Above-ground and below-ground competition between the willow Salix caprea and its understorey

Questions Has the observed decline in the willow Salix caprea L., one of the most common colonizers of post-mining sites in Central Europe, contributed to changes in the rest of the plant community on post-mining sites? How does S.caprea modify its understorey? How is the understorey vegetation affected by above-ground and below-ground competition with S.caprea? LocationHeaps after coal mining near Sokolov, Czech Republic (50 degrees 1421N, 12 degrees 4045E). MethodsThe effects of above-ground and below-ground competition with thewillow S.caprea on its understorey plant community were studied in un-reclaimed post-mining sites. Below-ground competition was evaluated by comparing (1) frames inserted into the soil that excluded woody roots (frame treatment), (2) frames that initially excluded woody root growth but then allowed regrowth of the roots (open-frame treatment), and (3) undisturbed soil (no-frame treatment). These treatments were combined with S.caprea thinning to assess the effect of above-ground competition. ResultsThree years after the start of the experiment, above-ground competition from S.caprea (as modified by thinning of the S.caprea canopy) had not affected understorey biomass or species number, but had affected species composition. In contrast, below-ground competition significantly affected both the above-ground and below-ground biomass of the understorey. The above-ground biomass of the understorey was larger in the frame treatment (which excluded woody roots) than in the other two treatments. The below-ground biomass of the understorey was larger in the frame than in the open-frame treatment. Unlike above-ground competition (light availability), below-ground competition did not affect understorey species composition. ConclusionsOur results suggest that S.caprea is an important component during plant succession on post-mining sites because it considerably modifies its understorey plant community. Below-ground competition is a major reason for the low cover and biomass of the herbaceous understorey in S.caprea stands on post-mining sites.