Earthworms increase plant biomass more in soil with no earthworm legacy than in earthworm-mediated soil, and favour late successional species in competition
Abstrakt
1. As ecosystem engineers, earthworms greatly affect plant communities. They create persistent soil structures enriched by nutrients that improve the conditions for plant growth and modify competition between plant species. We therefore hypothesized that earthworm activity would be more important in early stages of the primary succession, when the soil is not modified by earthworms, than in the late stages of the succession, when the soil is already improved by earthworms. On the other hand, earthworms also affect plants via many other effects such as seed predation or excreting hormone-like compounds, which could make earthworm presence important in late successional soil.
2. To explore earthworm effects on plant community succession, we performed a laboratory microcosm experiment without and with earthworms (Lumbricus rubellus and Aporrectodea caliginosa), with early successional plants (Poa compressa, Medicago lupulina and Daucus carota) and late successional plants (Arrhenatherum elatius, Lotus corniculatus and Plantago lanceolata) and with soil previously unaffected by earthworms (young soil) and soil substantially affected by earthworms (developed soil). These soils were taken from the early and late successional post-mining sites of the Sokolov coal mining district (northwest Czech Republic).
3. When both early and late successional plants were grown separately, earthworms increased plant biomass proportionally more in the young soil than in the developed soil, indicating that earthworm activity is more important in undeveloped than in developed soil.
4. When early and late successional plants were competing each other, the biomass of the early successional plants was reduced. In the young soil, the reduction was independent of earthworm presence. In the developed soil, the reduction was promoted by the earthworms. Late successional plants profited from the reduction of early successional plants and increased their biomass. This increase was promoted by the earthworm presence.
5. Our results indicate that the direct effects of earthworm presence on plants decrease during succession because of the cumulative effects of earthworm activity on soil conditions. Such ecosystem engineering effects favour late successional competitors and therefore promote the replacement of species during succession.