The native–invasive balance: implications for nutrient cycling in ecosystems

We conducted single- and mixed-litter experiments in a hardwood forest in Long Island, New York, using leaf litter from phylogenetically paired native and invasive species. We selected long-established, abundant invasive species with wide-ranging distributions in the eastern United States that likely make substantial contributions to the litter pool of invaded areas. Overall, leaf litter from invasive species differed from native litter, though differences varied by phylogenetic grouping. Invasive litter had lower carbon:nitrogen ratios (30.9 ± 1.96 SE vs. 32.8 ± 1.36, P = 0.034) and invasive species lost 0.03 ± 0.007 g of nitrogen and had 23.4 ± 4.9 % of their starting mass remaining at the end of 1 year compared with a loss of 0.02 ± 0.003 g nitrogen and 31.1 ± 2.6 % mass remaining for native species. Mixing litter from two species did not alter decomposition rates when native species were mixed with other native species, or when invasive species were mixed with other invasive species. However, mixing litter of native and invasive species resulted in significantly less mass and nitrogen loss than was seen in unmixed invasive litter. Mixtures of native and invasive litter lost all but 47 ± 2.2 % of initial mass, compared to 37 ± 5.8 % for invasive litter and 50 ± 5.1 % for native litter. This non-additive effect of mixing native and invasive litter suggests that an additive model of metabolic characteristics may not suffice for predicting invasion impacts in a community context, particularly as invasion proceeds over time. Because the more rapid decomposition of invasive litter tends to slow to rates typical of native species when native and invasive litters are mixed together, there may be little impact of invasive species on nutrient cycling early in an invasion, when native leaf litter is abundant (providing litter deposition is the dominant control on nutrient cycling).