Ecological impact of Prosopis species invasion in Turkwel riverine forest, Kenya

The impact of Prosopis species invasion in the Turkwel riverine forest in Kenya was investigated under three contrasting: Acacia, Prosopis and Mixed species (Acacia and Prosopis) canopies. Variation amongst canopies was assessed through soil nutrients and physical properties, tree characteristics and canopy closure. Invasion impact was evaluated by comparing herbaceous species cover and diversity, and occurrence of indigenous tree seedlings. Soil characteristics under Prosopis and Mixed species canopies were similar except in pH and calcium content, and had lower silt and carbon contents than soil under Acacia canopy. Tree density was higher under Prosopis intermediate under Mixed and lower under Acacia canopies. Prosopis trees had lower diameters than Acacia tortilis trees. Diameter classes' distribution in Mixed species canopy revealed invasion of Prosopis into mature A. tortilis stands. Herbaceous species cover and diversity were negatively correlated to Prosopis tree density; thus explaining the lower herbaceous species cover and diversity under Prosopis than under Acacia and Mixed species canopies. The study suggests a gradual conversion of herbaceous rich A. tortilis woodland to herbaceous poor Prosopis species woodland or thickets, through indiscriminate Prosopis invasion. ► We assessed herbs and tree regeneration under Prosopis and Acacia tortilis canopies. ► Herbs diversity and productivity were lower under Prosopis than under A. tortilis. ► We also found lack of A. tortilis seedlings under Prosopis canopy. ► Apparently, Prosopis invasion has severe repercussions to riverine forest ecology. ► We recommend Prosopis management to mitigate ecological and livelihoods threats.