Non‐additive effects of invasive tree litter shift seasonal N release: a potential invasion feedback

Abstract

Many invasive plant species strongly alter ecosystem processes by producing leaf litter that decomposes faster and releases N more quickly than that of native species. However, while most studies of invasive species litter impacts have only considered the decomposition of species in monoculture, forest litter layers typically contain litter from many species. Many litter mixtures decompose in a non‐additive manner, in which the mixture decomposes more quickly (synergistic effect) or more slowly (antagonistic effect) than would be expected based on decomposition of the component species’ litters in isolation. We investigated the potential for non‐additive effects of invasive species’ litter by conducting a one‐year litter bag experiment in which we mixed the litters of four native tree species with each of four invasive species. Litter mixtures frequently lost mass at non‐additive rates, although not at every loading ratio, and the presence, sign, and strength of effects depended on species composition. Non‐additive effects on N loss occurred in more litter combinations, and were almost always antagonistic at 90 days and synergistic at 365 days. Invasive species litter with lower C:N led to more strongly synergistic N loss with time. During the growing season, non‐additive patterns of N loss almost always resulted in increased N release – up to six times greater than would be expected based on single‐species decomposition. Consequently, we suggest that invasive species may further synchronize N release from the litter layer with plant N demand, enhancing any positive litter feedback to invasion. These results highlight the need to consider non‐additive effects of litter mixing in invaded forest communities, and suggest that estimates of invasive species’ impacts on ecosystem processes would be improved by considering these effects.

The potential for mass ratio and trait divergence effects to explain idiosyncratic impacts of non‐native invasive plants on carbon mineralization of decomposing leaf litter

“…One potential mechanism driving non-additivity in litter mixtures is differences in nutrient composition between litters, where nutrient-rich litters accelerate decomposition of nutrient-poor litters through nutrient transfer (Gartner & Cardon, 2004;Hättenschwiler et al, 2005). Because non-native invasive plants are, on average, of higher litter quality than co-occurring native species (van Kleunen et al, 2010;Ordonez et al, 2010), it is then suggested that non-native invasives should have synergistic effects on litter decomposition (Heneghan et al, 2002;Swan et al, 2008;Blair & Stowasser, 2009;Poulette & Arthur, 2012;Chen et al, 2013;Schuster & Dukes, 2014, but see Hickman et al, 2013;Schuster et al, 2017).…”

“…Because non-native invasive plant leaves typically have higher nutrient concentrations and specific leaf area than native litters (van Kleunen, Weber, & Fischer, 2010;Ordonez, Wright, & Olff, 2010), which are traits strongly linked to leaf decomposability, it may be expected that they should synergistically increase decomposition rates relative to native species. However, again, there is variability, with litter from non-native invasive plants having antagonistic (Hickman, Ashton, Howe, & Lerdau, 2013;Schuster & Dukes, 2014;Swan, Healey, & Richardson, 2008), synergistic (Chen, Peng, D'Antonio, Li, & Ren, 2013;Heneghan, Clay, & Brundage, 2002;Schuster & Dukes, 2014;Schuster et al, 2017) or negligible effects (Blair & Stowasser, 2009;Poulette & Arthur, 2012;Schuster & Dukes, 2014) on mixed-litter decomposition dynamics. Surprisingly, few studies have considered potential drivers of non-additive effects of litter mixing in regard to non-native invasions, but there is evidence that both the relative proportion of non-native litter (Chen et al, 2013) and trait differences (Schuster & Dukes, 2014;Schuster et al, 2017) of mixed litters can partially explain non-additive effects.…”

Invasive plant species in the Swedish flora: developing criteria and definitions, and assessing the invasiveness of individual taxa

“…through effects on nutrient pools and circulation, pollinator communities, soil biota or a third keystone species) rather than direct, and therefore difficult to identify and evaluate (Pyšek et al , Barney et al ). Further, biodiversity is much more than species and includes other physical entities such as genes and taxa, but also processes and functions (Hawksworth ), which may, as well as ecosystem services, be affected by invasive species (Martina et al , Schuster and Dukes ). For these reasons, we consider the Norwegian approach to assess the actual effect of each recently immigrated species on particular native taxa both too limited and too difficult for a wider practical application.…”

The Relationship between the Distribution of Invasive Plant Alternanthera philoxeroides and Soil Properties is Scale-Dependent

“…Invasive species with lower C/N ratios are associated with more synergistic N loss over time [38]. It has been suggested that invasive species may further synchronise the release of N from the litter layer with plant N demand, enhancing positive feedback to invasion.…”