Thicket Formation and Light Limitation Drive Patch-Scale Vegetation Transitions in a Subtropical Savanna

Bush encroachment, in which C₃ woody vegetation invades historically C₄ grass-dominated landscapes, is common in savannas worldwide. This process may lead to changes in the balance of tree and herbaceous species as well as transitions from open savannas to closed woodland or thicket states. When savannas are open, with high light availability, they support a high diversity of herbaceous species which maintain the overall function and resilience of the ecosystem. We analysed woody cover changes between 1949 and 2021 across 51 sites in a subtropical savanna in the Eastern Cape Province of South Africa (MAP ca. 700 mm). Most sites showed an increase in woody cover over the years. Low woody cover sites were significantly associated with Vachellia karroo trees, together with grass species such as Sporobolus fimbriatus, Digitaria eriantha, Eragrostis chloromelas and the forb Hibiscus aethiopicus. Densely wooded sites were characterised by woody species including Carissa macrocarpa, Canthium ciliatum, Scolopia mundii, Dovyalis zeyheri and Afrocanthium mundianum, alongside herbaceous species Stipa dregeana and Berkheya onopordifolia. The relationship between herbaceous basal cover and canopy conditions showed two data-derived breakpoints, indicating that herbaceous basal cover changed at different rates along the tree-density gradient. The first breakpoint was estimated at 19% woody cover (87% light transmittance; LAI 0.14), where basal cover began a sharp decline at the onset of encroachment and a second at 51% woody cover (63% light transmittance; LAI 0.88), where basal cover decreased steeply, coinciding with a compositional shift from Vachellia karroo-dominated savanna toward evergreen thicket vegetation. Herbaceous species richness declined steeply in the densest canopies, indicating progressive loss of understory diversity as canopies closed. Together, these results suggest that maintaining woody cover below ca. 50% is crucial for preventing progression to a persistent closed-canopy thicket state with a less flammable herbaceous layer.