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Fire is a fundamental process in the rangelands of the Great Plains, and dynamic mosaics of areas burned at different times are essential for maintaining and supporting biodiversity, livestock production, and wildlife habitats, among other ecosystem services. The semi-arid rangelands of the Southern Great Plains typically have strong spatial heterogeneity in vegetation structure, which can affect the pattern of fire and its impact on plant diversity and spatial pattern but have received little attention in research. In this study, we investigated the effect of prescribed fires on plant a- and UA7B5-diversity and their spatial pattern in a mesquite-oak savanna landscape in the Edwards Plateau. In an 182.2 ha (450-acre) burn unit in a research ranch managed with a pyric herbivory regime, we sampled 288 randomly located 1-m2 plots in both the pre-fire and post-fire seasons and collected data on plant species composition and abundance. We also mapped the areas burned within the burn unit using high-resolution (21 cm) multispectral data and machine-learning classification. Plant α-diversity was measured using species richness, evenness, and Shannon's H index, and β-diversity using the Sørensen index of dissimilarity and its turnover and nestedness components. Our results show that the prescribed fire appeared to promote α-diversity in soils with sufficient moisture but weakened the overall spatial structure of α- diversity. The prescribed fire negatively affected β-diversity, likely through both direct effects of fire on plants and indirect effects of intensified selective grazing after fire. Differential changes in species composition of forbs and grasses had an important influence on β-diversity. Burn pattern significantly influenced spatial patterns of post-fire ꞵ-diversity. These complex effects of prescribed fire on plant α-and β-diversities and their spatial pattern likely have implications for these heterogeneous savanna landscapes' ecosystem functions and services.