Controlling the abundance of a native invasive plant does not affect species richness or functional diversity of wet grasslands

By Marie-Therese Krieger, Julia Ditton, Harald Albrecht, Barteline Martina Baaij, Johannes Kollmann & Leonardo Henrique Teixeira

Marsh ragwort (Jacobaea aquatica) is a poisonous plant naturally occurring in pre-alpine wet grasslands in Central Europe. Over the last decades, this plant became a serious problem for farmers and nature conservation agencies. It has considerably increased in numbers and rapidly spread to other grassland sites where it did not occur before. This resulted in many negative impacts on agricultural activities and grassland management, because the whole plant contains highly toxic components (pyrrolizidine alkaloids) that accumulate in the liver of animals consuming it. Because of that, farmers are not willing to manage conservation sites since they have trouble using the hay of productive sites, thus resulting in the abandonment of such sites. Therefore, we developed several mowing regimes to control the spread of marsh ragwort, aimed at reducing light availability for the rosettes of this plant. This can be done by reducing mowing intensity and frequency, thus strengthening the competition exerted by surrounding plants.

To test the effects of the new management regimes, we conducted experiments in 20 different wet grasslands around Bavaria and Baden-Württemberg in Southern Germany. During four years, we monitored the effects of temporary abandonment and decreased mowing intensity in very low- and low-productive sites (2018–2021), as well as the effect of decreased mowing and fertilization at moderately productive sites (2017–2020). We recorded the cover of marsh ragwort plants as well as the number, identity, and cover of all other plant species occurring in our experimental sites. To evaluate the changes resulting from the management regimes tested, we used data collected in the first and last years of the experiments. At all productivity levels, the number of marsh ragwort plants declined with less frequent mowing. Meanwhile, the occurrence of other plants was not strongly affected. However, the abundance of grasses increased, while the abundance of herbs decreased and, when grassland sites were not mown for several consecutive years, less abundant plant species disappeared. To be most efficient in controlling the poisonous species and to avoid negative effects on plant diversity, management has to be adapted to the productivity level of grassland sites. We conclude that to control the spread of marsh ragwort, mowing has to be reduced but not completely stopped, thus also allowing for the maintenance of plant diversity in pre-alpine wet grasslands.

This is a plain language summary for the paper of Krieger et al. (2022) published in the journal Applied Vegetation Science (https://doi.org/10.1111/avsc.12676). This post was prepared by Marie-Therese Krieger and Leonardo Henrique Teixeira.