Abstract
A common hypothesis to explain the effect of litter mixing is based on the difference in litter N content between mixed species. Although many studies have shown that litter of invasive non-native plants typically has higher N content than that of native plants in the communities they invade, there has been surprisingly little study of mixing effects during plant invasions. We address this question in south China where Mikania micrantha H.B.K., a non-native vine, with high litter N content, has invaded many forested ecosystems. We were specifically interested in whether this invader accelerated decomposition and how the strength of the litter mixing effect changes with the degree of invasion and over time during litter decomposition. Using litterbags, we evaluated the effect of mixing litter of M. micrantha with the litter of 7 native resident plants, at 3 ratios: M1 (1∶4, = exotic:native litter), M2 (1∶1) and M3 (4∶1, = exotic:native litter) over three incubation periods. We compared mixed litter with unmixed litter of the native species to identify if a non-additive effect of mixing litter existed. We found that there were positive significant non-additive effects of litter mixing on both mass loss and nutrient release. These effects changed with native species identity, mixture ratio and decay times. Overall the greatest accelerations of mixture decay and N release tended to be in the highest degree of invasion (mix ratio M3) and during the middle and final measured stages of decomposition. Contrary to expectations, the initial difference in litter N did not explain species differences in the effect of mixing but overall it appears that invasion by M. micrantha is accelerating the decomposition of native species litter. This effect on a fundamental ecosystem process could contribute to higher rates of nutrient turnover in invaded ecosystems.
Highlights
In nature, litter of different plant species is typically mixed and the mixtures decompose together
Non-additive effects are often calculated by comparing observed dynamics of decomposition in litter mixtures with predictions of mass loss, or nutrient concentration change calculated from measured decay rates for each component litter decaying alone [2]
What we do not understand is how mixing effects change over the time course of an invasion. We address these questions in forest environments in south China where Mikania micrantha H.B.K., a non-native vine in the Asteraceae, has abundantly invaded many forests where it alters ecosystem processes [22]
Summary
Litter of different plant species is typically mixed and the mixtures decompose together. Most studies have shown that mixing different litter species causes litter mixtures to lose mass at different rates than expected from component species incubated in isolation [1,2,3]. Non-additive effects are often calculated by comparing observed dynamics of decomposition in litter mixtures with predictions of mass loss, or nutrient concentration change calculated from measured decay rates for each component litter decaying alone [2]. A review by Gartner & Cardon [2] concluded that mixing of litter often accelerates decomposition with differences between observed and expected mass loss ranging from 1% to 65%. Some studies showed that litter mixing can slow decomposition from
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