Invasions by alien species are a major threat to biodiversity and ecosystem functioning. For example, invasion of temperate deciduous broadleaf forests by Acacia tree species may be particularly troublesome. Acacia species can use atmospheric nitrogen and grow fast, quickly colonizing disturbed areas before the native vegetation has time to establish. However, the effects of forest invasion by alien trees on streams are poorly studied, despite the strong dependency of these aquatic systems on their riparian vegetation. We present a conceptual model to predict the effects of forest invasion by alien nitrogen-fixing trees on streams, taking into account the characteristics of native and invasive tree species and the extensive literature on stream responses to nutrient enrichment, water scarcity and changes in litter inputs. Our model predicts that the effects of these invasions include changes in water quality (increasing nitrogen concentration through e.g. decomposition of nitrogen rich litter in streams), decreases in water flow (due to high evapotranspiration by dense stands of evergreen Acacia trees) and changes in litter inputs (due to the replacement of diverse forests by nearly monospecific Acacia stands). We determine that the magnitude of these changes in streams will depend on the differences between the native and invasive trees, the stream characteristics, and the extent and duration of the invasion, and can affect both stream communities and processes. Consequently, forest invasion by nitrogen-fixing trees can jeopardize stream ecosystem services, with relevant social and economic consequences.

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Ferreira V., Figueiredo A., Graça M.A.S., Marchante E. & Pereira A. 2021. Invasion of temperate deciduous broadleaf forests by N-fixing tree species – consequences for stream ecosystems. Biological Reviews, https://doi.org/10.1111/brv.12682

Text written by Verónica Ferreira and edited by Clara Ruiz and Félix Picazo