Larger trees facilitate understory herbaceous biomass but not diversity in a South African savanna
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Date
2025
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Publisher
Elsevier
Abstract
Large single-standing trees contribute to the structural diversity of savannas as they strongly influence their immediate surroundings including 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 assessed 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 large trees, likely due to the dominance of Megathyrsus maximus under large trees. Megathyrsus maximus dominated under large trees regardless of N-fixing ability. Grass cover increased under large V. tortilis and Z. mucronata, suggesting that evergreen species like S. lancea negatively impact grass cover. Standing grass biomass was higher under large tree canopies than under small trees or outside canopies, with N-fixing ability having no significant effect (p > 0.05), thus, indicating that large trees of both leguminous and non-leguminous woody plants improve grass productivity. 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.
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Keywords
Grass biomass, Grass cover, Invasive species, Species richness, Tree architecture
Citation
Monegi, P. and Samuels, I., 2025. Larger trees facilitate understory herbaceous biomass but not diversity in a South African savanna. South African Journal of Botany, 178, pp.178-186.