Non-additive decomposition of native-invasive tree litter mixtures

Background/Question/Methods

When leaf litter of multiple tree species decomposes in a common environment, litter mixtures can exhibit non-additive decomposition and N loss, in which the rate of decomposition differs from what would be expected based on the litter of each species independently. Non-additive decomposition is often attributed to the presence of high quality leaf litter (low C:N), and invasive plant species often produce high quality litter. In this study, we hypothesized that litter mixtures containing invasive and native tree litter would experience synergistic, non-additive decomposition. To test this, we conducted a litter bag experiment using two-species mixtures of litter from four invasive small tree or shrub species (Acer ginnala, Elaeagnus umbellata, Lonicera maackii, and Morus alba) and four native tree species (Carya glabra, Cercis canadensis, Liriodendron tulipifera, and Quercus palustris) commonly found in Indiana, USA. To examine possible effects of evenness, bags containing mixed-species litter were filled according to loading ratios of 10:90, 50:50, and 90:10 percent native:invasive  tree litter. Litter bags were collected after 90 or 365 days, and were measured for mass loss and nitrogen loss.

Results/Conclusions

After 90 days of decomposition, non-additive effects on mass loss were detected in most species pairings, although not at every loading ratio. Mixtures containing L. tulipifera generally experienced synergistic non-additive effects, where decomposition occurred more rapidly than would be expected based on single-species litter bags. Mixtures containing Q. palustris and C. canadensis experienced antagonistic non-additive effects, where decomposition occurred more slowly than would be expected. Non-additive effects on N loss were almost always antagonistic in nature, and were more common. Non-additive effects were more common in mixtures containing greater proportions of invasive litter. These results suggest that non-additive effects are species dependent, but also that invasive plant species may act to reduce nutrient release in the early stages of decomposition.