Yves P. Klinger1, R. Lutz Eckstein2, Annette Otte1 and Kristin Ludewig1
1Institute of Landscape Ecology and Resource Management, Research Centre for Biosystems, Land Use and Nutrition (iFZ), Justus Liebig University Giessen, Heinrich-Buff-Ring 26-32, D-35392 Giessen, Germany
2Department of Environmental and Life Sciences, Biology, Karlstad University, SE- 651 88 Karlstad, Sweden
The legume Lupinus polyphyllus is among the 15 most common non-native plant species in Germany. As an ecosystem engineer, it is capable of altering nitrogen turnover and vegetation structure of invaded sites. It was introduced to the Rhön Biosphere Reserve in the 1930s in spruce woods and along newly built roads and has persisted in the region since then. Its large-scale spread into species-rich mountain meadows began after the annual mowing schedule was postponed for the protection of ground-nesting birds. Studies showed that the area invaded by L. polyphyllus doubled between 1998 and 2016 (Klinger et al., submitted). Overall, management in the Biosphere Reserve is currently not adequate to control the invader and little is known concerning the optimal cutting date to avoid seed maturation.
We analyzed the effect of the cutting date on the germinability of L. polyphyllus in order to develop management recommendations. To this end, we collected lupine seeds from five locations in the Rhön Biosphere Reserve during six weeks and let each seed batch germinate in climate chambers. We assessed easy-to-distinguish seed traits such as seed color and seed hardiness at each harvest date, these ranged from green and soft seeds to black and very hard seeds. Based on this information, we assessed germination percentage, mean germination time (MGT), and synchrony of germination of L. polyphyllus seeds.
Our results showed that germination behavior of L. polyphyllus seeds varied between the cutting dates. While on the first cutting date, when lupine seeds were soft and green, germination percentage was low (9%), during the second and third week of our experiment, lupine seeds showed high (16–26%) and rapid germination (MGT: 98 days). During the last week, after L. polyphyllus seeds had turned black and hard, germination percentage was low (13%) and germination rapid (MGT: 74 days), indicating physical dormancy. Furthermore, the germination of L. polyphyllus was highly asynchronous over all weeks and locations, indicating that L. polyphylus is a “pessimist” concerning germination. Overall, our results underline that L. polyphyllus stands should be managed during flowering and when seed pods are already present on the plants, mowing should be undertaken when the seeds are still green and soft.