Does the developmental stage and composition of riparian forest stand affect ecosystem functioning in streams?

A common watershed restoration practice to improve water quality and stream ecosystem functions and services is replanting riparian corridors with plant species that may differ from those of natural communities. This restoration practice may have consequences on the aquatic ecosystem processes because organisms obtain energy from leaf litter inputs of the riparian zones. Leaf litter decomposition in streams is a vital ecosystem-level process, which depends on the activity of microorganisms and invertebrates. In the current study, we examined whether the type and developmental stage of riparian forest affect stream ecosystem functioning. We selected three widespread tree species in the Northwest Portugal, namely alder (Alnus glutinosa (L.) Gaertn.), oak (Quercus robur L.) and eucalyptus (Eucalyptus globulus Labill.) and conducted stream litter decomposition experiments with leaf litter from trees differing in developmental stage to assess leaf mass loss, fungal and invertebrate biomass and diversity. Both type and developmental stage of riparian stand significantly affected leaf mass loss, biomass of fungi and benthic invertebrates, sporulation of fungi, and abundance of invertebrates. However, only developmental stage of the riparian stand had an impact on the richness and diversity of fungi, whereas invertebrate diversity and richness was influenced by both stage and type classes. Overall our study provides the novel information that stream ecosystem processes are dictated not only by the composition but also by the developmental stage of the riparian stand. Moreover, this study provides an insight into how by altering riparian forest community composition through restoration practices may have an impact on a key ecosystem process and may have implications for successfully implementing future management strategies.