Grass and woody species composition and diversity beneath mistletoe-infected and uninfected Sclerocarya birrea (marula) trees in two South African semi-arid savanna protected areas

• Mistletoe impacts in protected areas across rainfall sites are understudied. • Woody species richness was greater in the wetter site and beneath infected trees. • RaoQ was higher in the wetter site and beneath mistletoe-infected trees. • Rainfall had greater effects with mistletoe effects being mostly additive. • Rainfall and mistletoe infection shape understorey plant heterogeneity. Mistletoes increase understorey resource heterogeneity by altering animal visitations, litterfall, and soil moisture, thus support diverse understorey plants. However, their impact on understorey species composition and diversity in protected areas with small to large-sized herbivores remains unclear. We compared grass and woody species under mistletoe-uninfected and infected marula ( Sclerocarya birrea subsp. caffra ) trees in the Sabie and Sabi Sand Game Reserves, with differing mean annual rainfall, 500 mm and 610 mm, respectively. Species composition varied slightly between sites and with infection level. Total species evenness was 8% higher in the wetter site, where woody species diversity indices were up to 52% higher. In contrast, grass species richness and diversity were up to 33% higher in the drier site. Functional dissimilarity (FDis), Functional divergence (FDiv), and Rao's quadratic entropy (RaoQ) were up to 23% higher in the wetter site. Infected trees had 4% greater total cover abundance, and 32% higher woody species richness than uninfected trees. Additionally, eight woody and five grass species were exclusive to infected trees, compared to two woody and three grass species under uninfected trees indicating mistletoephily. RaoQ was 8% lower beneath uninfected than infected trees. Tree height significantly influenced total cover abundance and evenness and grass species richness and evenness. Overall, rainfall had a greater effect than mistletoe infection on understorey plants, suggesting moisture limitations and that mistletoe effects are additive. Thus, by enhancing resource availability and supporting mistletoephily, the interaction of mistletoes with other factors such as rainfall influence plant heterogeneity in these low-altitude semi-arid savannas.