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Large single-standing trees contribute to the structural diversity of savannas as they strongly influence their immediate surroundings such as soils and understory plant communities. The influence of woody vegetation at a stand level on the understory vegetation has been extensively studied; however, the understanding of the role of single large trees is limited. The objectives of the study were to 1) evaluate the impact of large trees on understory plant species diversity and composition, herbaceous cover and grass standing biomass, and 2) to establish if plant size and functional qualities such as N-fixing ability modulate understory vegetation responses to overstory trees over two growing seasons (January 2022 and 2023). Vachellia tortilis (a leguminous tree) and non-leguminous woody species (Searsia lancea and Ziziphus mucronata) were studied. We systematically selected 30 trees for each woody species and divided them into two size classes (i.e. small and large trees). Understory plant vegetation was asse ssed using quadrats under and outside the tree canopies. Plant species diversity and abundance were highest under small tree canopies and outside tree canopies compared to under large trees. Panicum maximum was the dominant grass species under large trees regardless of N-fixing ability. Nonetheless, grass cover was enhanced under large V. tortilis and Z. mucronata. Standing grass biomass was higher under the canopies of large trees compared to small trees and outside canopies, with N-fixing ability having no significant effect (p > 0.05). The current findings imply that in agrosilvopastoral systems and game farming, where ecological conservation is a priority for farmers, it is essential to have an overstorey composed of both small and large trees to sustain understory diversity and biomass.