Fungi without borders: cross-ecosystem decomposition of riparian leaf litter

Fungi are key drivers of leaf litter decomposition in riparian zones, with communities made up of phyllosphere fungi (fungi present on leaves prior to senescence), and fungi that colonize from the environment. While phyllosphere fungi dominate early decomposition in both submerged and terrestrial litter, their contribution to fungal community structure and growth across these environments remains unclear. We used quantitative stable isotope probing to quantify taxon-specific growth rates of phyllosphere fungi and fungi colonizing from the environment, and to differentiate growing fungi from dormant or dead fungi. We quantified the growth rates of phyllosphere and colonizing fungi on two leaf types in aquatic and terrestrial environments over three time points. Growing fungal communities showed substantial overlap between aquatic and terrestrial environments, although this overlap decreased over time. Phyllosphere fungi were more likely to be shared between environments than colonizing fungi. Leaf type exerted a stronger influence on community composition than the environment. Phyllosphere fungi dominated relative abundance and growth during the study, and all fungi grew consistently faster on terrestrial litter. These findings demonstrate the central role of phyllosphere fungi in fungal communities across both aquatic and terrestrial ecosystems, as well as the importance of leaf species.