The decline of metallophyte vegetation in floodplain grasslands: Implications for conservation and restoration

Abstract

AbstractQuestion: Which biogeochemical processes are responsible for the decline of endemic metallophyte vegetation in floodplain grasslands?Location: Floodplain grasslands along the River Geul (the Netherlands) and metalliferous mine spoils near the River Geul (Belgium).Methods: In order to find factors and soil processes that have caused a decline of metallophytes and an increase of pseudo‐metallophytes in floodplain grasslands, a soil study was done at locations currently and formerly dominated by metallophytes. In addition, changes in soil chemistry in recent decades in floodplain grasslands were investigated. Finally, a 2‐year plant growth experiment was performed in the field, to test the effects of improving soil conditions by topsoil removal.Results: Metallophytes only occur on acidic floodplain soils (pH‐H2O 5.0‐5.5) with relatively high Zn availability (total Zn >40 μmol g−1; Zn/Ca>0.8; Zn‐H2O>59 μmol kg−1) combined with low phosphate availability (Olsen‐P≪1250 μmol kg−1). The Olsen‐P and total Ca concentrations were relatively high in topsoil (0‐20 cm), while total Zn was high throughout the soil profile (0‐50 cm). Removal of topsoil led to recovery of P and Zn availability. Under the new soil conditions there was almost no growth and expansion of pseudo‐metalliferous grasses in time, wheras metallophytes easily established, maintained and reproduced with significant increases in cover over time.Conclusions: On a global scale, metallophyte vegetation types are increasingly under threat of extinction. Our study shows that factors leading to higher soil alkalinity inhibit Zn availability at the expense of the metallophyte vegetation. Factors leading to a higher Olsen‐P concentration stimulate the growth of more competitive pseudo‐metallophyte grasses. Both eutrophication and alkalinisation have contributed to the decline of metallophyte vegetation in floodplains of the River Geul. Removal of the alkaline‐ and phosphate‐enriched soil top layer restores the original soil chemistry and enables re‐colonisation by the metallophyte vegetation. The results can be applied in conservation and restoration strategies for such sites.