The invasive riparian species Salix spp. (willows) have caused alterations in riparian areas, degrading ecosystems and affecting ecosystem function and biota in adjacent waterbodies. Rehabilitation of invaded areas to remove willows can have both positive and negative effects, although impacts on ecosystem function post-removal are poorly understood. The decomposition of allochthonous leaf litter is a vital function that supports carbon cycling and exchange of nutrients between terrestrial and aquatic systems.
Here, we 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 either eucalypt, blackwood, willow or blackberry leaves, and placed in river systems within each of three differing riparian vegetation categories; willow-lined, remnant and revegetated. Sampling occurred after 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, although 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 among sites was seen in mass loss, invertebrate and microbial assemblages, suggesting that other factors, such as leaf traits or adjacent land -use, were influencing decomposition. 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 were degraded.
These findings add to the body of knowledge regarding the impact of willow removal on 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.