Morphological and productivity responses of Guinea grass (Panicum maximum) under large trees in a South African savanna

• Large trees reduce grass tillering and tuft sizes in savannas. • Grass vegetative height, leaf blade size and biomass improve under tree canopies. • Evergreen woody species diminish reproductive capabilities of grasses. • Tree canopy architecture, not N-fixing ability, is key to predicting P. maximum responses to microsites created by woody species. While herbaceous productivity typically decreases as woody plants become more abundant, there have also been reports of positive feedback, especially from single-standing larger woody species. The shade-tolerant and nitrophilous grass, Panicum maximum (Jacq) has been documented to colonise and flourish under tree canopies, especially large trees. However, information is lacking about the impact of large trees on P. maximum morphological and productivity responses under woody species with different functional traits (i.e. N-fixing ability) and canopy architectures. This study aimed to 1) evaluate the impact of large trees on morphological characteristics and biomass production of P. maximum , 2) determine the effect of deciduous vs non-deciduous trees on P. maximum morphological dynamics and production, and 3) to establish if functional qualities such as N-fixing ability modulate P. maximum responses under leguminous trees over two growing seasons (January 2022 and 2023). The study findings showed that P. maximum plants growing outside tree canopies had a significantly greater number of tillers and tuft sizes compared to the ones growing under tree canopies. No significant differences were found on P. maximum plant height among the microsites. However, P. maximum plant's vegetative height, leaf sizes and biomass production were enhanced under tree canopies. Additionally, the inflorescence length and the percentage of reproductive tillers of P. maximum plants recorded under Searsia lancea (evergreen tree) canopies were substantially reduced. The study findings demonstrated that structural changes created by the expansion and establishment of woody species do not only modify the micro-environment but can also influence the growth and development of both vegetative and reproductive parts of understory grasses.