Conclusion

Our research highlighted the variability of impact of alien invasive plant species on soil and ecosystem processes. We showed that this variability has two components. The first component is what we called the "site factor" which represents the role of the environment in determining the direction and amplitude of impact. For the seven species tested, the impact of invasion on soil properties depended on the environment in a predictable way. The concentration of nutrients in soil increased following the invasion in initially oligotrophic sites while it decreased in more eutrophic sites. This leads to a strong homogenisation of soil properties across invaded landscapes contributing to the ongoing global ecosystem homogenization.

The second source of variability was the identity of the invasive species. Even for similar growth form, differences of impact were observed. The contrast was particularly obvious for Solidago gigantea and Fallopia japonica. The first had only an important impact on soil P while the second had an impact on all cations and P. The impact of Solidago is mediated by a modification of chemical equilibrium and by the mineralisation of dead fine roots and do not affect the total soil P pool. On the other hand, the impact of Fallopia implies the transport of nutrients from the deep soil to the topsoil through the deep rooting system of the plant. This process increases the total pool of nutrient in the topsoil. Fallopia can be considered as an ecosystem engineer. In addition to these chemical soil modifications, it profoundly modifies the structure and functioning of invaded ecosystems. The plant produces a high amount of slow decomposing litter in which soil mineral N is then sequestered. This, combined with a very conservative management of N, gives a competitive advantage to the invasive plant. The canopy of the plant also modifies the soil microclimate with repercussions on soil fauna which shift from a typical herbaceous community to a typical forest community.

The resilience of all these ecosystem impacts after an eventual eradication of the invasive plants should be measured.

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