Post-thinning fire effects on savanna understory vegetation and ecosystem structure

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Abstract Background Selective tree thinning is a common method for controlling bush encroachment in savanna rangelands. Yet its impact is often temporary because of inadequate post-thinning strategies to suppress woody regeneration. Using fire as a post-thinning measure can be a useful tool to control regeneration of the woody species. This study aimed to (1) evaluate the effects of fire as a post-thinning treatment on herbaceous vegetation (species diversity, biomass, and rangeland condition); (2) assess the impact of thin-burn treatments on tree-seedling establishment; and (3) determine their influence on woody plant mortality. Selective tree removal was applied to reduce woody density (4065 ± 76 plants ha −1 ) of the control (0% woody plant removal intensity (WPRI)) to 10, 20, 50, 75, and 100% WPRIs in 2018. These plots were later burned between 2023 and 2024. Results We found that herbaceous species diversity was consistent across treatments, likely due to dominance by a few species. However, standing grass biomass and herbaceous cover significantly increased in the 20%, 50%, 75%, and 100% thin-burn treatments compared to burn only. These treatments also promoted desirable perennial grasses such as Digitaria eriantha Steud. and Panicum maximum Jacq. While thin-burn treatments had minimal impact on tree-seedling recruitment via coppicing, seed-based recruitment increased notably in the 50%, 75%, and 100% thin-burn treatments compared to burn only. Juvenile tree mortality from fire remained largely unchanged, but large tree mortality increased significantly in the 50% and 75% treatments, this including the large palatable microphyllous species like Vachellia spp. compared to less palatable macrophyllous species such as Euclea and Searsia spp. Conclusion Although a combination of moderate to high-intensity thinning and burning improve herbaceous forage conditions, their use should be informed by careful consideration of ecosystem trade-offs, particularly regarding large tree mortality, species composition, biomass fuel provision and long-term vegetation dynamics.

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