations in species composition and diversity are poorly understood, especially in Amazonian floodplain forests. We investigated tree species composition, richness and alpha diversity in the Amazonian white-water (varzea) forest, looking particularly at: (1) the flood-level gradient, (2) the successional stage (stand age), and (3) the geographical location of the forests.
Location: Eastern Amazonia, central Amazonia, equatorial western Amazonia and the southern part of western Amazonia.
Methods: The data originate from 16 permanent varzea forest plots in the central and western Brazilian Amazon and in the northern Bolivian Amazon. In addition, revised species lists of 28 varzea forest inventories from across the Amazon Basin were used. Most important families and species were determined using importance values. Floristic similarity between plots was calculated to detect similarity variations between forest types and over geographical distances. To check for spatial diversity gradients, alpha diversity (Fisher) of the plots was correlated with stand age, longitudinal and latitudinal plot location, and flood-level gradient.
Results: More than 900 flood-tolerant tree species were recorded, which indicates that Amazonian varzea forests are the most species-rich floodplain forests worldwide. The most important plant families recorded also dominate most Neotropical upland forests, and c. 31% of the tree species listed also occur in the uplands. Species distribution and diversity varied: (1) on the flood-level gradient, with a distinct separation between low-varzea forests and high-varzea forests, (2) in relation to natural forest succession, with species-poor forests in early stages of succession and species-rich forests in later stages, and (3) as a function of geographical distance between sites, indicating an increasing alpha diversity from eastern to western Amazonia, and simultaneously from the southern part of western Amazonia to equatorial western Amazonia.
Main conclusions: The east-to-west gradient of increasing species diversity in varzea forests reflects the diversity patterns also described for Amazonian terra firme. Despite the fine-scale geomorphological heterogeneity of the floodplains, and despite high disturbance of the different forest types by sedimentation and erosion, varzea forests are dominated by a high proportion of generalistic, widely distributed tree species. In contrast to high-varzea forests, where floristic dissimilarity increases significantly with increasing distance between the sites, low-varzea forests can exhibit high floristic similarity over large geographical distances. The high varzea may be an important transitional zone for lateral immigration of terra firme species to the floodplains, thus contributing to comparatively high species richness. However, long-distance dispersal of many low-varzea trees contributes to comparatively low species richness in highly flooded low varzea.
