Transport of leaf litter in upland streams of Eucalyptus and Nothofagus forests in south-eastern Australia

Abstract

Leaf transport - especially differences among species from diverse taxonomic groups - is generally less well understood than are the other phenomena that influence the fate of leaves in streams, such as conditioning by bacteria and fungi and fragmentation and consumption by invertebrates. To address this topic, we compared the transport behaviour of entire leaves from five indigenous species of tree in a naturally forested, upland stream in south-eastern Australia: two cool temperate rainforest taxa (Nothofagus cunninghamii (HOOK) OERST. and Atherosperma moschatum LABIL.), two sclerophyllous taxa (Acacia melanoxylon R. BR. and Eucalyptus regnans F. MUELL.) and one taxon from the ecotone between the two forest types (Lomatia fraseri R. BR.). Laboratory experiments indicated that, irrespective of flow regime, rainforest leaves sank markedly more slowly than did sclerophyllous leaves. Eucalyptus regnans leaves, for example, in moving water typically sank within 2 days of immersion, whereas Nothofagus cunninghamii leaves in moving water had a mean time before sinking of 8 to 67 days. These differences in flotation behaviour were reflected in field experiments that used marked leaves to quantify transport down a first-order stream in the study area. The field experiments showed the stream to be highly retentive, with leaves from no taxon travelling more than 100m in six hours. Leaves from rainforest taxa, however, were transported longer distances in a given time than were sclerophyllous leaves, and in some of the latter cases (e.g., Acacia melanoxylon) retention commenced in distances as short as 5 m. There was some evidence that small leaves were transported greater distances than were large leaves; leaf texture and flexibility (a reflection of leaf morphology) also influenced transport distance, but the characteristics of the leaf margins seemingly did not. The ecological significance of these findings is that differential transport will influence the relative contribution made by various species to the pool of coarse particular leaf matter occurring in a given reach of a stream. It will also influence the amount of material available to downstream ecosystems and the formation of allochthonous fossil leaf assemblages.