Comparison of Leaf Litter Breakdown Rates Among Rivers with Varying Riparian Vegetation

The invasive riparian species Salix spp. (willows) have caused alterations in riparian areas, shifting them towards more degraded ecosystems, with impacts on ecosystem function and biota in adjacent waterbodies. Rehabilitation of invaded areas to remove willows can have both positive and negative effects on aquatic systems, although impacts on ecosystem function post- removal are poorly understood. The decomposition of allochthonous leaf litter, from riparian areas, is a vital function that supports carbon cycling and exchange of nutrients between terrestrial and aquatic systems. Therefore, this study aimed to evaluate the decomposition rates of invasive and native leaf types within Victoria, and to quantify the associated microbial and invertebrate assemblages. To understand decomposition processes, leaf litter packs were constructed using litter from either eucalypt, blackwood, willow, or blackberry leaves and placed in river systems within three differing riparian vegetation categories; willow-lined, remnant and revegetated. Sampling occurred 14- and 28- days for mass loss and invertebrates, and 21 days for microbial assemblages. Results indicated that decomposition rate was significantly slower in native, than invasive leaf types, though mass loss did not differ over riparian vegetation categories. This, combined with seasonal loss of invasive leaves, suggests that allochthonous input from willows may be an inadequate source of carbon to facilitate ecosystem function year-round. Small-scale variation was seen in mass loss, invertebrate and microbial assemblages, suggesting that other factors, such as leaf litter traits or adjacent land-use, were influencing decomposition processes. Correlations between invertebrate assemblages and water quality variables were also identified, with pollution tolerant taxa more likely to occur in willow-lined sites, anecdotally suggesting those systems are somewhat degraded. These findings add to the body of knowledge regarding the impact of removal and restoration processes within Victorian river systems and can help inform future management strategies. They also provide a knowledge basis for future research in the importance of microbial decomposition in freshwater systems.