Distribution and β-diversity of tree species in igapó forests (Negro River basin, Brazilian Amazon)

Abstract

QUESTIONS: How are the tree species of the periodically flooded black water forest distributed along the Negro River basin (igapo)? Can we classify the tree communities into similarity groups? Are oligarchic species present? METHODS: We analysed available data on tree species composition in vegetation plots located in the Negro River basin (igapo forests) in Brazil and built a similarity matrix based on Sorensen's similarity index. We applied principal coordinates analysis, established community groups (a posteriori) and applied both multi‐response permutation procedures and indicator species analysis. RESULTS: Among 19 surveys conducted in igapo forests, at least 604 woody taxa were recognized, and 384 taxa were identified to species level. Igapo tree species were not as frequent or abundant as were other tree species of the Amazon upland forest, although they were widely distributed in other forest types. The Sorensen similarity index was low (Sₛ = 0.16). Generally, the highest similarity values were recorded between surveys that were spatially close. The multivariate analyses suggested the existence of a strong vegetation gradient along the Negro River. The two groups established a posteriori showed 25 species with an indicator value (IV) >50%, and 68 species presented IndValues in both groups. CONCLUSIONS: Our results show a strong turnover of tree species replacement along the course of the Negro River, with 57.5% of species restricted to one survey of occurrence. This trend is particularly notable from the point at which the Branco River strongly discharges into the Negro River. No species show high values for range distribution or local abundance, although four were considered moderately frequent. The upper parts of the main rivers have indicator species that are less frequent downstream.