Some aspects of the interactions between forest litters and aquatic or terrestrial ecosystems

We have examined the processes responsible for the disturbing effects of dead leaves in freshwater environments. The first is a biochemical deoxygenation caused by the polyphenoloxidase-polyphenol system, which remains active in dead leaves. The intensity of this effect considerably varies from species to species. It depends on the quantity and nature of the phenolic substrates, as well as of the phenoloxidase activities, the presence of variable amounts of phenoloxidase inhibitors such as tanins, and the aqueous extractibility of these agents. The deoxygenating effect of dead leaves macerates is immediate and intense for some species, for example Poplar and Ash, but weak for others, such as Oak and Birch. This first process is followed by two others : 1 ) microbiological deoxygenation and 2) spontaneous precipitation of phytomelanins resulting from their oxidative polycondensation. These phytomelanins have ion exchange and sequestration properties. Nevertheless their disturbing effects o n the aquatic environment are poorly understood at the present time. A fourth process is the toxicity of leaf macerates. This toxicity is probably due to their phenolic compounds, their oxidation products, or other compounds such as saponosides. Thus the Birch, in spite of a very weak deoxygenating effects is very toxic for juvenile roaches. We studied these four different processes in ten different species, and we established their respective role in the pollution of freshwater by dead leaves. These roles are discussed in terms of the phylogeny of the tree species studied, as well as of the functionning of temperate forest ecosystems