Effects of the invasion of riparian forest by Acacia spp. trees on leaf litter decomposition in streams

Abstract

Streams depend on riparian forests (for example, as a source of plant detritus that sustains aquatic communities), therefore, they are susceptible to changes in the species that compose them. The riparian forests in central Portugal are threatened by invasive species, including species of the Acacia genus that have characteristics distinct from those of native species. In recent years, several studies have been carried out in streams of Serra da Lousã to understand what effects (if any) the invasion of riparian forests have on aquatic communities and processes. Here we report some results relating to the comparison of the decomposition of plant detritus in three streams in forests with native species and three streams in forests invaded by Acacia spp. Invasion can slow the microbial decomposition of plant detritus, and consequently the recycling of nutrients, mainly by altering the type of detritus entering streams (which are more recalcitrant as a result of the greater contribution of invasive species). Increased concentrations of dissolved nutrients in the water had a small stimulatory effect on detritus decomposition and the access of fragmenting macroinvertebrates mitigated differences in detritus decomposition between stream types. The effects may worsen as the extent of the invasion increases.