Seed Dispersal in Flood Plain Forests of Amazonia

Abstract

Seed dispersal in the seasonally inundated flood plain forests of whitewater (varzea) and blackwater rivers (igap6) of Central Amazonia was investigated in a two year field study with particular attention being paid to the phenology of the constituent tree species and the role of water and fish in the dissemination of their diaspores. Maximum fruiting of the trees coincides with the period of inundation, and the specific timing of diaspore release appears related to special dispersal mechanisms, dormancy, and/or requirements for germination and seedling establishment. Due to the possession of specific tissues or other devices that provide buoyancy, the diaspores of most tree species of the inundated forests are capable of floating for prolonged periods. A comparison of diaspore characteristics between hydrochorous tree species and their congeners in noninundated habitats reveals recurrent patterns: some lineages that in noninundated habitats possess dehiscent fruits with anemochorous or zoochorous seeds, in inundatable habitats switch to the production of hydrochorous seeds. Other lineages that in noninundated habitats have dehiscent fruits, in inundatable habitats switch to the production of indehiscent hydrochorous fruits. Lineages that in noninundated habitats produce indehiscent fruits remain indehiscent when switching to hydrochory in inundatable habitats. In the inundated forests virtually all diaspores that fall into the water are consumed by fish, with rates of destruction differing greatly. Catfish cannot disrupt hard protective layers of seeds and fruits, while characins are destructive and act largely as seed predators. Very small diaspores, such as those of Cecropia, pass the gut of characin fish unimpaired and viable; for larger diaspores the rate of destruction seems to decrease with the age of the consuming fish. Most hydrochorous diaspores can be dispersed by fish, if they are not destroyed by them (facultative ichthyochory). Others depend on fish for dispersal (obligatory ichthyochory), for example, because their diaspores are very heavy and would sink to the ground under the parent tree, or because their seeds are enclosed in a hard shell from which they can be freed only by the jaws of characins. Ultimately, the diaspores dispersed by water or fish sink to the bottom of the inundated forests from where they start germinating shortly after the end of the submersion. It is most likely that the dormancy of many kinds of seeds is broken by their exposure to the hypoxic conditions that prevail in the still water. Evolutionarily, the synchronization of fruiting at high water must have preceded the origin of the adaptations for hydrochory, ichthyochory, and germination.