Abstract
Biological invasions are a major threat to biodiversity and ecosystem functioning. Forest invasion by alien woody species can have cross-ecosystem effects. This is especially relevant in the case of stream-riparian forest meta-ecosystems as forest streams depend strongly on riparian vegetation for carbon, nutrients and energy. Forest invasion by woody species with dissimilar characteristics from native species may be particularly troublesome. The invasion of temperate deciduous broadleaf forests with low representation of nitrogen (N)-fixing species by N-fixers has the potential to induce ecosystem changes at the stream level. Although effects of tree invasion on stream ecosystems have been under assessed, knowledge of native and invasive tree characteristics allows prediction of invasion effects on streams. Here we present a conceptual model to predict the effects of forest invasion by alien N-fixing species on streams, using as a background the invasion of temperate deciduous broadleaf forests by leguminous Acacia species, which are among the most aggressive invaders worldwide. Effects are discussed using a trait-based approach to allow the model to be applied to other pairs of invaded ecosystem-invasive species, taking into account differences in species traits and environmental conditions. Anticipated effects of N-fixing species invasions include changes in water quality (increase in N concentration) and quantity (decrease in flow) and changes in litter input characteristics (altered diversity, seasonality, typology, quantity and quality). The magnitude of these changes will depend on the magnitude of differences in species traits, the extent and duration of the invasion and stream characteristics (e.g. basal nutrient concentration). The extensive literature on effects of nutrient enrichment of stream water, water scarcity and changes in litter input characteristics on aquatic communities and processes allows prediction of invasion effects on stream structure and function. The magnitude of invasion effects on aquatic communities and processes may, however, depend on interactions among different pathways (e.g. effects mediated by increases in stream nutrient concentration may contrast with those mediated by decreases in water availability or by decreases in litter nutritional quality). A review of the literature addressing effects of increasing cover of N-fixing species on streams suggests a wide application of the model, while it highlights the need to consider differences in the type of system and species when making generalizations. Changes induced by N-fixing species invasion on streams can jeopardize multiple ecosystem services (e.g. good quality water, hydroelectricity, leisure activities), with relevant social and economic consequences.
Highlights
Since effects are discussed on the basis of plant traits, the potential effects of invasion by Acacia species can likely be extrapolated to other invasive species, taking into account differences in native and invasive species traits and in environmental conditions
Several Acacia species produce allelopathic compounds, which may inhibit the germination of seeds or growth of other species (Souto et al, 2001; Lorenzo et al, 2010, 2011), the role of allelopathy in the process of invasion has been considered mostly negligible under field conditions (Souza-Alonso et al, 2017)
Based on the assumption that it is possible to anticipate the effects of Acacia invasion on stream ecosystems by considering the characteristics of deciduous forest trees and of Acacia species (A. dealbata and A. melanoxylon), we propose a conceptual model to predict the effects of Acacia invasion on stream water quality and quantity, litter inputs to streams, aquatic communities and litter decomposition
Summary
Anticipated effects of Acacia invasion of riparian areas on streams – a conceptual model (1) Changes in water quality (2) Changes in water quantity (3) Changes in litter inputs to streams (a) Changes in the diversity of litter inputs (b) Changes in the phenology of litter inputs (c) Changes in the typology of litter inputs (d) Changes in the quantity of litter inputs (e) Changes in the quality of litter inputs (f) Changes in allelopathic compounds (4) Changes in stream litter decomposition and aquatic communities (a) Response to changes in water quality (b) Response to changes in water quantity (c) Response to changes in litter input characteristics. V. Conceptual framework to predict effects of invasion of riparian areas by N-fixing species on streams
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