Plant invasions may profoundly affect ecosystem processes mediated by species interactions in the recipient communities. While many studies have investigated interactions between invasive plants and natural enemies, effects on decomposers have received less attention although decomposition is an important component of biogeochemical cycles. Invasive plants may affect decomposition in different ways: First, they frequently possess acquisitive traits that do not only enable fast growth but are also known to facilitate decomposition. Second, many invasive plants also contain novel secondary compounds with antibiotic effects which may in turn decelerate decomposition rate. Based on the relatively small number of published case studies, it is still impossible to conclude whether there are consistent and predictable effects of plant invasions on ecosystems via decomposition. Therefore, we compared decomposition rate, functional traits and palatability of leaf litter of 13 woody species native to central Europe, and 13 exotic woody species. The experiments included a phylogenetically controlled subset of native and exotic congeners, but also exotics without native relatives. This allowed us to test the hypothesis that decomposition of exotic leaf litter is, on average, lower than that of native litter, thereby disentangling ecological effects from effects of phylogenetic relatedness.
Results/Conclusions
We conducted a decomposition experiment using litter bags of two mesh sizes to separate the effect of invertebrates from microbial effects, and included three populations of each species to account for intraspecific variation. First-order exponential decomposition rate varied substantially between species and populations, but in contrast to our hypothesis, we found no consistent difference between exotic and native leaf litter. This was neither the case when comparing all species, nor in the phylogenetically controlled comparison, nor when exotics without native relatives were considered. Decomposition of exotic and native litter was not differentially affected by mesh size, i.e. by presence or absence of invertebrates. None of the litter functional traits measured differed consistently between exotic and native species. Decomposition rate was strongly dependent on interactions between functional traits, but there was no difference between exotic and native species in these relationships. Feeding experiments with woodlice revealed a close correlation between their relative growth rate and leaf C/N ratio as well as decomposition rate, but again showed no difference between exotic and native leaf litter. We thus conclude that the influence of plant invasions on litter decomposition and matter cycling is governed by idiosyncratic effects, i.e. it depends on what particular species invades a native ecosystem.
