Leaf litter decomposition and litter fauna in urban forests: Effect of the degree of urbanisation and forest size

Urbanisation is an important driver of environmental change and has the potential to alter the functioning of ecosystem processes. In urban forests, a key ecosystem process is litter decomposition, which is driven by the litter quality and composition of litter fauna. We examined whether the degree of urbanisation and forest size influence litter decomposition rates by the mesofauna in Basel (Switzerland). In order to assess the impact of the litter mesofauna on decomposition rates, we exposed litterbags with three mesh sizes (100 μm, 2 mm, and 4 mm) filled with a mixture of Acer pseudoplatanus, Fagus sylvatica, and Fraxinus excelsior litter for a period of 9 months in 17 forests of different size located in areas with different degrees of urbanisation. We recovered the litterbags after 3, 6, and 9 months, extracted the fauna and assessed the decomposition rates. We found that litter decomposition rates were negatively affected by the degree of urbanisation after 6 months, but not impacted by forest size. In terms of litter fauna, oribatid mites and Collembola were differently affected by urbanisation. Oribatid mites were most frequent in forests in moderately urbanised areas, whereas Collembola were more abundant in forests in highly urbanised areas. Abundances of Acari and Oribatida were lowest in medium-sized forests, while the abundance of Collembola was highest in large forests. Collembola species composition did not shift with urbanisation. Considering Collembola traits, species with globular bodies were most abundant in forests of medium-size and situated in low urbanised areas, whereas species with an eudaphic life form tended to be less abundant in small forests. We showed that urbanisation and forest size have the potential to impact litter fauna abundance and species with certain morphological and life form traits, which may alter the important ecosystem service of litter decomposition.