Pioneer trees in Amazonian floodplains: Three key species form monospecific stands in different habitats

Abstract

Three pioneer tree species - Salix humboldtiana, Cecropia latiloba, Senna reticulata - form monospecific stands in the Central Amazonian white-water flood plain. In contrast to terra firma forests where species composition is unpredictable even for pioneer species, in Central Amazonian várzea the occurrence of the main colonizing species seems to be predictable. This predictability is linked to characteristic habitat conditions and the low number of pioneer species. This preference for different habitats is reflected by different germination and early growth, by the structural and physiological characteristics, as well as by the reproductive and morphological adaptations of the three main species. The germination rate was above 90% in all species, and the duration until germination ranged between one day in Salix and 14 days in Cecropia. Stem elongation was more than 10 cm per month in Salix and Cecropia, and about 50 cm per month in Senna. Wood specific gravity ranged from 0.33 g cm-3 in Cecropia to 0.45 g cm-3 in Senna. The annual wood increment increased by 1.20 (Cecropia), 1.23 (Salix) to 2.14 cm per year (Senna). All species produced adventitious roots, lenticels and/or stem hypertrophy. Leaf photosynthesis was between 17 and 20 μmol m-2s-1, and reached a maximum of 30 μmol m-2s-1 in Senna. Flowering and fruiting in Salix occurred throughout the year, whereas in Cecropia and Senna they were concentrated in the flooded period. Salix humboldtiana occurs mainly at low sites subjected to long periods of inundation and high sedimentation rates. Once Salix has formed dense forest stands, sedimentation and water currents are reduced at these sites and Cecropia latiloba may take over. This species grows on low to middle elevations in the flooding gradient at sites with lower current and sedimentation rates. Senna reticulata does not tolerate submergence and colonizes habitats that may have strong currents and high sedimentation on higher levels in the flooding gradient.