Roots of pioneer trees in the Amazonian rain forest

On low fluvial terraces of the Caqueta River, Araracuara region, Colombia, tree root systems were ex- amined with regard to their structure, underground strati- fication and biomass distribution. Excavations of skele- ton roots and microscopic observation of terminal fine roots included ten species belonging to the Cecropia, Vismia, Miconia, Goupia, Clathrotropisand Brosimum genera. Roots of particular species varied in the external features of their periderm and structure of freshly cut slash. Coarse skeleton roots differed in the shape and po- sition of the plagiotropic branches and positively geotro- pic taproots or sinkers. None of the observed species could be identified as a shallow-rooted tree, in spite of the general fine root concentration in the upper soil hori- zon. In the two seasons of study, the terminal roots dis- played prevailingly secondary anatomical structure and did not maintain much primary anatomical tissue in their apices. Neither ectomycorrhizas nor endomycorrhizas were detected in the samples. In a set of regeneration stages the amount of tree roots linearly increased with the age of growth. However, fine roots below 2 mm in diameter shared 80-90% of all roots in the upper 20 cm layer of all sample plots. In an old-growth forest, the to- tal tree root biomass amounted to 39 ton/ha, thus being comparable to the underground biomass observed in sim- ilar tropical forests.