Spreading of the cup plant (Silphium perfoliatum) in northern Bavaria (Germany) from bioenergy crops

Invasive species can be the cause of severe problems for biodiversity, economy and human health. The cup plant (Silphium perfoliatum) is native to eastern North America and is increasingly cultivated in Germany as a new bioenergy crop. Its growth characteristics and autecology do not exclude a possible invasive potential. However, there are hardly any studies on this to date. In this study, habitat requirements for spontaneous colonization and establishment of the cup plant were investigated. Therefore, a 15 m radius around eleven cup plant fields in northern Bavaria (Germany) was examined. Data on cup plant colonization, habitat type, vegetation structure, ground cover, and further site conditions using the Ellenberg indicator values were collected and analyzed by logistic regression models. Spontaneously colonized cup plants were found in a wide range of habitats. Open habitats and human settlement areas were particularly suitable, especially field margins and agricultural paths. A portion of open soil of about 25% was preferentially colonized. Cup plants occurred predominantly within the first few meters of the field margin and increasingly around cup plant fields that have existed for a longer period. Favorable for the development of stems and thus for flowering, fruiting and establishing are warmer sites with a high herb layer. Individual plants that had developed a stem persist for several years and increased their stem number over time. The number of stem-developing individuals also increased over time. Thus, there exist an enormous potential for spread in the future. However, an invasive potential could not be confirmed based on the present study, because a threat of biodiversity was not proven

Micro-habitat and season dependent impact of the invasive Impatiens glandulifera on native vegetation

The impact of invasive species is often difficult to assess due to species × ecosystem interactions. Impatiens glandulifera heavily invaded several habitat types in Central Europe but its impact on native plant communities is rated ambiguously. One reason could be that the impact differs between habitat types or even between environmentally heterogeneous patches (micro-habitats) within one habitat type. In the present study a vegetation survey was performed within heterogeneous riverside habitats in Germany investigating the impact of I. glandulifera on native vegetation in dependence of environmental conditions. The vegetation was recorded in summer and spring because of seasonal species turnover and thus potentially different impact of the invasive plant. We found that the cover of I. glandulifera depended on environmental conditions resulting in a patchy occurrence. I. glandulifera did not have any impact on plant alpha-diversity but reduced the cover of the native vegetation, especially of the dominant species. This effect depended on micro-habitat and season. The native vegetation was most affected in bright micro-habitats, especially those with a high soil moisture. Not distinguishing between micro-habitats, plant species composition was not affected in summer but in spring. However, environmental conditions had a higher impact on the native vegetation than I. glandulifera. We conclude that within riparian habitats the threat of I. glandulifera to the native vegetation can be rated low since native species were reduced in cover but not excluded from the communities. This might be due to patchy occurrence and year-to-year changes in cover of I. glandulifera. The context-dependency in terms of micro-habitat and season requires specific risk assessments which is also an opportunity for nature conservation to develop management plans specific to the different habitats. Particular attention should be given to habitats that are bright and very wet since the effect of I. glandulifera was strongest in these habitats