Co-variation of introduced common buckthorn (Rhamnus cathartica L.) and European earthworms in upland deciduous oak forests

Background/Question/Methods

Common buckthorn (Rhamnus cathartica L.) and European earthworms have been posited to facilitate each other’s invasion into deciduous forests of the Midwest United States. Both have been shown to cause changes in belowground processes and can negatively affect herbaceous and woody plant species abundance and richness. Moreover, they are both implicated in a larger invasion cascade that causes widespread agricultural damage. However, the patterns and mechanisms behind their facilitation are still unclear. This study examined the co-variation of common buckthorn and European earthworms across a gradient of common buckthorn invasion in Minnesota, USA. We established twenty-four plots in an upland, mesic oak forest undergoing invasion by common buckthorn. Buckthorn abundance and environmental plot characteristics were surveyed in the summer of 2012. In addition, we collected 1,712 earthworms using a mustard extraction and subsequently identified, weighed, and measured them in the lab. Data was examined using multiple regression to investigate relationships between earthworms and measured plot variables. Interactions were further investigated using path analysis.

Results/Conclusions

Earthworm biomass increased across a gradient of low to high buckthorn (p = 0.01), providing novel evidence for a potential relationship between these two invasive species. Multiple regression showed that the presence of earthworms was best predicted by a model including buckthorn, soil moisture, and the interaction between the two (p=0.01, R²=0.40), suggesting that buckthorn and environmental characteristics may both play a part in determining earthworm presence. Furthermore, path analysis showed that buckthorn had a positive indirect effect on earthworm biomass through its effect on soil moisture (p=0.010), highlighting a possible mechanism of facilitation between the two species. The results of this study have implications for managing invaded forests and suggest that removal of buckthorn may help to decrease earthworm abundance in deciduous forests. While this study highlights a one-way facilitation, future studies should investigate other mechanisms behind these patterns, in particular, the possible facilitation of buckthorn by earthworms.