Invasive Acacia Tree Species Affect Instream Litter Decomposition Through Changes in Water Nitrogen Concentration and Litter Characteristics

Abstract

Non-native nitrogen-fixing Acacia species have been invading riparian ecosystems worldwide, potentially threatening stream communities that strongly depend on allochthonous litter. We examined the effects of the invasion of native deciduous temperate forests by Acacia species on litter decomposition and associated fungal decomposers in streams. Litter of native (Alnus glutinosa and Quercus robur) and invasive (Acacia melanoxylon) species were enclosed in fine-mesh bags and immersed in three native and three invaded streams, for 14-98 days. Litter decomposition rates, fungal biomass, and aquatic hyphomycete sporulation rates were higher in invaded than in native streams, likely due to the higher water nitrogen concentration found in invaded streams. Alnus glutinosa litter had higher aquatic hyphomycete sporulation rates and species richness, and higher decomposition rates, probably because they were soft and nitrogen rich. Quercus robur litter also had high aquatic hyphomycete sporulation rates but lower decomposition rates than Al. glutinosa, probably due to high polyphenol concentration and carbon:nitrogen ratio. Acacia melanoxylon litter had lower aquatic hyphomycete sporulation rates and species richness, and lower decomposition rates, most likely because it was very tough. Thus, litter decomposition rates varied in the order: Al. glutinosa > Q. robur > Ac. melanoxylon. The aquatic hyphomycete community structure strongly differed between native and invaded streams, and among litter species, suggesting that microbes were sensitive to water nitrogen concentration and litter characteristics. Overall, increases in water nitrogen concentration and alterations in litter characteristics promoted by the invasion of native riparian forests by Acacia species may affect the activity and community structure of microbial decomposers, and instream litter decomposition, thus altering the functioning of stream ecosystems.

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

“…Litter decomposition rates and microbial decomposer activity were higher in the invaded than in the native stream, most likely because water N concentration was significantly higher in the former stream. Invasion of riparian forests by N‐fixing species has been shown to increase water N concentration (Compton et al., 2003; Goldstein et al., 2009; Shaftel et al., 2012; Singh et al., 2019; Stewart et al., 2019; Wiegner et al., 2013), and the same trend has recently been observed in the case of the invasion of temperate deciduous riparian forests by Acacia species (Pereira et al., 2021). The increase in water N concentration in the invaded stream is most likely attributed to the invasion of the native forest by N‐fixing Acacia species, because agricultural and industrial activity is almost non‐existent and there are no other known N sources in the stream basin.…”

“…Additionally, since Acacia species can fix atmospheric N, through their association with root‐nodule bacteria (e.g., Bradyrhizobium spp. ; Souza‐Alonso et al., 2017), water N concentration will most likely increase, as observed recently in streams with riparian forests invaded by Acacia species (Pereira et al., 2021), or in streams flowing through forests composed by other N‐fixing species (Compton et al., 2003; Goldstein et al., 2009; Shaftel et al., 2012; Singh et al., 2019; Stewart et al., 2019; Wiegner et al., 2013).…”

“…sativa and Q . robur trees, with 1% Acacia species in the riparian area (Pereira et al., 2021). Sotão stream (40°07′54.1″N, 8°09′08.3″W, 373 m a.s.l.…”

Quantifying the response of a functional indicator of ecosystem health to disturbance gradients in New Zealand riverine environments: a meta-analysis

“…The stimulatory effects of nutrient enrichment on OM breakdown rates has been observed to be humpshaped, with low breakdown rates present in both low and high nutrient conditions, reaching a threshold when eutrophication becomes increasingly toxic and consequently inhibitory Clapcott et al, 2012;Woodward et al, 2012). The stimulatory effects of nutrient enrichment have also been suggested to be stronger in materials which have low nutrient concentrations (Gulis & Suberkropp, 2003;Gulis et al, 2006;Pereira et al, 2021), such as cotton strips.…”

“…Further, effects can also depend on the various mitigation practices managers can use such as preservation of riparian boundaries and felling techniques (Baillie & Neary, 2015). Disruption and alteration of food webs is common due to changes in types, timing and rates of leaf-litter inputs and the different community compositions they promote (Graça et al, 2002;Martínez et al, 2013;Ferreira et al, 2016;Pereira et al, 2021). Microbial driven breakdown has been shown to exhibit varying responses across different stages post-clearfelling (Mckie & Malmqvist, 2009;Burrows et al, 2014), attributed to the myriad of changes highlighted above.…”