A meta-analysis on the effects of changes in the composition of native forests on litter decomposition in streams

Native forest replacement by plantations and invasions by exotic plant species are occurring worldwide. We conducted a meta-analysis of studies reporting the effects of these forest changes on litter decomposition in streams. Overall, forest change significantly inhibited litter decomposition rate by ca 18%. However, only the replacement of native forests by deciduous broadleaf or eucalyptus (Eucalyptus globulus) plantations resulted in significant inhibition of litter decomposition (26% and 22%, respectively) whereas conifer plantations or the invasion by exotic species did not significantly affect litter decomposition. The replacement of native forests by eucalyptus plantations was the most common type of forest change in our database. The effect of eucalyptus plantations on litter decomposition was significant when they were present simultaneously at the catchment and riparian scales and when native litter input was replaced by eucalyptus litter input in a manipulative experiment at the stream reach level. This suggests that a strong effect of eucalyptus plantations on ecosystem functioning is mediated by changes on litter inputs to streams. Eucalyptus plantations significantly inhibited the decomposition of high quality litter and total leaf litter decomposition but not the decomposition of wood and low quality leaves, or microbial-driven leaf litter decomposition. This indicates that eucalyptus plantations inhibit (likely through changes in litter inputs) litter decomposition by affecting detritivores. Eucalyptus plantations may thus have stronger negative effects on decomposition rates in streams receiving high quality litter and where detritivores are abundant. The presence of native tree species in the riparian area may mitigate the negative effects of eucalyptus plantations on stream processes. The inhibitory effect of deciduous broadleaf plantations on litter decomposition was likely due to beech (Fagus sylvatica) being used in the majority of the plantations, and beech litter is of low quality for detritivores and decomposers. The lack of significant effects of conifer plantations and invasions might be due to contrasting effects of the different conifer or invasive species on decomposers. This meta-analysis also identified several research gaps that may guide future studies on the effect of forest change on stream functioning.