Calotropis procera (Apocynaceae) shrub intrusion on wildlife foraging rangelands in the Ruaha National Park

The relationships between the encroachment of Calotropis procera (Apocynaceae) and understorey herbaceous cover and various environmental variables of recipient grasslands were studied in the north-eastern Ruaha National Park in southern Tanzania. Three hypotheses were investigated. (1) C. procera exerts a negative influence on the cover of understorey herbaceous plants. (2) C. procera exerts a negative influence on environmental variables (humidity, light intensity, soil pH, air temperature, and soil temperature). (3) The native shrub canopy exerts a negative influence on understorey herbaceous plant cover and the environmental variables. We identified two sampling areas, one encroached by C. procera, the other with only native tree/shrub species. In each area, 21 5 m × 5 m subplots were established, and within each, two 1 m × 1 m sample plots were established, one 20 cm from the stem of a C. procera or native trees/shrub, the other at 5 m distance, and herbaceous plant cover and environmental variables were measured. Variables were measured on three occasions (December 2022, March, and July 2023) on the 42 subplots in each sampling area. Results demonstrated a negative influence of C. procera on understorey herbaceous cover, with a significantly lower percentage (16 ± 1.7) of herbaceous cover for plots at 20 cm than 5 m (72.4 ± 1.4) from C. procera stems. Moreover, soil pH was significantly lower at 20 cm than at 5 m distance from the stems in the encroached area, whereas in the non-encroached area, there was no recorded difference in the environmental variables. These results emphasised the necessity for a prompt intervention to contain the spread of C. procera in suitable grazing areas. A detailed study to find out the most practical and cost-effective control approaches appears warranted.