Invasion of temperate deciduous broadleaf forests by N‐fixing tree species – consequences for stream ecosystems

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 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.

Litter Quality Is a Stronger Driver than Temperature of Early Microbial Decomposition in Oligotrophic Streams: a Microcosm Study

“…Among the high-quality categories, F. insipida has been reported to decompose at rates similar to alder in temperate sites [49] or even higher in the tropics [50]. As previously stated, forest changes can affect litter composition and decomposition rates, altering C flows [19], and this is not only a projection of climate change effects [13][14][15], but also natural forests suffering different sources of alternation [16][17][18]. Furthermore, these changes tend to reduce litter diversity, with repercussions for instream litter decomposition at the global scale [51].…”

“…Another factor that can alter microbial-mediated litter decomposition is litter quality, which is expected to be reduced due to changed riparian community composition [13] and also due to increased atmospheric C dioxide (CO 2 ) and warming [14,15]. Additionally, afforestation and other human-related impacts such as the substitution of natural forests by plantations [16], biological invasions [17], or emerging diseases [18] can affect the composition of litter and its decomposition rate, thus altering C flows [19]. In general, these alterations render litter inputs characterized by lower nutrient concentrations, as well as higher concentrations of structural compounds and inhibitory secondary metabolites, which often reduce microbial decomposition [20,21] compared to softer and more nutritious litter [e.g., 22,23].…”

Increasing inputs of invasive N‐fixing Acacia litter decrease litter decomposition and associated microbial activity in streams

“…In this stream, 94% of the riparian forest comprises A. dealbata (Pereira et al., 2021), and the supply of N to stream water may occur via: (a) direct input of Acacia litter into the stream and their decomposition in water; (b) leaching of N from the Acacia litter decomposing in the riparian Acacia stands through surface water runoff; and/or (c) flow of soluble N‐rich root exudates from Acacia stands through ground water and hyporheic zones, as observed for other streams that flow through forests with high cover of N‐fixing species (Compton et al., 2003; Goldstein et al., 2009; Shaftel et al., 2012; Singh et al., 2019; Stewart et al., 2019). Nevertheless, we should take into account that, even though water N concentration is still in the oligotrophic range in the invaded stream, probably it will increase in the future, as Acacia species become even more dominant in the stream basin, and the accumulation of organic material from Acacia trees in the riparian zone and the leaching of their soluble N compounds continues through time (Ferreira et al., 2021).…”

Invasive forest pathogens affect the characteristics, microbial colonisation, and decomposition of leaf litter in streams

“…Forests and their streams are tightly connected by the bidirectional transfer of matter and energy (Marks, 2019; Tolkkinen et al., 2020). In particular, forest streams strongly depend on terrestrial organic matter for their functioning (Marks, 2019; Wallace et al., 1997), and are thus potentially susceptible to changes in the forest ecosystem that lead to altered litter inputs (Ferreira et al., 2021; Hladyz et al., 2011; Larrañaga et al., 2021; Mineau et al., 2012). The incorporation of litter inputs into aquatic food webs is mediated by microbial decomposers, mostly aquatic hyphomycetes (Gulis & Suberkropp, 2003; Hieber & Gessner, 2002).…”