Influence of two co-occurring invasive plant species on resident woody species and surface soil properties in Chipinge Safari Area, Zimbabwe

Savanna ecosystems contain multiple invasive woody plant species yet most ecological studies focus largely on single invader species. This study investigated the combined and single impacts of Lantana camara L. (sensu lato) and Dichrostachys cinerea (L.) Wight & Arn. on soil nutrient concentrations and resident woody species structure and diversity in Chipinge Safari Area, Zimbabwe. We laid 60 sample plots of 30x20 m in patches invaded by L. camara (L-invaded), D. cinerea (D-invaded), L. camara and D. cinerea (mixed) and in patches lacking both species (control). Soil C and N in mixed plots were twice greater than in L- or D-invaded (monoculture) plots and three times greater than in control plots. Soil P, K and pH did not differ between mixed and monoculture plots but were significantly higher compared to control plots. Soil moisture decreased on average by 64% in mixed and monoculture plots compared to control plots. Woody plant height, diameter, basal area as well as density of trees, shrubs and seedlings showed no significant difference between mixed and monoculture plots. The distributions of resident woody plant diameter and height conformed to the reverse J structure in control plots and bell-shaped structure in mixed and monoculture plots. The results suggest that co-invasion effects of L. camara and D. cinerea on resident woody vegetation and ecosystem functions can be greater or neutral than impacts of either invader species in isolation. We suggest that co-invaded sites should be prioritized for invasive plant management over single species invaded sites.