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

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.