Aboveground roots in Amazonian floodplain trees

Sediment-rich rivers seasonally flood central Amazonian várzea forests, leading to periodic anoxic conditions in the rhizosphere and requiring morphological and structural adaptations, such as aboveground root systems. We investigated some possible relationships between root types and environmental factors in forest plots covering 3.1 ha of várzea in the Mamirauá Sustainable Development Reserve, Brazil. Digital elevation models of the study sites were obtained; sedimentation and soil texture were investigated to check relationship between position of trees on the flood gradient, soil conditions, and aboveground root systems. Different types of aboveground roots were closely related to flooding duration and habitat dynamics. Species subjected to higher and more prolonged floods tended to produce more aboveground roots than species subjected to lower and shorter inundations. Plank-buttressing species increased with decreasing flood height and/or flood duration, and with increasing growth height and basal area. Habitats inundated for long periods were dominated by species with low growth heights and low basal areas, which formed stilt roots and aerial roots. Root system and sediment deposition showed a close relationship, plank buttressing being more common in sites subjected to lower sediment rates. In the disturbed sites close to the main river channel colonized by pioneer species, the occurrence of buttresses was lower than in less disturbed climax stages. No clear relationship was found between root systems and sediment grain sizes