Contextualizing Parthenium abundance with hydro-edaphic conditions and native plant species in different land systems in Baringo, Kenya

Invasive species are a threat to biodiversity, altering ecosystem balances by affecting native species. Parthenium hysterophorus is a highly invasive weed that was introduced recently to Kenya, where it is rapidly spreading, particularly in the lowland areas around Lake Baringo in the semi-arid Rift Valley. The recent conversion of rangelands to crop fields created new habitats for the establishment of Parthenium, while the development of infrastructures (roads and irrigation canals) constituted seed dispersal corridors. The emergence of these new spatial-temporal patterns of land systems may differentially affect the dynamics of Parthenium spread and its ecological and economic implications. To understand the determinants and dynamics of Parthenium spread and their effects on native plant communities, we assessed the floristic composition in rangelands, croplands, and key infrastructures, and the hydro-edaphic properties associated with Parthenium abundance. Subsequently, we related Parthenium infestation to changes in the floristic composition in different land systems and seasons, based on 525 observation plots. We recorded 63 species from 19 families. Perennial monocotyledonous species dominated the rangelands, while annual dicotyledonous species were associated with highly disturbed croplands and roadsides. Invasive Parthenium was recorded in all types of land use and infrastructures, except for the wet sodic grasslands. Hydro-edaphic conditions strongly affected the abundances of Parthenium. Densities and relative cover of Parthenium were highest during the wet season, in irrigated croplands, and along irrigation canals (38 individuals/ m2), with compacted poorly drained Gleysols and Fluvisols soils and shallow water tables. At high levels of Parthenium invasion, we observed a 50% reduction in the cover of native vegetation, with a differential displacement of specific native species. While several annual weedy species co-occurred with Parthenium, high abundance of Parthenium and C4-grasses tended to be mutually exclusive. Thus, Parthenium not only affected the biomass and cover of the native vegetation but also reduced the quality of rangeland and fallows for ruminant grazing. With progressing land system shifts towards intensified irrigated agriculture, and with the projected wetter futures with climate change in East Africa, Parthenium will increasingly affect the native vegetation in Baringo and in similar environments of East Africa.