Non-native nitrogen-fixing Acacia species have been invading riparian ecosystems worldwide, potentially threatening stream communities that strongly depend on allochthonous litter. We examined the effects of the invasion of native deciduous temperate forests by Acacia species on litter decomposition and associated fungal decomposers in streams. Litter of native (Alnus glutinosa and Quercus robur) and invasive (Acacia melanoxylon) species were enclosed in fine-mesh bags and immersed in three native and three invaded streams, for 14-98 days. Litter decomposition rates, fungal biomass, and aquatic hyphomycete sporulation rates were higher in invaded than in native streams, likely due to the higher water nitrogen concentration found in invaded streams. Alnus glutinosa litter had higher aquatic hyphomycete sporulation rates and species richness, and higher decomposition rates, probably because they were soft and nitrogen rich. Quercus robur litter also had high aquatic hyphomycete sporulation rates but lower decomposition rates than Al. glutinosa, probably due to high polyphenol concentration and carbon:nitrogen ratio. Acacia melanoxylon litter had lower aquatic hyphomycete sporulation rates and species richness, and lower decomposition rates, most likely because it was very tough. Thus, litter decomposition rates varied in the order: Al. glutinosa > Q. robur > Ac. melanoxylon. The aquatic hyphomycete community structure strongly differed between native and invaded streams, and among litter species, suggesting that microbes were sensitive to water nitrogen concentration and litter characteristics. Overall, increases in water nitrogen concentration and alterations in litter characteristics promoted by the invasion of native riparian forests by Acacia species may affect the activity and community structure of microbial decomposers, and instream litter decomposition, thus altering the functioning of stream ecosystems.
Changes in forest composition and litter inputs to streams due to invasion by exotic tree species can affect the functioning of freshwater ecosystems. Acacia dealbata is an important invasive tree species in Mediterranean areas, and often replaces the native riparian vegetation. In this study, we assessed the chemical characteristics of three litter types produced by the invasive Ac. dealbata (leaflets, flowers and pods) and leaf litter produced by two native tree species with contrasting litter characteristics (Quercus robur and Alnus glutinosa). We then assessed litter decomposition and associated microbial activity (i.e., overall microbial metabolism as respiration, fungal growth as biomass accumulation, and reproduction by aquatic hyphomycetes as conidial production), and the aquatic hyphomycetes community structure, in laboratory microcosms. In general, Ac. dealbata pods supported lower microbial activity and decomposed slower than all other litter types, due to their low nutrient concentrations and high carbon:nutrients molar ratio. Alnus glutinosa leaf litter supported high microbial activity and decomposed fast, due to its relatively high nutrient concentrations, low carbon:nutrients molar ratios and low lignin concentration. Acacia dealbata leaflets and flowers and Q. robur leaf litter generally had similar microbial activity and decomposition rates, intermediate between those of Ac. dealbata pods and Al. glutinosa leaf litter, likely due to trade-offs between nutrient concentrations and concentrations of structural and secondary compound among litter types. Aquatic hyphomycetes community structure also differed among litter types. For instance, Ac. dealbata pods had the lowest species richness per sampling date, but due to high dissimilarity among replicates, total species richness over the incubation period was comparable to that of other litter types. The invasion of native riparian forests by Ac. dealbata can affect the quality of litter inputs into streams, potentially affecting the community structure and activity of microbial decomposers, thus altering the functioning of stream ecosystems.
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